Bi-Sb alloys irradiated with swift heavy ions as potential topological amorphous superconductors

Bi-Sb alloys irradiated with swift heavy ions as potential topological amorphous superconductors | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Bi-Sb alloys irradiated with swift heavy ions as potential topological amorphous superconductors Alberto Andrino-Gómez, Gema Tabares, Diego Ramírez, Andrés Redondo-Cubero, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8743396/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Crystalline Bi 100− x Sb x alloys are well known as the first experimentally realized topological insulators, in addition to their promising thermoelectric properties. In contrast, their amorphous counterparts have been reported to exhibit superconductivity with critical temperatures exceeding 6 K. However, the strong tendency of Bi and Bi-Sb alloys to crystallize, even at very low temperatures, has hindered both systematic studies and practical applications of these amorphous phases. To explore the possibility of obtaining amorphous superconducting states and enhancing thermoelectric performance, we investigated ion-beam irradiation as a method to induce amorphization in Bi 100− x Sb x alloys. We performed irradiation experiments on pure Bi and Bi-Sb melt-spun ribbons using iodine ions with energies between 25 and 40 MeV, achieving estimated vacancy damage levels of 40–80%. Structural characterization by X-ray diffraction and electrical resistivity measurements in the range 2–300 K revealed that, although amorphization and superconductivity were not achieved, ion-induced disorder led to significant conductivity improvements, particularly in Bi 90 Sb 10 Furthermore, interesting correlations were observed between the resistivity values and the semiconducting gap with the Sb content, both before and after irradiation. These results provide new insights into the interplay between structural disorder, electrical transport, and topological properties in Bi-Sb alloys. bismuth ion beam modification of materials amorphous solids topological materials electrical resistivity superconductivity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1 Introduction Bismuth (Bi) is a diamagnetic semimetal that has long attracted attention due to its unusual electronic properties when pure or lightly doped [ 1 , 2 ]. Its alloys with antimony (Bi-Sb) form narrow-gap intrinsic semiconductors, which have been extensively studied for their promising thermoelectric performance [2−9]. The high figure of merit of these materials makes them candidates for efficient energy conversion applications, particularly in low-temperature regimes. Interest in Bi-Sb systems was dramatically renewed when it was experimentally demonstrated [ 10 , 11 ] that Bi-Sb crystals behave as topological insulators (TIs), confirming earlier theoretical predictions [ 12 ]. This discovery opened a new research frontier in condensed matter physics, positioning Bi-Sb alloys together with Bi 2 Se 3 compounds [13−16] at the center of studies on topological phases, which remain a highly active field today [17−20]. Beyond their topological properties, it is less widely known that superconductivity in Bi and its alloys with Sb, Pb, and Tl was reported more than seventy years ago, although only in amorphous state, with critical temperatures exceeding 6 K [21−23]. However, these systems exhibit a strong tendency to crystallize even at very low temperatures, limiting the stability of the superconducting phase and hindering practical applications. Recent literature has explored the possibility of topological superconductivity (TS), either intrinsic or induced, in different systems [24−26]. On the other hand, amorphous topological phases have been proposed as an emerging concept [ 20 ]. Interestingly, both issues merge in Bi-Sb materials, since superconductivity in Bi-based alloys appears to occur only in the amorphous state [21−23]. Achieving amorphous Bi-Sb is therefore of great interest, not only to enable superconductivity but also to potentially enhance thermoelectric properties. However, previous studies have highlighted significant challenges in amorphizing these alloys, even under extreme processing conditions [ 21 , 22 , 27 ]. In previous work, we investigated different sample preparation techniques and ion irradiation strategies, primarily using Bi ions [ 28 , 29 ]. Our findings indicated that melt-spun ribbons provide superior structural and electrical characteristics compared to other fabrication methods. Building on these results, the present study focuses exclusively on melt-spun Bi-Sb ribbons and investigates the effect of iodine ion irradiation, which offers higher beam currents and deeper penetration than Bi ions. We systematically examine the influence of irradiation dose across a broader composition range of Bi 100− x Sb x ( x = 0, 5, 7, 10, 15), using ion energies between 25 and 40 MeV. The objectives of this work are twofold: (i) to assess the structural modifications induced by iodine ion irradiation in Bi-Sb ribbons; (ii) to analyze the evolution of electrical resistivity at low temperatures as a function of composition and irradiation fluence. To this end, we compare resistivity–temperature curves and X-ray diffraction patterns for all the different compositions before and after inducing substantial structural damage via iodine ion irradiation. From there, we discuss the experimental results, draw conclusions, and outline ideas for future experiments. The rest of the paper is structured as follows. Section 2 describes the experimental techniques employed, including sample preparation, followed in Section 3 by the experimental results and their discussion, and concludes with the general findings of the work in Section 4. 2 Experimental 2.1 Melt spinning and sample preparation Bi 100−x Sb x ribbons were fabricated using the melt-spinning technique in two home-built systems [ 30 ]. First, master-alloy pellets were prepared from the pure elements (Bi and Sb) in the appropriate weight ratios. The elements were mixed, pressed, and subsequently melted at 800°C. For this process, the pressed elements were placed in a non-contaminating boron nitride crucible located inside a graphite container. The assembly was introduced into an inert atmosphere enclosure and heated by induction currents generated by a radiofrequency electromagnetic source. This process resulted in different Bi 100−x Sb x alloy ingots. Each of these ingots were then remelted in an N 2 atmosphere chamber containing a copper wheel. The molten alloy was ejected onto the cylindrical surface of the rotating wheel, using different surface velocities selected between 20 and 40 m/s. Ribbons with thicknesses typically in the range of 10–30 µm were thus produced, depending on the speed and were subsequently used for irradiation experiments. Using this procedure, long (> 10 cm) ribbons of pure Bi were also obtained (Fig. 1 b). However, polycrystalline ribbons of the Bi-Sb alloys were much more brittle and only shorter pieces were obtained (Fig. 1 a). For electrical conductivity measurements, pairs of ribbons were mounted on a copper sample holder, with an insulating Kapton film placed in between, as shown in Fig. 1 c. 2.2 Ion beam irradiation Irradiation was performed with iodine ions at the Centre of Microanalysis of Materials (CMAM), Madrid [ 31 ], using the Implantation beamline of the 5 MV tandem accelerator. Negative iodine ions I − were extracted from a sputtering source and accelerated both before and after passing through a stripper, which removes electrons to produce the desired charge states (+ 7 or + 8). The Implantation beamline, located at the 20° port of the switching magnet, includes a raster-scanner system with two electrostatic deflectors operating at non-commensurable sweep frequencies. This setup enables homogeneous irradiation of the sample surface by scanning a pencil beam of mm² dimensions horizontally and vertically. The irradiated area is defined by adjusting the scan amplitudes. Beam quality and shape are verified by ionoluminescence on a Si wafer with thermal silica prior to irradiation (see Fig. 2 of Ref. [ 29 ]). Samples are arranged as a mosaic on the sample holder and surrounded by luminescent reference samples to visualize the irradiated area during exposure. Beam flux is determined by measuring the current at a Faraday cup upstream of the chamber (raster off) and then estimating the irradiated area from images of reference samples with a 1 cm grid (raster on). Flux is checked before and after short irradiations, and every 15 min for long runs, ensuring accurate dose control thanks to the beam’s high stability. Irradiations were carried out in a multi-energy scheme at three energies (25, 35 and 40 MeV) to produce a smooth defect profile with depth. Fluences were selected to create flat regions with ~ 40% or 80% of nominal vacancies per atom over several µm (see Fig. 2 ), which we will label hereafter as f 40 and f 80 , respectively. Penetration depth and vacancy density profiles were calculated using SRIM [ 32 ] with the “Quick Calculation of Damage” option (instead of using the “Full Cascade” method, which has been shown to overestimate the vacancy production [33−35]). Calculated profiles for Bi-Sb films show minimal variation with Sb content [ 29 ]. Heavy iodine ions penetrate only a few µm, and damage profiles shift nearly linearly with energy. Ion currents ranged from 300 to 1000 nA, depending on energy and charge state. Duration of irradiations varied from 5 minutes to 1 hour. The CMAM accelerator’s excellent voltage stability ensures precise ion energies and reproducible conditions. 2.3. Low-temperature electrical conductivity The electrical resistance of the samples was measured from room temperature down to ~ 2 K using a standard 4 He cryostat and the four-probe method [ 28 , 29 ]. Resistivity was calculated from the measured resistance and sample geometry. To minimize offsets and thermoelectric effects, the current polarity was periodically reversed. A Keithley 224 programmable current source supplied identical current to both stripes (connected in series), while voltage drops were recorded independently with dedicated Keithley microvoltmeters. Samples were mounted on a holder designed for electrical connections (see Fig. 1 in Ref. [ 29 ]). Contacts between the Bi-Sb alloy and Au electrodes were made with 0.05 mm Cu wires fixed with silver paint. A CCS carbon thermometer, calibrated over the full range, monitored temperature via a LakeShore 350 controller. High vacuum was achieved before cooling using a diffusion pump. Cooling proceeded in stages: first to 77 K with liquid N 2 , then to 4.2 K with liquid 4 He, and finally to ~ 2 K by pumping the 4 He bath. Temperature was decreased slowly to ensure thermal equilibrium. Resistance was also measured during heating up to room temperature at rates below 1 K/min. Overlap between cooling and heating curves confirmed proper thermalization (see Fig. 5 b in Ref. [ 28 ]). Further experimental details are provided in Ref. [ 29 ]. 