Evidence of a Bulk Interlayer Excitonic Insulator Phase in the Oxypnictide CeMnAsO1-xFx

Evidence of a Bulk Interlayer Excitonic Insulator Phase in the Oxypnictide CeMnAsO1-xFx | 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 Article Evidence of a Bulk Interlayer Excitonic Insulator Phase in the Oxypnictide CeMnAsO 1-x F x Abbie Mclaughlin, Struan Simpson, Gaynor Lawrence, Clemens Ritter, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7419291/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Excitonic insulators are a remarkable class of insulators that can exhibit a condensate of electron-hole pairs below a critical temperature T C and are predicted to give rise to exotic quantum phenomena. Thus far, the excitonic insulator phase has been predominantly explored in chalcogenide-based systems. However, the nature of the excitonic order parameter governing the EI transitions in these candidate systems is still controversial as there are few candidate materials that exhibit an EI phase without an accompanying symmetry-breaking lattice distortion. The discovery of new chemical systems that can host the EI phase is hence very important. Here we report evidence of a tuneable, bulk correlated interlayer excitonic insulator in the oxypnictide CeMnAsO 1 − x F x ( x ≥ 0.035) with a maximum T C of 104 K. Crucially, no symmetry-breaking lattice distortion or charge density wave is observed across the transition which will enable further study of the EI phase diagram. The proposed excitonic insulator (EI) phase emerges when the electronic band gap is tuned below a critical threshold through chemical doping and is marked by a significant upturn in the resistivity. First-principles calculations reveal the formation of bound excitons between spatially separated electrons and holes in distinct layers within the crystal structure with a binding energy, E b = 50 meV. A reversal in the Hall and Seebeck coefficients is observed below T C as holes from the CeO/F layer bind with electrons in the Mott insulating As-Mn-As block. Neutron diffraction shows that T C can be further controlled by reducing the interlayer distance and enhancing the electronic coupling between layers. This work identifies CeMnAsO 1 − x F x as a promising chemical platform for exploring novel quantum phases arising from excitons and expands the range of materials considered to host the EI phase. Physical sciences/Materials science/Condensed-matter physics/Electronic properties and materials Physical sciences/Physics/Condensed-matter physics/Electronic properties and materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 INTRODUCTION Electron correlations can stabilise a plethora of emergent electronic states with fascinating properties. The excitonic insulator (EI) is one such interaction-driven many-body state which is predicted to arise in small-gap semiconductors and semimetals. In this scenario, the Coulomb interaction between holes and electrons is sufficiently strong to induce the spontaneous formation of excitons (bound electron-hole pairs) , , producing an exotic insulating ground state – the excitonic insulator. The electronic phase diagram predicted for the EI phase is shown in Fig. 1 (a). Below a critical temperature T C , a many-body gap 2Δ E opens which is related to the excitonic binding energy E b . In a semiconductor, the excitonic insulating state is realised when E b is greater than the charge band gap E g . On the semiconductor side of the phase diagram, a Bose-Einstein (BE) condensate of tightly bound excitons is predicted whereas on the semimetal side of the phase diagram a BCS-like condensate of loosely bound excitons is expected below T C . The excitonic state is most stable and the maximum transition temperature is predicted when E g is zero. The EI phase has attracted considerable interest due to its potential in developing novel electronic devices based on the manipulation of exciton flux, such as excitonic transistors and switches . To date, several bulk chalcogenide materials have been proposed to demonstrate the EI phase including Ta 2 NiSe 5 and 1 T -TiSe 2 . However, the nature of the excitonic order parameter governing the EI transitions in these candidate systems can be controversial owing to the confounding influence of symmetry-breaking lattice distortions. For example, a second-order band Jahn-Teller effect have been suggested to explain the electronic and structural properties of 1 T -TiSe 2 , . In Ta 2 NiSe 5 , the semiconductor-insulator transition is accompanied by a structural phase transition from orthorhombic to monoclinic symmetry, leading to controversy over whether the order parameter has predominantly structural or electronic character , . Recent research has focussed on WTe 2 , which is a layered semimetal. The properties of WTe 2 change remarkably when exfoliated to a monolayer, transforming into a two-dimensional topological insulator .. There is now evidence that the insulating