2.4. Structural characterization A comprehensive structural characterization of the samples was carried out by scanning electron microscopy (SEM) at the Instituto de Micro y Nanotecnología (IMN, CSIC). The measurements were performed using a FEI Verios 460 microscope operated at an accelerating voltage of 2.0 kV, a beam current of 13 pA, and a working distance ranging from 3.0 to 4.5 mm. Images were acquired in secondary electron mode, and the detector was tilted between − 10° and 10° when appropriate. In parallel, energy-dispersive X-ray spectroscopy (EDX) analyses were performed at an accelerating voltage of 20 kV over surface areas ranging from 5–25 × 3.6–18 µm². Cross-sectional spectra (80 × 55 µm²) as well as compositional maps (250 × 86–117 µm²) were also obtained. Additionally, X-ray diffraction (XRD) measurements were carried out using Bragg–Brentano (θ–2θ) geometry, where θ is the incident angle and 2θ the diffraction angle. The experiments were performed with a PANalytical X’Pert PRO θ–2θ diffractometer using Cu Kα radiation (λ = 1.5418 Å). The incident beam was generated by a Cu X-ray tube, focused by a Göbel mirror, and collimated using a 0.04 rad Soller slit. The diffracted beam passed through a 0.18° parallel-plate collimator, a graphite (002) monochromator, and an additional 0.04 rad Soller slit before detection with a Xe gas scintillation detector. Diffraction patterns were collected in the 2θ range from 20° to 80°, with particular attention paid to the region between 20° and 50°. A step size of 0.02° and an acquisition time of 4 s per step were used. 3 Results and Discussion Table 1 summarizes the main parameters of the different Bi 100 - x Sb x samples investigated in this work. The first column lists the sample composition, while the second indicates whether the samples are pristine (as grown) or iodine-ion–irradiated. In the latter case, it is specified whether a multi-energy irradiation scheme corresponding to f 40 (dashed line in Fig. 2, with a characteristic vacancy density of approximately 40%) or f 80 (solid black line in Fig. 2, with a vacancy density twice that of f 40 ) was applied. In one sample, the initial fluences were further tripled within the same scheme ( f 120 ). The following three columns report the geometric dimensions of the ribbons used in the experiments (for the pristine Bi sample, the dimensions of the two ribbons measured from the same batch are provided). The penultimate column presents the electrical resistivity values obtained for each sample at room temperature (left) and at 50 K (right). Finally, the last column reports the semiconductor band gap extracted from least-squares fitting procedures, as described below in the text. Table 1 Main data of the samples studied in this work. For each composition there are samples either pristine or with two different I-ion irradiations, as specified in the main text. Their geometric dimensions are provided in the central columns (for the pristine Bi sample, the dimensions of the two ribbons measured from the same batch are provided). The last but one column shows the electrical resistivity values obtained for each sample at room temperature (left) and at 50 K (right). The last column presents the semiconductor band-gap values obtained from least-squares fittings, as described in the main text. Firstly, we present and discuss the morphological and structural characterization of samples with different compositions, both before (pristine) and after iodine-ion irradiation ( f 80 ). Figure 3 shows representative SEM images of Bi and Bi 90 Sb 10 ribbons. The left panels correspond to the as-grown samples, whereas the right panels show samples irradiated with iodine ions following the f 80 fluence scheme. Cross-sectional images reveal a polycrystalline structure composed of grains with varying sizes and morphologies. The sample thickness was also determined, yielding typical values in the range of 10–60 µm. It should be noted that the thickness is not uniform across these ribbons, owing to the intrinsic difficulty in controlling this parameter in the melt-spinning growth technique. For irradiation and electrical resistivity measurements, the thinnest available samples were therefore selected. Top-view surface images reveal a layered, directional, and relatively homogeneous growth. Instead of well-defined polyhedral crystalline grains, a more compact and diffuse morphology is observed. After iodine-ion irradiation, noticeable surface modifications can be detected, with a tendency toward a more homogeneous microstructure and less well-defined grain boundaries. Samples with all other compositions were also examined in both cross-sectional and top-view configurations. As their morphological features do not differ significantly from those shown in Fig. 3, only these images are presented as representative examples. Fig. 3 Comparison of SEM cross-section and top-view surface images of some pure Bi and Bi 90 Sb 10 ribbons as grown (ASG, left panels) and after f 80 irradiation with iodine ions (IRR, right panels). Thickness of each sample is indicated in the cross-section images. We also performed an elemental analysis of the alloys as a cross-check of their nominal compositions. Figure 4 presents representative EDX spectra from four different samples: pristine and iodine-ion–irradiated ( f 80 ) pure Bi, and pristine and irradiated ( f 80 ) Bi 90 Sb 10 . The only significant difference between the spectra of pristine Bi and Bi 90 Sb 10 is the appearance of the characteristic Sb peaks, highlighted with blue arrows. When comparing the irradiated Bi 90 Sb 10 sample with its pristine counterpart, the spectra show no substantial differences. Data were collected from all measured samples; however, as the results are essentially identical to those shown in Fig. 4, only these spectra are included as representative examples. Quantitatively, the Sb-to-Bi atomic ratios in the alloyed samples fall within the expected range, with small deviations from the nominal compositions that are typical of EDX measurements, given their limited precision. These results confirm that the samples were grown with compositions close to the intended values and without any significant concentration of impurities. Moreover, no evidence of irradiation-induced compositional changes was observed. To further support these conclusions, EDX spectra were acquired from spatially separated surface regions using different electron accelerating voltages (8 kV and 20 kV), thereby probing different information depths. In addition, cross-sectional compositional maps were obtained. All these measurements consistently indicate a reasonably homogeneous and isotropic distribution of Bi and Sb throughout the samples, with no detectable compositional inhomogeneities. Fig. 4 Comparison of EDX spectra of different samples: as grown Bi (black), as grown Bi 90 Sb 10 (green) and irradiated ( f 80 ) Bi (grey) and Bi 90 Sb 10 (red). Characteristic X-ray emission lines of Bi and Sb are indicated by their corresponding peaks. From the internal structural point of view, all XRD patterns (Fig. 5) indicate that the as-grown samples consist of polycrystalline grains with random orientations. The diffraction peaks can be indexed to the rhombohedral crystal structure of Bi described in the hexagonal setting, in good agreement with previously reported results for thin films [36–40], ribbons [41], and nanoparticles [42,43]. The (012) reflection is the most intense, indicating that this plane is the dominant crystallographic orientation, as also observed in Refs. [36,39,41,42], although several additional reflections with lower intensity are present. Fig. 5 Comparison of XRD patterns for (a) pure Bi, (b) Bi 95 Sb 5 , (c) Bi 90 Sb 10 and (d) Bi 85 Sb 15 , both before (as grown, green spectra) and after irradiation with iodine ions (red spectra) with a fluence corresponding to about 80% of vacancies ( f 80 ), as indicated in the legends. As grown Bi 85 Sb 15 corresponds to a different series of fabrication (and different set of measurements) than the rest of the samples. The incorporation of antimony appears to have a negligible effect on the diffraction profiles. Consequently, the alloyed samples were analyzed using the same reference pattern ( Match! program, database entry 96‑712‑3353) as that employed for pure Bi. Minor variations in peak positions and relative intensities among samples with different compositions are likely attributable to slight differences in sample preparation and growth conditions. Fig. 5 also displays the XRD patterns of samples after iodine-ion irradiation following the f 80 scheme. Samples irradiated using the f 40 protocol were likewise examined; however, because their diffraction patterns are nearly identical to those obtained with f 80 , only the latter are shown for clarity. Any differences relative to the as-grown samples are more readily discernible at the higher irradiation dose. In all cases, irradiation results in a slight broadening of the diffraction peaks, as reflected in the increased full width at half maximum (FWHM), indicating a partial degradation of crystalline order. No additional diffraction peaks are observed, ruling out the formation of new crystalline phases induced by irradiation. Likewise, no diffuse scattering background is detected that would suggest the presence of an amorphous phase [44]. This absence also may be attributed to the fact that any amorphized material is expected to be confined to a relatively shallow subsurface region, approximately 4–7 µm below the surface, as Fig. 2 shows. Although the measurements were performed in the θ–2θ configuration, the diffraction signal from such a thin amorphous layer is likely overwhelmed by the contribution from the he polycrystalline region closer to the surface, which is less damaged. Therefore, the lack of detectable amorphization signatures suggests that permanent amorphization was either not achieved or is limited to a spatial region beyond the sensitivity threshold of our experimental setup. A very similar behavior has been reported following irradiation with bismuth ions, both in comparable melt-spun ribbons and in thermally evaporated samples [29]. Fig. 6 Plots of the resistivity coefficients versus temperature for different melt spun ribbons Bi 100 - x Sb x ( x = 0, 5, 7, 10, 15) ribbons: as grown (green curves), with a set of fluences f 40 (blue curves) or f 80 (red curves), as well as f 120 (brown curve) for the case x = 7, as indicated in the legends. For pure Bi, in addition to the linear graph (a), a log-log plot is also depicted (b). A crystallite size analysis was also performed. Each diffraction peak was fitted using a Gaussian function in order to extract the full width at half maximum (FWHM). The crystallite size was then estimated using the Scherrer equation with a shape factor K = 0.94. The resulting crystallite sizes range from approximately 100 to 500 nm, in good agreement with the submicrometer grain sizes observed in the SEM images. We now present and discuss the results of the electrical resistivity measurements as a function of temperature, r ( T ), for ribbons of the various studied compositions, including both as-grown (pristine) samples and ribbons irradiated with iodine ions in the energy range of 25–40 MeV, following the irradiation procedures described above. As previously observed in earlier work [29], mainly using bismuth self-ions, melt-spun ribbons exhibit significantly lower electrical resistivity than polycrystalline samples prepared by other methods. In particular, pure Bi samples display a semimetallic behavior –similar to bulk Bi single crystals– instead of the semiconducting behavior reported for other types of Bi samples [28,29]. In the present study, we therefore focus exclusively on melt-spun samples, employ iodine (I) ions of higher energy and penetration depth in thinner ribbons –thus optimizing the modified region– and extend the range of investigated compositions to Bi 100 - x Sb x with x = 0, 5, 7, 10, 15. The corresponding r ( T ) curves are shown in Fig. 6 on a linear scale, except for pure Bi, whose semimetallic behavior and lower resistivity values are also depicted on a log–log scale (Fig. 6b). In all cases, resistivity was obtained directly from the geometric dimensions of each ribbon, without accounting for the fact that the deepest regions of the samples are barely affected by ion irradiation. Consequently, the actual effect of irradiation-induced disorder on the electrical conductivity is likely more pronounced than observed. Nonetheless, given the varying ribbon thicknesses and to avoid introducing the bias of a specific model, we present the raw r ( T ) data, which still allow for at least a qualitative discussion. For all Bi 100 - x Sb x alloys with x ≠ 0, the expected narrow-gap semiconducting behavior described by r = r 0 exp( E g /2 k B T ) is observed, where E g is the full thermal bandgap. As extensively discussed in the literature [4,6–8,28,29], at low temperatures -typically below 50–100 K depending on the sample- the resistivity ceases to increase exponentially and tends to saturate due to the dominant contribution of charge carriers originating from ionized impurity levels, rather than from the intrinsic energy bands of Bi 100 - x Sb x . It is worth noting that the room‑temperature resistivity values obtained ( ρ = 0.13 mΩ·cm for semimetallic Bi and ρ ≈ 0.2 mΩ·cm for the Bi 100 - x Sb x semiconducting alloys) are very similar to those previously measured in bulk single crystals [7,45]. Figure 7 shows the same curves as Fig. 6 (excluding pure Bi, which does not display semiconducting behavior), plotted as the natural logarithm of resistivity versus inverse temperature, enabling determination of the semiconductor bandgap Δ . Owing to the aforementioned low‑temperature saturation effect associated with ionized impurity levels, least‑squares fits were performed only in the relatively high‑temperature ranges where the expected exponential dependence is not affected by this additional contribution. The extracted bandgap values, on the order of 10–20 meV, are listed in the last column of Table 1. These values are smaller than those obtained using other sample‑preparation methods [28], and instead closely resemble those reported in the literature for foils [7] and even for bulk crystals [2]. Although the various irradiation treatments did not succeed in fully amorphizing a region of the samples -as apparently confirmed by XRD-, they presumably introduced a certain degree of disorder into the crystalline structure. While no direct correlation with the calculated damage level (see Table 1) is observed, in many cases the disorder does reduce the electrical resistivity of Bi 100 - x Sb x ribbons, including the clear case of pure Bi, whose resistivity reaches nearly half that of a pristine single crystal after the highest applied iodine‑ion irradiation. Fig. 7 Natural logarithm of the resistivity coefficients versus 1000/ T for the same data points shown in Fig. 6 for Bi 100 - x Sb x ( x = 5, 7, 10, 15), excluding pure Bi. The solid lines show the linear fits performed at higher temperatures to determine the semiconducting gaps. Insets: the same plot in the whole temperature range, where the saturation of the semiconducting behavior at the lowest temperatures is observed. Fig. 8 Values of the resistivity coefficients at 50 K in logarithmic scale (a) and of the semiconducting gap (b) as a function of composition for Bi 100 - x Sb x ( x = 5, 7, 10, 15), for pristine samples (green symbols) and after f 80 irradiations (red symbols). See all available data in Table 1. Attempting to identify possible correlations, Fig. 8 shows the values obtained for the low‑temperature (50 K) resistivity coefficient and the semiconductor bandgap as a function of alloy composition, for both pristine samples and those irradiated with the f 80 fluence set –selected as the most representative case for clarity. Although no single unambiguous correlation can be established, and despite the discussed uncertainties associated with the absolute values, some well‑defined and similar trends appear for both electrical properties. Both quantities increase with x (Sb content) up to approximately x = 10, after which the trend reverses. Moreover, ion irradiation tends to decrease both ρ and Δ , i.e., to improve the electrical performance of the material, particularly for samples with higher Sb content. This seems to be in agreement with recent theoretical findings in two-dimensional alloys of Bi 100 - x Sb x [46]. 4 Conclusion In summary, we have systematically investigated the impact of ion-irradiation-induced damage and disorder −produced by tens-of-MeV iodine ions− on melt-spun Bi 100− x Sb x ribbons in the search for routes toward amorphous and potentially superconducting material. Structural characterization (SEM, XRD) indicates that the thermodynamic driving force toward crystallization remains sufficiently strong to prevent full amorphization, even in the most heavily irradiated regions where simulations predict atomic displacements exceeding 80%. Nevertheless, the melt-spun ribbons exhibit electrical resistivity values remarkably close to those reported for bulk single crystals, in contrast with other sample types such as thermally evaporated films. Moreover, the irradiation-induced disorder in these polycrystalline ribbons leads to a slight but reproducible enhancement of electrical conductivity, observed both in semimetallic Bi and in narrow-gap (10–20 meV) Bi-Sb alloys. These results highlight the robustness of the crystalline state in these materials under extreme ion damage, while revealing that controlled irradiation may serve as a viable tool to tune their transport properties. Declarations Conflict of interest The authors declare no conflict of interest. Author Contribution NG and MAR managed the project and carried out the conceptualization of the experiments. AAG and DR performed low-temperature measurements. GT and AAG conducted characterization experiments and analysis. VM, CF and JV fabricated the samples. AAG, ARC, GGL, NG and MAR did the ion-beam irradiation experiments. MAR wrote the draft and all authors supervised the writing of the manuscript. Acknowledgement We are grateful to the CMAM technical staff for their support and the beam time access with the proposal codes IMP021/23, IMP024/23, IMP005/24, and IMP013/24.We acknowledge financial support from the Spanish Ministry of Science, Innovation and Universities (MCIN/AEI/10.13039/501100011033) through the grant PID2021-127498NB-I00, and within the “María de Maeztu” Program for Units of Excellence in R&D (CEX2023-001316-M). The authors also acknowledge the funding from Comunidad de Madrid through project TEC-2024/TEC-380 "Mag4TIC” and support from the COST action CA21144 “Superqumap”. 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Gordillo, P.C. Gutiérrez, J. Olivares, R. Pérez, Casero, M.D. Ynsa, Current status and future developments of the ion beam facility at the centre of micro-analysis of materials in Madrid. Eur. Phys. J. Plus. 136 , 175 (2021) J.F. Ziegler, J.P. Biersack, U. Littmark (eds.), The Stopping and Ranges of Ions in Solids (Pergamon, New York, 1985). http://www.srim.org/ R.E. Stoller, M.B. Toloczko, G.S. Was, A.G. Certain, S. Dwaraknath, F.A. Garner, On the use of SRIM for computing radiation damage exposure. Nucl. Instr Meth Phys. Res. B 310 (2013) Y. Agarwal, C. Lin, R.E. Li, Stoller, S.J. Zinkle, On the use of SRIM for calculating vacancy production: Quick calculation and full-cascade options. Nucl. Instr Meth Phys. Res. B 503 (2021) Y.-R. Lin, S.J. Zinkle, C.J. Ortiz, J.-P. Crocombette, R. Webb, R.E. Stoller, Predicting displacement damage for ion irradiation: Origin of the overestimation of vacancy production in SRIM full-cascade calculations. Curr. Opin. Solid State Mater. Sci. 27 , 101120 (2023) D.-H. Kim, S.-H. Lee, J.-K. Kim, G.-H. Lee, Structure and electrical transport properties of bismuth thin films prepared by RF magnetron sputtering. Appl. Surf. Sci. 252 , 3525–3531 (2006) L. Kumari, S.-J. Lin, J.-H. Lin, Y.-R. Ma, P.-C. Lee, Y. Liou, Effects of deposition temperature and thickness on the structural properties of thermal evaporated bismuth thin films. Appl. Surf. Sci. 253 , 5931–5938 (2007) J. Chang, H. Kim, J. Han, M.H. Jeon, W.Y. Lee, Microstructure and magnetoresistance of sputtered bismuth thin films upon annealing. J. Appl. Phys. 98 , 023906 (2006) P.M. Vereecken, L. Sun, P.C. Searson, M. Tanase, D.H. Reich, C.L. Chien, Magnetotransport properties of bismuth films on p-GaAs. J. Appl. Phys. 88 , 6529–6535 (2000) A. Dauscher, M.O. Boffoué, B. Lenoir, R. Martin-Lopez, H. Scherrer, Unusual growth of pulsed laser deposited bismuth films on Si (100). Appl. Surf. Sci. 138 , 188–194 (1999) K. Kang, Y.F. Hu, L.H. Lewis, Q. Li, A.R. Moodenbaugh, Y.S. Choi, Large magnetoresistance in rapidly solidified bismuth. J. Appl. Phys. 98 , 073704 (2005) Z. Wang, C. Jiang, R. Huang, H. Peng, X. Tang, Investigation of optical and photocatalytic properties of bismuth nanospheres prepared by a facile thermolysis method. J. Phys. Chem. C 118 , 1155–1160 (2014) A.C. Gandhi, S.S. Gaikwad, J.C. Peng, C.W. Wang, T.S. Chan, S.Y. Wu, Strong electron-phonon coupling in superconducting bismuth nanoparticles. APL Mater. 