behaviour observed below 100 K in monolayer WTe 2 is a result of an excitonic insulator state , in the clean limit without a structural phase transition. A new approach to realizing the EI phase has been to construct charge transfer interlayer excitons by separating the electrons and holes in distinct layers. Exciton condensation has now been reported under thermal non equilibrium conditions in two-dimensional atomic double layers , . For example, a correlated interlayer excitonic insulator has been reported in heterostructures of monolayer WSe 2 and moiré WS 2 /WSe 2 , . Here, interlayer excitons form when electrons were added to the Mott insulator in the WS 2 /WSe 2 bilayer and holes were added to the WSe 2 monolayer under an out-of-plane electric field. Excitonic condensation was observed below 60 K 16 . More recently, a charge transfer excitonic insulator was reported in the organic-inorganic superlattice Co(Cp) 2 –SnSe 2 through self-assembly intercalation of Co(Cp) 2 molecules into the inorganic chalcogenide SnSe 2 . Given that excitonic insulators are predicted to host exotic quantum phenomena including unconventional superconductivity, it is important to identify further tuneable chemical systems in which the transition to the EI state can be decoupled from any structural symmetry-breaking to enable a deeper understanding of the EI phase. Recently, we reported a novel quantum insulating phase in the layered oxypnictide CeMnAsO 1− x F x (0.035 ≤ x ≤ 0.075) and in Ce non-stoichiometric phases Ce y MnAsO 0.95 F 0.05 ( y = 0.96, 0.97) . Above T C , the samples are semiconducting but at the transition, their resistivities abruptly increase by more than two orders of magnitude over a narrow temperature interval and rapidly become immeasurably large. It was tentatively suggested that the semiconductor-insulator transition could be a result of many-body localisation (MBL). However, it is not apparent yet if MBL is possible in greater than one-dimensional systems , , and so more investigation of the exotic quantum insulating state is needed. The results from electrical measurements, neutron diffraction analysis and first principal calculations shown here strongly suggest that the semiconductor-insulator transition in CeMnAsO 1 − x F x is a result of a transition to an excitonic insulator phase where holes in the CeO/F layer bind with electrons in the Mott insulating As-Mn-As block below T C (Fig. 1 (b) inset). The remaining charge carriers are deeply localised so that the resistivity increases by orders of magnitude below T C . High-resolution neutron diffraction studies show that there is no change in crystal symmetry at T C in CeMnAsO 1 − x F x which is important to facilitate deeper study of the EI state. T C can be tuned by both band gap engineering and reducing the Ce-As distance so that CeMnAsO 1 − x F x is an excellent chemical platform for further investigating the exotic quantum phases of excitons. RESULTS AND DISCUSSION The electronic phase diagram for an excitonic insulator is shown in Figure 1(a). On the semiconducting side of the phase diagram, a transition from a semiconductor to an excitonic insulator is observed when the bandgap, E g , is reduced to a critical level (so that E b is greater E g ). The transition temperature ( T C ) from the semiconductor to the EI phase is observed to increase as E g is reduced reaching a maximum at E g = 0 eV. The electronic phase diagram of CeMnAsO 1- x F x shown in Figure 1(b). On the semiconducting side, the observed phase boundaries closely match theoretical predictions for an excitonic insulator (Fig 1(a)) which is possible if the F ‑ doping value x has a monotonic relationship with ( E b - E g ). CeMnAsO is a Mott insulator; substituting F - for O 2- in CeMnAsO 1- x F x reduces the electronic bandgap, E g , by almost an order of magnitude from 0.31(5) eV to 0.04(2) eV 19 . Upon cooling, CeMnAsO 1- x F x phases with x ³ 0.035 undergo a transition into a quantum insulating phase below a critical temperature T C . The emergence of an insulating phase, only when E g is tuned below a critical threshold by carrier doping, alongside a T C that increases from 18 K ( x = 0.035) to 82 K ( x = 0.075) as the system approaches the semi metallic regime (Figure 1b) is consistent with the theoretical expectations for EI formation (Fig. 1(a)). This suggests that the quantum insulating phase observed in CeMnAsO 1- x F x could be the result of a transition to an excitonic insulator below T C . To investigate if interlayer excitons are supported in CeMnAsO, first-principles calculations were performed. Using density functional theory (DFT) Kohn-Sham single-particle wave function as a starting point, we utilised the G 0 W 0 method to calculate the band structure. The obtained band gap was 1.0 eV, which was larger than experimental electronic band gap, so we applied scissors correction to match the experimental band gap of 0.31 eV. The resulting band structure is presented