7 , 031111 (2019) A. Benyagoub, Investigations by X-ray diffraction of swift heavy ion induced effects in inorganic materials. Nucl. Instrum. Methods Phys. Res. B 225 , 88–96 (2004) A.L. Jain, Temperature Dependence of the Electrical Properties of Bismuth-Antimony Alloys. Phys. Rev. 114 , 1518–1528 (1959) A.J. Uría Álvarez, J.J. Palacios, Amorphization-induced topological and insulator-metal transitions in bidimensional Bi x Sb 1–x alloys. Phys. Rev. Res. 7 , 043263 (2025) Tables Table 1 is available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files Table1.pdf Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 19 Mar, 2026 Reviews received at journal 24 Feb, 2026 Reviews received at journal 14 Feb, 2026 Reviewers agreed at journal 06 Feb, 2026 Reviewers agreed at journal 03 Feb, 2026 Reviewers invited by journal 03 Feb, 2026 Editor assigned by journal 01 Feb, 2026 Submission checks completed at journal 31 Jan, 2026 First submitted to journal 30 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8743396","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":585566799,"identity":"16f433e6-47ab-480f-8478-98b952a28471","order_by":0,"name":"Alberto Andrino-Gómez","email":"","orcid":"","institution":"Autonomous University of Madrid","correspondingAuthor":false,"prefix":"","firstName":"Alberto","middleName":"","lastName":"Andrino-Gómez","suffix":""},{"id":585566800,"identity":"eb9f4e5c-af37-4b53-9c9c-200a4708fc5f","order_by":1,"name":"Gema Tabares","email":"","orcid":"","institution":"Autonomous University of Madrid","correspondingAuthor":false,"prefix":"","firstName":"Gema","middleName":"","lastName":"Tabares","suffix":""},{"id":585566801,"identity":"85988e56-199c-4f2e-b2c7-a847d62f8f74","order_by":2,"name":"Diego Ramírez","email":"","orcid":"","institution":"Autonomous University of Madrid","correspondingAuthor":false,"prefix":"","firstName":"Diego","middleName":"","lastName":"Ramírez","suffix":""},{"id":585566802,"identity":"84185ffc-020c-475e-9e49-0efebb8e24fe","order_by":3,"name":"Andrés Redondo-Cubero","email":"","orcid":"","institution":"Autonomous University of Madrid","correspondingAuthor":false,"prefix":"","firstName":"Andrés","middleName":"","lastName":"Redondo-Cubero","suffix":""},{"id":585566803,"identity":"a0a8ce0b-be1b-4b8c-95ad-21510e16171e","order_by":4,"name":"Vicente Madurga","email":"","orcid":"","institution":"Universidad Publica de Navarra","correspondingAuthor":false,"prefix":"","firstName":"Vicente","middleName":"","lastName":"Madurga","suffix":""},{"id":585566804,"identity":"36941073-88e9-49a4-9fc1-ced3da4f9b69","order_by":5,"name":"Cristina Favieres","email":"","orcid":"","institution":"Universidad Publica de Navarra","correspondingAuthor":false,"prefix":"","firstName":"Cristina","middleName":"","lastName":"Favieres","suffix":""},{"id":585566805,"identity":"d55c6876-a621-4a2d-8f69-f20d929ebd74","order_by":6,"name":"José Vergara","email":"","orcid":"","institution":"Universidad Publica de Navarra","correspondingAuthor":false,"prefix":"","firstName":"José","middleName":"","lastName":"Vergara","suffix":""},{"id":585566806,"identity":"acd8c309-c71d-4c6b-b44d-c9eb108e2385","order_by":7,"name":"Gastón García-López","email":"","orcid":"","institution":"Autonomous University of Madrid","correspondingAuthor":false,"prefix":"","firstName":"Gastón","middleName":"","lastName":"García-López","suffix":""},{"id":585566807,"identity":"e94a4c8a-a2f1-4922-86de-59979e853dc9","order_by":8,"name":"Nuria Gordillo","email":"","orcid":"","institution":"Autonomous University of Madrid","correspondingAuthor":false,"prefix":"","firstName":"Nuria","middleName":"","lastName":"Gordillo","suffix":""},{"id":585566808,"identity":"ea36e035-674c-48c4-96d2-380ef98c960a","order_by":9,"name":"Miguel Ángel Ramos","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2ElEQVRIiWNgGAWjYDACZhBRAOV8IF6LAYTNOIN4q6BamHmIUazbzn5N4oMBQz7/tMOHP9u22UUzsLc/wKvF7DBPmeQMAwbLGbfT0qRz25JzG3jOGBDSkibNYwB02u0cM+bcNubcBokc/A4Da/kD1CJ/O8f4s2VbfW6D/HNCDmM/Jg20w8Dgdo6BNGPbYaAtDAQdxmzZYyBhYAj0i2TPueO5bTw5BLScP/7wxo8KGwO528mHP/woq87tZz+O32EMDDwgMyUQfDYC6oGAnZCZo2AUjIJRMOIBAFsdP9kF8K5pAAAAAElFTkSuQmCC","orcid":"","institution":"Autonomous University of Madrid","correspondingAuthor":true,"prefix":"","firstName":"Miguel","middleName":"Ángel","lastName":"Ramos","suffix":""}],"badges":[],"createdAt":"2026-01-30 16:39:24","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8743396/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8743396/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102065950,"identity":"bc2ffb7b-7cf1-4dbf-ac20-d632c88b2c0a","added_by":"auto","created_at":"2026-02-06 18:12:35","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":201506,"visible":true,"origin":"","legend":"\u003cp\u003ePhotographs of typical melt-spun ribbons fabricated and used in the experiments: (a) Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e ribbons, as obtained; (b) pure Bi ribbons, as obtained; (c) two ribbons of pure Bi, mounted in the experimental setup to measure resistivity at low temperatures.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/c14bca2ed28e954133920348.jpg"},{"id":102295749,"identity":"afc877a3-72a7-434e-8e8c-138128aa99b1","added_by":"auto","created_at":"2026-02-10 10:14:38","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":602667,"visible":true,"origin":"","legend":"\u003cp\u003eSRIM simulations of the profile of vacancy density produced by multi-energy irradiations with I ions onto Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e films, with the ion energies (expressed in MeV) and corresponding fluences (´ 10\u003csup\u003e13\u003c/sup\u003e ion/cm\u003csup\u003e2\u003c/sup\u003e) employed in the experiments for each energy (curve) indicated in the left-top legend. The total accumulated damage profiles for the sets of irradiations, with plateaux of about 80% and 40% of vacancies (the latter using half of the indicated fluences), are shown by black solid lines and dashed lines, respectively.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/eab14b3d00d1d9476de47869.jpg"},{"id":102065959,"identity":"fad6ba20-444d-4156-a96f-28bf7701ee83","added_by":"auto","created_at":"2026-02-06 18:12:36","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1179155,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of SEM cross-section and top-view surface images of some pure Bi and Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e ribbons as grown (ASG, left panels) and after \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e irradiation with iodine ions (IRR, right panels). Thickness of each sample is indicated in the cross-section images.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/b60395bc671229f5375669f1.jpg"},{"id":102065956,"identity":"9d4f2ec5-b03d-40b0-ac61-98dc3e3ce5f8","added_by":"auto","created_at":"2026-02-06 18:12:36","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":546835,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of EDX spectra of different samples: as grown Bi (black), as grown Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e (green) and irradiated (\u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e) Bi (grey) and Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e (red). Characteristic X-ray emission lines of Bi and Sb are indicated by their corresponding peaks.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/5992bf6ff46a67794bd7e4c9.jpg"},{"id":102295655,"identity":"96054787-2f33-4e51-9a9a-ddeeb3439dc0","added_by":"auto","created_at":"2026-02-10 10:13:35","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1795207,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of XRD patterns for (a) pure Bi, (b) Bi\u003csub\u003e95\u003c/sub\u003eSb\u003csub\u003e5\u003c/sub\u003e, (c) Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e and (d) Bi\u003csub\u003e85\u003c/sub\u003eSb\u003csub\u003e15\u003c/sub\u003e, both before (as grown, green spectra) and after irradiation with iodine ions (red spectra) with a fluence corresponding to about 80% of vacancies (\u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e), as indicated in the legends. As grown Bi\u003csub\u003e85\u003c/sub\u003eSb\u003csub\u003e15\u003c/sub\u003e corresponds to a different series of fabrication (and different set of measurements) than the rest of the samples.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/43c51d2b6b334910dc57f55a.jpg"},{"id":102065958,"identity":"24acbeb4-2b3a-43a0-bcdb-2f669a7f5ab2","added_by":"auto","created_at":"2026-02-06 18:12:36","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2432697,"visible":true,"origin":"","legend":"\u003cp\u003ePlots of the resistivity coefficients versus temperature for different melt spun ribbons Bi\u003csub\u003e100-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eSb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003ex\u003c/em\u003e = 0, 5, 7, 10, 15) ribbons: as grown (green curves), with a set of fluences \u003cem\u003ef\u003c/em\u003e\u003csub\u003e40\u003c/sub\u003e (blue curves) or \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80 \u003c/sub\u003e(red curves), as well as \u003cem\u003ef\u003c/em\u003e\u003csub\u003e120 \u003c/sub\u003e(brown curve) for the case \u003cem\u003ex\u003c/em\u003e = 7, as indicated in the legends. For pure Bi, in addition to the linear graph (a), a log-log plot is also depicted (b).\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/a0e4a103f12f8dc6d2bb0c97.jpg"},{"id":102065954,"identity":"d1f93543-c8e3-4557-b65f-0c4bb4f105de","added_by":"auto","created_at":"2026-02-06 18:12:36","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":161060,"visible":true,"origin":"","legend":"\u003cp\u003eNatural logarithm of the resistivity coefficients versus 1000/\u003cem\u003eT\u003c/em\u003e for the same data points shown in Fig. 6 for Bi\u003csub\u003e100-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eSb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003ex\u003c/em\u003e = 5, 7, 10, 15), excluding pure Bi. The solid lines show the linear fits performed at higher temperatures to determine the semiconducting gaps. Insets: the same plot in the whole temperature range, where the saturation of the semiconducting behavior at the lowest temperatures is observed. \u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/90f7daa698e427029bef009a.jpg"},{"id":102065957,"identity":"64f5a8ff-666e-43e4-af0a-623129d1cd87","added_by":"auto","created_at":"2026-02-06 18:12:36","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":603713,"visible":true,"origin":"","legend":"\u003cp\u003eValues of the resistivity coefficients at 50 K in logarithmic scale (a) and of the semiconducting gap (b) as a function of composition for Bi\u003csub\u003e100-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eSb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003ex\u003c/em\u003e = 5, 7, 10, 15), for pristine samples (green symbols) and after \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e irradiations\u003csub\u003e \u003c/sub\u003e(red symbols). See all available data in Table 1.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/ccb7230853f9a69094be0a6c.jpg"},{"id":102299003,"identity":"60f46748-3ac9-400e-a0e2-9630f83f7f87","added_by":"auto","created_at":"2026-02-10 11:02:08","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8243904,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/19147b0e-f11b-4535-b56a-1aade8c545fa.pdf"},{"id":102065951,"identity":"b57fdfff-deae-4ed6-ae34-cc852487c48f","added_by":"auto","created_at":"2026-02-06 18:12:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":60897,"visible":true,"origin":"","legend":"","description":"","filename":"Table1.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8743396/v1/c0a705723d084cb250a9c699.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Bi-Sb alloys irradiated with swift heavy ions as potential topological amorphous superconductors","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eBismuth (Bi) is a diamagnetic semimetal that has long attracted attention due to its unusual electronic properties when pure or lightly doped [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Its alloys with antimony (Bi-Sb) form narrow-gap intrinsic semiconductors, which have been extensively studied for their promising thermoelectric performance [2\u0026minus;9]. The high figure of merit of these materials makes them candidates for efficient energy conversion applications, particularly in low-temperature regimes.\u003c/p\u003e \u003cp\u003eInterest in Bi-Sb systems was dramatically renewed when it was experimentally demonstrated [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] that Bi-Sb crystals behave as topological insulators (TIs), confirming earlier theoretical predictions [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. This discovery opened a new research frontier in condensed matter physics, positioning Bi-Sb alloys together with Bi\u003csub\u003e2\u003c/sub\u003eSe\u003csub\u003e3\u003c/sub\u003e compounds [13\u0026minus;16] at the center of studies on topological phases, which remain a highly active field today [17\u0026minus;20]. Beyond their topological properties, it is less widely known that superconductivity in Bi and its alloys with Sb, Pb, and Tl was reported more than seventy years ago, although only in amorphous state, with critical temperatures exceeding 6 K [21\u0026minus;23]. However, these systems exhibit a strong tendency to crystallize even at very low temperatures, limiting the stability of the superconducting phase and hindering practical applications.\u003c/p\u003e \u003cp\u003eRecent literature has explored the possibility of topological superconductivity (TS), either intrinsic or induced, in different systems [24\u0026minus;26]. On the other hand, \u003cem\u003eamorphous\u003c/em\u003e topological phases have been proposed as an emerging concept [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Interestingly, both issues merge in Bi-Sb materials, since superconductivity in Bi-based alloys appears to occur only in the amorphous state [21\u0026minus;23]. Achieving amorphous Bi-Sb is therefore of great interest, not only to enable superconductivity but also to potentially enhance thermoelectric properties. However, previous studies have highlighted significant challenges in amorphizing these alloys, even under extreme processing conditions [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn previous work, we investigated different sample preparation techniques and ion irradiation strategies, primarily using Bi ions [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Our findings indicated that melt-spun ribbons provide superior structural and electrical characteristics compared to other fabrication methods. Building on these results, the present study focuses exclusively on melt-spun Bi-Sb ribbons and investigates the effect of iodine ion irradiation, which offers higher beam currents and deeper penetration than Bi ions. We systematically examine the influence of irradiation dose across a broader composition range of Bi\u003csub\u003e100\u0026minus;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eSb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003ex\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0, 5, 7, 10, 15), using ion energies between 25 and 40 MeV.\u003c/p\u003e \u003cp\u003eThe objectives of this work are twofold: (i) to assess the structural modifications induced by iodine ion irradiation in Bi-Sb ribbons; (ii) to analyze the evolution of electrical resistivity at low temperatures as a function of composition and irradiation fluence. To this end, we compare resistivity\u0026ndash;temperature curves and X-ray diffraction patterns for all the different compositions before and after inducing substantial structural damage via iodine ion irradiation. From there, we discuss the experimental results, draw conclusions, and outline ideas for future experiments.\u003c/p\u003e \u003cp\u003eThe rest of the paper is structured as follows. Section \u003cspan refid=\"Sec2\" class=\"InternalRef\"\u003e2\u003c/span\u003e describes the experimental techniques employed, including sample preparation, followed in Section 3 by the experimental results and their discussion, and concludes with the general findings of the work in Section 4.\u003c/p\u003e"},{"header":"2 Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Melt spinning and sample preparation\u003c/h2\u003e \u003cp\u003eBi\u003csub\u003e100\u0026minus;x\u003c/sub\u003eSb\u003csub\u003ex\u003c/sub\u003e ribbons were fabricated using the melt-spinning technique in two home-built systems [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. First, master-alloy pellets were prepared from the pure elements (Bi and Sb) in the appropriate weight ratios. The elements were mixed, pressed, and subsequently melted at 800\u0026deg;C. For this process, the pressed elements were placed in a non-contaminating boron nitride crucible located inside a graphite container. The assembly was introduced into an inert atmosphere enclosure and heated by induction currents generated by a radiofrequency electromagnetic source. This process resulted in different Bi\u003csub\u003e100\u0026minus;x\u003c/sub\u003eSb\u003csub\u003ex\u003c/sub\u003e alloy ingots. Each of these ingots were then remelted in an N\u003csub\u003e2\u003c/sub\u003e atmosphere chamber containing a copper wheel. The molten alloy was ejected onto the cylindrical surface of the rotating wheel, using different surface velocities selected between 20 and 40 m/s. Ribbons with thicknesses typically in the range of 10\u0026ndash;30 \u0026micro;m were thus produced, depending on the speed and were subsequently used for irradiation experiments. Using this procedure, long (\u0026gt;\u0026thinsp;10 cm) ribbons of pure Bi were also obtained (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). However, polycrystalline ribbons of the Bi-Sb alloys were much more brittle and only shorter pieces were obtained (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). For electrical conductivity measurements, pairs of ribbons were mounted on a copper sample holder, with an insulating Kapton film placed in between, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Ion beam irradiation\u003c/h2\u003e \u003cp\u003eIrradiation was performed with iodine ions at the Centre of Microanalysis of Materials (CMAM), Madrid [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], using the Implantation beamline of the 5 MV tandem accelerator. Negative iodine ions I\u003csup\u003e\u0026minus;\u003c/sup\u003e were extracted from a sputtering source and accelerated both before and after passing through a stripper, which removes electrons to produce the desired charge states (+\u0026thinsp;7 or +\u0026thinsp;8).\u003c/p\u003e \u003cp\u003eThe Implantation beamline, located at the 20\u0026deg; port of the switching magnet, includes a raster-scanner system with two electrostatic deflectors operating at non-commensurable sweep frequencies. This setup enables homogeneous irradiation of the sample surface by scanning a pencil beam of mm\u0026sup2; dimensions horizontally and vertically. The irradiated area is defined by adjusting the scan amplitudes. Beam quality and shape are verified by ionoluminescence on a Si wafer with thermal silica prior to irradiation (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e of Ref. [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]).\u003c/p\u003e \u003cp\u003eSamples are arranged as a mosaic on the sample holder and surrounded by luminescent reference samples to visualize the irradiated area during exposure. Beam flux is determined by measuring the current at a Faraday cup upstream of the chamber (raster off) and then estimating the irradiated area from images of reference samples with a 1 cm grid (raster on). Flux is checked before and after short irradiations, and every 15 min for long runs, ensuring accurate dose control thanks to the beam\u0026rsquo;s high stability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIrradiations were carried out in a multi-energy scheme at three energies (25, 35 and 40 MeV) to produce a smooth defect profile with depth. Fluences were selected to create flat regions with ~\u0026thinsp;40% or 80% of nominal vacancies per atom over several \u0026micro;m (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), which we will label hereafter as \u003cem\u003ef\u003c/em\u003e\u003csub\u003e40\u003c/sub\u003e and \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e, respectively. Penetration depth and vacancy density profiles were calculated using SRIM [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e] with the \u0026ldquo;Quick Calculation of Damage\u0026rdquo; option (instead of using the \u0026ldquo;Full Cascade\u0026rdquo; method, which has been shown to overestimate the vacancy production [33\u0026minus;35]). Calculated profiles for Bi-Sb films show minimal variation with Sb content [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Heavy iodine ions penetrate only a few \u0026micro;m, and damage profiles shift nearly linearly with energy. Ion currents ranged from 300 to 1000 nA, depending on energy and charge state. Duration of irradiations varied from 5 minutes to 1 hour. The CMAM accelerator\u0026rsquo;s excellent voltage stability ensures precise ion energies and reproducible conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Low-temperature electrical conductivity\u003c/h2\u003e \u003cp\u003eThe electrical resistance of the samples was measured from room temperature down to ~\u0026thinsp;2 K using a standard \u003csup\u003e4\u003c/sup\u003eHe cryostat and the four-probe method [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Resistivity was calculated from the measured resistance and sample geometry. To minimize offsets and thermoelectric effects, the current polarity was periodically reversed. A Keithley 224 programmable current source supplied identical current to both stripes (connected in series), while voltage drops were recorded independently with dedicated Keithley microvoltmeters.