in Figure 2 (a). Although the S-point had a slightly narrower band gap, the band structure indicates that a Γ – Γ exciton can be hosted. To characterise excitons, based on the G 0 W 0 method, we performed the Bethe-Salpeter equation (BSE) calculation. The optical response function and joint density of states are presented in Figure 2 (b). Two exciton peaks are clearly discernible around 0.27 and 0.31 eV, which correspond to exciton binding energies E b = 0.05 and 0.01 eV, respectively, confirming the presence of interlayer excitons for the first time in CeMnAsO. The crystal structure of the CeMnAsO 1- x F x phases is shown in the inset of Figure 2(b). DFT calculations have previously shown that the electronic structure of CeMnAsO 0.94 F 0.06 is highly unusual 19 . Upon electron doping CeMnAsO, by substitution of F - for O 2- , the Ce 4f band is destabilised in the vicinity of F - , so that upon F - doping some of the Ce 3+ oxidises to Ce 4+ , hence creating holes in the 4 f band. At the same time there is reduction of the Mn 3d band by the electrons added by substitution of F - for O 2- and the simultaneous oxidation of Ce 3+ 19 . Hence CeMnAsO 1- x F x phases intrinsically host spatially separated charge carriers with holes in the CeO/F layer and electrons in the As-Mn-As block. The emergence of excitons in CeMnAsO 1- x F x could then arise in a similar fashion to that recently reported in heterostructures of monolayer WSe 2 and moiré WS 2 /WSe 2 15 , 16 where interlayer excitons form when electrons are added to the Mott insulator in the WS 2 /WSe 2 bilayer and holes are added to the WSe 2 monolayer. Analogously, we propose heavy holes from the CeO/F layer bind together with the electrons from the Mott insulating As-Mn-As block to form a correlated interlayer EI state through the Coulomb interaction (Figure 3 (a)). The holes and electrons are then separated in real space which stabilises the electron-hole pair, resulting in an excitonic insulator below T C when E g is reduced to a critical value 4 . In further corroboration, the calculated exciton binding energy of CeMnAsO ( E b = 50 meV) is comparable to the binding energy reported for the strongly correlated excitonic insulator consisting of atomic double layers of MoSe 2 and WSe 2 15 , 16 ( E b ≈ 25 meV) which has a T C of 60 K. The calculated exciton binding energy of CeMnAsO is also lower than the calculated bandgap ( E b = 0.05 eV; E g = 0.31 eV) which explains why the EI phase is only realised as the band gap is reduced by an order of magnitude upon F - doping CeMnAsO 1-x F x . Further verification of the interlayer EI state below T C is provided by Hall coefficient measurements on Ce 0.97 MnAsO 0.95 F 0.05 (Figure 3 (b), Supplementary Section 1) where the presence of both electron and hole carriers are observed. Electrons dominate the Hall response above T C , consistent with the presence of heavy/immobile holes in the 4 f band and more mobile electrons in the 3 d band above T C. Below T C , when the electrons and holes are fully condensed into bound excitons, the Hall coefficient will then be sensitive to any small imbalance in hole and electron concentration. In Ce 0.97 MnAsO 0.95 F 0.05 , a crossover from a negative to a positive Hall coefficient is observed below T C (Figure 3 (b)), from the residual 4 f hole carriers that remain unbound following exciton formation. This reflects an extremely low density of free charge carriers (holes) in the condensed excitonic phase. Above T C the charge carriers are localised as a result of Anderson localisation, and variable range hopping is observed. Below T C the exciton condensate and disorder most likely block all thermalisation channels which would explain the gradual decoupling of electrons and phonons in CeMnAsO 1- x F x and significant reduction in Hall mobility 19 . The residual charge carriers are then deeply localised (potentially many body localised) so that the resistivity rapidly increases and become immeasurable below T C . The results from Seebeck coefficient measurements on two different CeMnAsO 0.965 F 0.035 samples are also consistent with this picture as a sign reversal and a colossal Seebeck coefficient are observed below T C 19 (Supplementary Section 1). To further explore structure property relationships of the potential EI phase in CeMnAsO 1- x F x , high resolution neutron diffraction experiments have been performed on the solid solution CeMnAsO 1- x F x ( x = 0 – 0.075). Rietveld fits, cell parameters, agreement factors, and atomic parameters obtained from the refinements are provided in Section 2 of the Supplementary Information. Selected bond lengths and angles are displayed in Supplementary Figures 2-4 and Supplementary Tables 1-4. Results show that T C increases as the interlayer electronic coupling between the CeO/F layer and Mott insulating As-Mn-As block is enhanced, giving further evidence of the interlayer EI state as described below. The Ce–O/F bond length expands