\u003c/p\u003e \u003cp\u003eSamples were mounted on a holder designed for electrical connections (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e in Ref. [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]). Contacts between the Bi-Sb alloy and Au electrodes were made with 0.05 mm Cu wires fixed with silver paint. A CCS carbon thermometer, calibrated over the full range, monitored temperature via a LakeShore 350 controller. High vacuum was achieved before cooling using a diffusion pump.\u003c/p\u003e \u003cp\u003eCooling proceeded in stages: first to 77 K with liquid N\u003csub\u003e2\u003c/sub\u003e, then to 4.2 K with liquid \u003csup\u003e4\u003c/sup\u003eHe, and finally to ~\u0026thinsp;2 K by pumping the \u003csup\u003e4\u003c/sup\u003eHe bath. Temperature was decreased slowly to ensure thermal equilibrium. Resistance was also measured during heating up to room temperature at rates below 1 K/min. Overlap between cooling and heating curves confirmed proper thermalization (see Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb in Ref. [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]).\u003c/p\u003e \u003cp\u003eFurther experimental details are provided in Ref. [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Structural characterization\u003c/h2\u003e \u003cp\u003eA comprehensive structural characterization of the samples was carried out by scanning electron microscopy (SEM) at the Instituto de Micro y Nanotecnolog\u0026iacute;a (IMN, CSIC). The measurements were performed using a FEI Verios 460 microscope operated at an accelerating voltage of 2.0 kV, a beam current of 13 pA, and a working distance ranging from 3.0 to 4.5 mm. Images were acquired in secondary electron mode, and the detector was tilted between \u0026minus;\u0026thinsp;10\u0026deg; and 10\u0026deg; when appropriate.\u003c/p\u003e \u003cp\u003eIn parallel, energy-dispersive X-ray spectroscopy (EDX) analyses were performed at an accelerating voltage of 20 kV over surface areas ranging from 5\u0026ndash;25 \u0026times; 3.6\u0026ndash;18 \u0026micro;m\u0026sup2;. Cross-sectional spectra (80 \u0026times; 55 \u0026micro;m\u0026sup2;) as well as compositional maps (250 \u0026times; 86\u0026ndash;117 \u0026micro;m\u0026sup2;) were also obtained.\u003c/p\u003e \u003cp\u003eAdditionally, X-ray diffraction (XRD) measurements were carried out using Bragg\u0026ndash;Brentano (θ\u0026ndash;2θ) geometry, where θ is the incident angle and 2θ the diffraction angle. The experiments were performed with a PANalytical X\u0026rsquo;Pert PRO θ\u0026ndash;2θ diffractometer using Cu Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.5418 \u0026Aring;). The incident beam was generated by a Cu X-ray tube, focused by a G\u0026ouml;bel mirror, and collimated using a 0.04 rad Soller slit. The diffracted beam passed through a 0.18\u0026deg; parallel-plate collimator, a graphite (002) monochromator, and an additional 0.04 rad Soller slit before detection with a Xe gas scintillation detector.\u003c/p\u003e \u003cp\u003eDiffraction patterns were collected in the 2θ range from 20\u0026deg; to 80\u0026deg;, with particular attention paid to the region between 20\u0026deg; and 50\u0026deg;. A step size of 0.02\u0026deg; and an acquisition time of 4 s per step were used.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cp\u003eTable 1 summarizes the main parameters of the different Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e Sb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e samples investigated in this work. The first column lists the sample composition, while the second indicates whether the samples are pristine (as grown) or iodine-ion\u0026ndash;irradiated. In the latter case, it is specified whether a multi-energy irradiation scheme corresponding to \u003cem\u003ef\u003c/em\u003e\u003csub\u003e40\u003c/sub\u003e (dashed line in Fig. 2, with a characteristic vacancy density of approximately 40%) or \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e (solid black line in Fig. 2, with a vacancy density twice that of \u003cem\u003ef\u003c/em\u003e\u003csub\u003e40\u003c/sub\u003e) was applied. In one sample, the initial fluences were further tripled within the same scheme (\u003cem\u003ef\u003c/em\u003e\u003csub\u003e120\u003c/sub\u003e).\u003c/p\u003e\n\u003cp\u003eThe following three columns report the geometric dimensions of the ribbons used in the experiments (for the pristine Bi sample, the dimensions of the two ribbons measured from the same batch are provided). The penultimate column presents the electrical resistivity values obtained for each sample at room temperature (left) and at 50 K (right). Finally, the last column reports the semiconductor band gap extracted from least-squares fitting procedures, as described below in the text.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e Main data of the samples studied in this work. For each composition there are samples either pristine or with two different I-ion irradiations, as specified in the main text. Their geometric dimensions are provided in the central columns (for the pristine Bi sample, the dimensions of the two ribbons measured from the same batch are provided). The last but one column shows the electrical resistivity values obtained for each sample at room temperature (left) and at 50 K (right). The last column presents the semiconductor band-gap values obtained from least-squares fittings, as described in the main text.\u003c/p\u003e\n\u003cp\u003eFirstly, we present and discuss the morphological and structural characterization of samples with different compositions, both before (pristine) and after iodine-ion irradiation (\u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e).\u003c/p\u003e\n\u003cp\u003eFigure 3 shows representative SEM images of Bi and Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e ribbons. The left panels correspond to the as-grown samples, whereas the right panels show samples irradiated with iodine ions following the \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e fluence scheme. Cross-sectional images reveal a polycrystalline structure composed of grains with varying sizes and morphologies. The sample thickness was also determined, yielding typical values in the range of 10\u0026ndash;60 \u0026micro;m. It should be noted that the thickness is not uniform across these ribbons, owing to the intrinsic difficulty in controlling this parameter in the melt-spinning growth technique. For irradiation and electrical resistivity measurements, the thinnest available samples were therefore selected.\u003c/p\u003e\n\u003cp\u003eTop-view surface images reveal a layered, directional, and relatively homogeneous growth. Instead of well-defined polyhedral crystalline grains, a more compact and diffuse morphology is observed. After iodine-ion irradiation, noticeable surface modifications can be detected, with a tendency toward a more homogeneous microstructure and less well-defined grain boundaries.\u003c/p\u003e\n\u003cp\u003eSamples with all other compositions were also examined in both cross-sectional and top-view configurations. As their morphological features do not differ significantly from those shown in Fig. 3, only these images are presented as representative examples.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 3\u003c/strong\u003e Comparison of SEM cross-section and top-view surface images of some pure Bi and Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e ribbons as grown (ASG, left panels) and after \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e irradiation with iodine ions (IRR, right panels). Thickness of each sample is indicated in the cross-section images.\u003c/p\u003e\n\u003cp\u003eWe also performed an elemental analysis of the alloys as a cross-check of their nominal compositions. Figure 4 presents representative EDX spectra from four different samples: pristine and iodine-ion\u0026ndash;irradiated (\u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e) pure Bi, and pristine and irradiated (\u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e) Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e. The only significant difference between the spectra of pristine Bi and Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e is the appearance of the characteristic Sb peaks, highlighted with blue arrows. When comparing the irradiated Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e sample with its pristine counterpart, the spectra show no substantial differences.\u003c/p\u003e\n\u003cp\u003eData were collected from all measured samples; however, as the results are essentially identical to those shown in Fig. 4, only these spectra are included as representative examples.\u003c/p\u003e\n\u003cp\u003eQuantitatively, the Sb-to-Bi atomic ratios in the alloyed samples fall within the expected range, with small deviations from the nominal compositions that are typical of EDX measurements, given their limited precision. These results confirm that the samples were grown with compositions close to the intended values and without any significant concentration of impurities. Moreover, no evidence of irradiation-induced compositional changes was observed.\u003c/p\u003e\n\u003cp\u003eTo further support these conclusions, EDX spectra were acquired from spatially separated surface regions using different electron accelerating voltages (8 kV and 20 kV), thereby probing different information depths. In addition, cross-sectional compositional maps were obtained. All these measurements consistently indicate a reasonably homogeneous and isotropic distribution of Bi and Sb throughout the samples, with no detectable compositional inhomogeneities.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 4\u003c/strong\u003e Comparison of EDX spectra of different samples: as grown Bi (black), as grown Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e (green) and irradiated (\u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e) Bi (grey) and Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e (red). Characteristic X-ray emission lines of Bi and Sb are indicated by their corresponding peaks.\u003c/p\u003e\n\u003cp\u003eFrom the internal structural point of view, all XRD patterns (Fig. 5) indicate that the as-grown samples consist of polycrystalline grains with random orientations. The diffraction peaks can be indexed to the rhombohedral crystal structure of Bi described in the hexagonal setting, in good agreement with previously reported results for thin films\u0026nbsp;[36\u0026ndash;40], ribbons\u0026nbsp;[41], and nanoparticles\u0026nbsp;[42,43]. The (012) reflection is the most intense, indicating that this plane is the dominant crystallographic orientation, as also observed in Refs.\u0026nbsp;[36,39,41,42], although several additional reflections with lower intensity are present.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 5\u003c/strong\u003e Comparison of XRD patterns for (a) pure Bi, (b) Bi\u003csub\u003e95\u003c/sub\u003eSb\u003csub\u003e5\u003c/sub\u003e, (c) Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e and (d) Bi\u003csub\u003e85\u003c/sub\u003eSb\u003csub\u003e15\u003c/sub\u003e, both before (as grown, green spectra) and after irradiation with iodine ions (red spectra) with a fluence corresponding to about 80% of vacancies (\u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e), as indicated in the legends. As grown Bi\u003csub\u003e85\u003c/sub\u003eSb\u003csub\u003e15\u003c/sub\u003e corresponds to a different series of fabrication (and different set of measurements) than the rest of the samples.\u003c/p\u003e\n\u003cp\u003eThe incorporation of antimony appears to have a negligible effect on the diffraction profiles. Consequently, the alloyed samples were analyzed using the same reference pattern (\u003cem\u003eMatch!\u003c/em\u003e program, database entry 96‑712‑3353) as that employed for pure Bi. Minor variations in peak positions and relative intensities among samples with different compositions are likely attributable to slight differences in sample preparation and growth conditions. Fig. 5 also displays the XRD patterns of samples after iodine-ion irradiation following the \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e scheme. Samples irradiated using the \u003cem\u003ef\u003c/em\u003e\u003csub\u003e40\u003c/sub\u003e protocol were likewise examined; however, because their diffraction patterns are nearly identical to those obtained with \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e, only the latter are shown for clarity. Any differences relative to the as-grown samples are more readily discernible at the higher irradiation dose.\u003c/p\u003e\n\u003cp\u003eIn all cases, irradiation results in a slight broadening of the diffraction peaks, as reflected in the increased full width at half maximum (FWHM), indicating a partial degradation of crystalline order. No additional diffraction peaks are observed, ruling out the formation of new crystalline phases induced by irradiation. Likewise, no diffuse scattering background is detected that would suggest the presence of an amorphous phase\u0026nbsp;[44]. This absence also may be attributed to the fact that any amorphized material is expected to be confined to a relatively shallow subsurface region, approximately 4\u0026ndash;7 \u0026micro;m below the surface, as Fig. 2 shows. Although the measurements were performed in the \u0026theta;\u0026ndash;2\u0026theta; configuration, the diffraction signal from such a thin amorphous layer is likely overwhelmed by the contribution from the he polycrystalline region closer to the surface, which is less damaged. Therefore, the lack of detectable amorphization signatures suggests that permanent amorphization was either not achieved or is limited to a spatial region beyond the sensitivity threshold of our experimental setup. A very similar behavior has been reported following irradiation with bismuth ions, both in comparable melt-spun ribbons and in thermally evaporated samples\u0026nbsp;[29].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 6\u003c/strong\u003e Plots of the resistivity coefficients versus temperature for different melt spun ribbons Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e (\u003cem\u003ex\u003c/em\u003e = 0, 5, 7, 10, 15) ribbons: as grown (green curves), with a set of fluences \u003cem\u003ef\u003c/em\u003e\u003csub\u003e40\u003c/sub\u003e (blue curves) or \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80 \u003c/sub\u003e(red curves), as well as \u003cem\u003ef\u003c/em\u003e\u003csub\u003e120 \u003c/sub\u003e(brown curve) for the case \u003cem\u003ex\u003c/em\u003e = 7, as indicated in the legends. For pure Bi, in addition to the linear graph (a), a log-log plot is also depicted (b).\u003c/p\u003e\n\u003cp\u003eA crystallite size analysis was also performed. Each diffraction peak was fitted using a Gaussian function in order to extract the full width at half maximum (FWHM). The crystallite size was then estimated using the Scherrer equation with a shape factor \u003cem\u003eK\u003c/em\u003e = 0.94. The resulting crystallite sizes range from approximately 100 to 500 nm, in good agreement with the submicrometer grain sizes observed in the SEM images.\u003c/p\u003e\n\u003cp\u003eWe now present and discuss the results of the electrical resistivity measurements as a function of temperature, \u003cem\u003er\u003c/em\u003e(\u003cem\u003eT\u003c/em\u003e), for ribbons of the various studied compositions, including both as-grown (pristine) samples and ribbons irradiated with iodine ions in the energy range of 25\u0026ndash;40 MeV, following the irradiation procedures described above. As previously observed in earlier work [29], mainly using bismuth self-ions, melt-spun ribbons exhibit significantly lower electrical resistivity than polycrystalline samples prepared by other methods. In particular, pure Bi samples display a semimetallic behavior \u0026ndash;similar to bulk Bi single crystals\u0026ndash; instead of the semiconducting behavior reported for other types of Bi samples [28,29].\u003c/p\u003e\n\u003cp\u003eIn the present study, we therefore focus exclusively on melt-spun samples, employ iodine (I) ions of higher energy and penetration depth in thinner ribbons \u0026ndash;thus optimizing the modified region\u0026ndash; and extend the range of investigated compositions to Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e with \u003cem\u003ex\u003c/em\u003e = 0, 5, 7, 10, 15. The corresponding \u003cem\u003er\u003c/em\u003e(\u003cem\u003eT\u003c/em\u003e) curves are shown in Fig. 6 on a linear scale, except for pure Bi, whose semimetallic behavior and lower resistivity values are also depicted on a log\u0026ndash;log scale (Fig. 6b). In all cases, resistivity was obtained directly from the geometric dimensions of each ribbon, without accounting for the fact that the deepest regions of the samples are barely affected by ion irradiation. Consequently, the actual effect of irradiation-induced disorder on the electrical conductivity is likely more pronounced than observed. Nonetheless, given the varying ribbon thicknesses and to avoid introducing the bias of a specific model, we present the raw \u003cem\u003er\u003c/em\u003e(\u003cem\u003eT\u003c/em\u003e) data, which still allow for at least a qualitative discussion.\u003c/p\u003e\n\u003cp\u003eFor all Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e alloys with \u003cem\u003ex\u003c/em\u003e \u0026ne; 0, the expected narrow-gap semiconducting behavior described by \u003cem\u003er\u003c/em\u003e = \u003cem\u003er\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e exp(\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e/2\u003cem\u003ek\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e\u003cem\u003eT\u003c/em\u003e) is observed, where \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e is the full thermal bandgap. As extensively discussed in the literature [4,6\u0026ndash;8,28,29], at low temperatures -typically below 50\u0026ndash;100 K depending on the sample- the resistivity ceases to increase exponentially and tends to saturate due to the dominant contribution of charge carriers originating from ionized impurity levels, rather than from the intrinsic energy bands of Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e. It is worth noting that the room‑temperature resistivity values obtained (\u003cem\u003e\u0026rho;\u003c/em\u003e = 0.13 m\u0026Omega;\u0026middot;cm for semimetallic Bi and \u003cem\u003e\u0026rho;\u003c/em\u003e \u0026asymp; 0.2 m\u0026Omega;\u0026middot;cm for the Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e semiconducting alloys) are very similar to those previously measured in bulk single crystals [7,45].\u003c/p\u003e\n\u003cp\u003eFigure 7 shows the same curves as Fig. 6 (excluding pure Bi, which does not display semiconducting behavior), plotted as the natural logarithm of resistivity versus inverse temperature, enabling determination of the semiconductor bandgap \u003cem\u003e\u0026Delta;\u003c/em\u003e. Owing to the aforementioned low‑temperature saturation effect associated with ionized impurity levels, least‑squares fits were performed only in the relatively high‑temperature ranges where the expected exponential dependence is not affected by this additional contribution. The extracted bandgap values, on the order of 10\u0026ndash;20 meV, are listed in the last column of Table 1. These values are smaller than those obtained using other sample‑preparation methods [28], and instead closely resemble those reported in the literature for foils [7] and even for bulk crystals [2].\u003c/p\u003e\n\u003cp\u003eAlthough the various irradiation treatments did not succeed in fully amorphizing a region of the samples -as apparently confirmed by XRD-, they presumably introduced a certain degree of disorder into the crystalline structure. While no direct correlation with the calculated damage level (see Table 1) is observed, in many cases the disorder does reduce the electrical resistivity of Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e ribbons, including the clear case of pure Bi, whose resistivity reaches nearly half that of a pristine single crystal after the highest applied iodine‑ion irradiation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 7\u003c/strong\u003e Natural logarithm of the resistivity coefficients versus 1000/\u003cem\u003eT\u003c/em\u003e for the same data points shown in Fig. 6 for Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e (\u003cem\u003ex\u003c/em\u003e = 5, 7, 10, 15), excluding pure Bi. The solid lines show the linear fits performed at higher temperatures to determine the semiconducting gaps. Insets: the same plot in the whole temperature range, where the saturation of the semiconducting behavior at the lowest temperatures is observed.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 8\u003c/strong\u003e Values of the resistivity coefficients at 50 K in logarithmic scale (a) and of the semiconducting gap (b) as a function of composition for Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e (\u003cem\u003ex\u003c/em\u003e = 5, 7, 10, 15), for pristine samples (green symbols) and after \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e irradiations (red symbols). See all available data in Table 1.\u003c/p\u003e\n\u003cp\u003eAttempting to identify possible correlations, Fig. 8 shows the values obtained for the low‑temperature (50 K) resistivity coefficient and the semiconductor bandgap as a function of alloy composition, for both pristine samples and those irradiated with the \u003cem\u003ef\u003c/em\u003e\u003csub\u003e80\u003c/sub\u003e fluence set \u0026ndash;selected as the most representative case for clarity. Although no single unambiguous correlation can be established, and despite the discussed uncertainties associated with the absolute values, some well‑defined and similar trends appear for both electrical properties. Both quantities increase with \u003cem\u003ex\u003c/em\u003e (Sb content) up to approximately \u003cem\u003ex\u003c/em\u003e = 10, after which the trend reverses. Moreover, ion irradiation tends to decrease both \u003cem\u003e\u0026rho;\u003c/em\u003e and \u003cem\u003e\u0026Delta;\u003c/em\u003e, i.e., to improve the electrical performance of the material, particularly for samples with higher Sb content. This seems to be in agreement with recent theoretical findings in two-dimensional alloys of Bi\u003csub\u003e100\u003c/sub\u003e\u003csub\u003e-\u003c/sub\u003e\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003eSb\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e [46].\u003c/p\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eIn summary, we have systematically investigated the impact of ion-irradiation-induced damage and disorder \u0026minus;produced by tens-of-MeV iodine ions\u0026minus; on melt-spun Bi\u003csub\u003e100\u0026minus;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eSb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e ribbons in the search for routes toward amorphous and potentially superconducting material. Structural characterization (SEM, XRD) indicates that the thermodynamic driving force toward crystallization remains sufficiently strong to prevent full amorphization, even in the most heavily irradiated regions where simulations predict atomic displacements exceeding 80%. Nevertheless, the melt-spun ribbons exhibit electrical resistivity values remarkably close to those reported for bulk single crystals, in contrast with other sample types such as thermally evaporated films. Moreover, the irradiation-induced disorder in these polycrystalline ribbons leads to a slight but reproducible enhancement of electrical conductivity, observed both in semimetallic Bi and in narrow-gap (10\u0026ndash;20 meV) Bi-Sb alloys. These results highlight the robustness of the crystalline state in these materials under extreme ion damage, while revealing that controlled irradiation may serve as a viable tool to tune their transport properties.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003cstrong\u003eConflict of interest\u003c/strong\u003e \u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eNG and MAR managed the project and carried out the conceptualization of the experiments. AAG and DR performed low-temperature measurements. GT and AAG conducted characterization experiments and analysis. VM, CF and JV fabricated the samples. AAG, ARC, GGL, NG and MAR did the ion-beam irradiation experiments. MAR wrote the draft and all authors supervised the writing of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe are grateful to the CMAM technical staff for their support and the beam time access with the proposal codes IMP021/23, IMP024/23, IMP005/24, and IMP013/24.We acknowledge financial support from the Spanish Ministry of Science, Innovation and Universities (MCIN/AEI/10.13039/501100011033) through the grant PID2021-127498NB-I00, and within the \u0026ldquo;Mar\u0026iacute;a de Maeztu\u0026rdquo; Program for Units of Excellence in R\u0026amp;D (CEX2023-001316-M). The authors also acknowledge the funding from Comunidad de Madrid through project TEC-2024/TEC-380 \"Mag4TIC\u0026rdquo; and support from the COST action CA21144 \u0026ldquo;Superqumap\u0026rdquo;. The authors are grateful to the service from the MiNa Laboratory at IMN and the funding from Comunidad de Madrid (project SpaceTec, S2013/ICE2822).A.A.G. thanks the Spanish Ministry of Science, Innovation and Universities for the fellowship under contract PRE2022-103066.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData Availability. The research data that support the findings of this study are available from the correspondingauthor upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eA.W. Smith, The Hall effect and some allied effects in alloys. Phys. Rev. \u003cb\u003e32\u003c/b\u003e, 178\u0026ndash;200 (1911)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH.J. Goldsmid, B.-A. Alloys, phys. stat. sol. (a) 1, 7\u0026ndash;28 (1970), and references therein\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG.E. Smith, R. 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Res. \u003cb\u003e7\u003c/b\u003e, 043263 (2025)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTable 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-low-temperature-physics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jltp","sideBox":"Learn more about [Journal of Low Temperature Physics](http://link.springer.com/journal/10909)","snPcode":"10909","submissionUrl":"https://submission.nature.com/new-submission/10909/3","title":"Journal of Low Temperature Physics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"bismuth, ion beam modification of materials, amorphous solids, topological materials, electrical resistivity, superconductivity","lastPublishedDoi":"10.21203/rs.3.rs-8743396/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8743396/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCrystalline Bi\u003csub\u003e100−\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eSb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e alloys are well known as the first experimentally realized topological insulators, in addition to their promising thermoelectric properties. In contrast, their amorphous counterparts have been reported to exhibit superconductivity with critical temperatures exceeding 6 K. However, the strong tendency of Bi and Bi-Sb alloys to crystallize, even at very low temperatures, has hindered both systematic studies and practical applications of these amorphous phases. To explore the possibility of obtaining amorphous superconducting states and enhancing thermoelectric performance, we investigated ion-beam irradiation as a method to induce amorphization in Bi\u003csub\u003e100−\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eSb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e alloys.\u003c/p\u003e\n\u003cp\u003eWe performed irradiation experiments on pure Bi and Bi-Sb melt-spun ribbons using iodine ions with energies between 25 and 40 MeV, achieving estimated vacancy damage levels of 40–80%. Structural characterization by X-ray diffraction and electrical resistivity measurements in the range 2–300 K revealed that, although amorphization and superconductivity were not achieved, ion-induced disorder led to significant conductivity improvements, particularly in Bi\u003csub\u003e90\u003c/sub\u003eSb\u003csub\u003e10\u003c/sub\u003e Furthermore, interesting correlations were observed between the resistivity values and the semiconducting gap with the Sb content, both before and after irradiation. These results provide new insights into the interplay between structural disorder, electrical transport, and topological properties in Bi-Sb alloys.\u003c/p\u003e","manuscriptTitle":"Bi-Sb alloys irradiated with swift heavy ions as potential topological amorphous superconductors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-06 18:12:27","doi":"10.21203/rs.3.rs-8743396/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-19T12:57:27+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-24T22:00:37+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-15T03:58:34+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"20272237606081298240153641121807294988","date":"2026-02-06T06:32:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"78028137157082404416728780459267706234","date":"2026-02-03T18:28:58+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-03T18:22:07+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-01T22:34:20+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-31T05:06:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Low Temperature Physics","date":"2026-01-30T16:21:58+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-low-temperature-physics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jltp","sideBox":"Learn more about [Journal of Low Temperature Physics](http://link.springer.com/journal/10909)","snPcode":"10909","submissionUrl":"https://submission.nature.com/new-submission/10909/3","title":"Journal of Low Temperature Physics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"951b93f2-8da7-4309-a546-348f25b9bcdc","owner":[],"postedDate":"February 6th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-11T13:19:37+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-06 18:12:27","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8743396","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8743396","identity":"rs-8743396","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}