from 2.3636(10) Å to 2.3670(12) Å as x increases from 0 – 0.075 so that the interlayer Ce-As distance reduces upon increasing x in CeMnAsO 1- x F x (Supplementary Figures 1 and 2) and the CeO/F layer moves closer to the semiconducting layer (As-Mn-As block). The same increase in the Ce-O/F bond length is observed in the superconducting CeFeAsO 1- x F x series 23 and has been reported to facilitate electron transfer from the Ce-O/F layer to the As-Fe-As block and hence enhances electronic coupling between the layers 23 . The temperature variation of the lattice parameters were determined from Rietveld fits to the P 4/ nmm structural model from neutron diffraction data for the CeFeAsO 1- x F x series. The linear coefficients of thermal expansion were calculated by fitting the temperature dependence of the lattice parameters to a third order polynomial, separately from 50 K - 300 K and 10 K - 30 K (above and below T SR , Supplementary Section 3) and are shown for CeMnAsO 0.965 F 0.035 in Figure 4. α ( a ) is approximately zero below 30 K whereas c appears to plateau at a constant (finite) value. Below T SR the elastic properties are anisotropic because of the magnetostriction observed in c (Supplementary Figure 8) as a result of the interlayer magnetic coupling between the Mn 2+ spins and Ce 3+ spins 24 . Unusually, at 300 K (well above T SR ), the anisotropy ratio a ( c )/ a ( a ) significantly increases from 1.08(3) to 1.20(3) as x is changed from 0.00 to 0.075 in CeMnAsO 0.965 F 0.035 . Figure 5(a) shows that the anisotropy in the thermal expansion increases as the Ce-As distance, and therefore the inter-layer distance, reduces. Reducing the Ce-As distance enhances charge transfer and electronic coupling between the Ce(O/F) and MnAs layers. The resulting change in anisotropy ratio with chemical doping reflects this stronger interlayer coupling, making it a quantitative proxy for the interlayer coupling strength between the CeO/F layer and the As–Mn–As block. We had previously shown that T C can also be tuned by varying the Ce non-stoichiometry in Ce y MnAsO 0.95 F 0.05 where T C = 34 K and 104 K for y = 0.00 and 0.96 respectively 19 . Figure 5 (b) shows that there is a clear correlation between the anisotropy ratio and T C across all CeMnAsO 1-x F x and Ce 0.96 MnAsO 0.95 F 0.05 phases reported here, so that T C monotonically increases as the interlayer electronic coupling is enhanced. Hence it would appear that the significant enhancement of T C with either F - doping or Ce non stoichiometry is related to the increased interlayer electronic coupling, consistent with the hypothesis that the transition to a novel insulating phase in CeMnAsO 1-x F x below T C is a result of interlayer excitonic condensation. Reducing the Ce-As distance and increasing inter-layer coupling will result in greater spatial overlap between the electrons and holes and hence enhance the inter-layer Coulomb interaction, leading to a greater exciton binding energy and higher transition temperatures, as observed 25 , 26 . DFT calculations show that CeMnAsO 1- x F x has a direct band gap 19 and so a change in crystal symmetry would not be anticipated at the EI transition. We find no evidence of any subtle peak splitting or superstructure reflections within the high resolution of our diffraction experiments to suggest there is any change in crystal symmetry upon cooling the samples from 300 K, with the ~ 10 K neutron diffraction data showing an excellent Rietveld fit to the aristotypical P 4/ nmm structural model (Supplementary Figure 1). One alternative possibility is that since the electronic band structure enables a Γ – Γ exciton to form, the EI transition may be accompanied by a Γ-point symmetry-breaking distortion which cannot be distinguished reliably by Rietveld refinement alone. However, group-theoretical analysis reveals the majority of Γ-point distortions of the aristotype structure should change the crystal symmetry (Supplementary Table 5), thus any hypothetical structural symmetry-breaking at the EI transition would be accompanied by some symmetry-breaking strain as well. Analysis of the peak shapes in our high-resolution powder neutron diffraction data shows there is no significant increase in microstrain both across the series as well as upon cooling below T C in each sample (Supplementary Table 6). This shows there is no significant deviation from the tetragonal lattice parameters across the EI transition, demonstrating that the exciton condensation is not strongly coupled to any change in crystal symmetry. CONCLUSIONS The combined experimental and theoretical evidence presented strongly supports the identification of CeMnAsO 1 − x F x as a tuneable correlated interlayer excitonic insulator that emerges from a Mott insulator upon reducing the electronic band gap, E g , to a critical level by carrier doping. Key signatures of the EI phase include a transition from a Mott insulator to an exotic insulating state when E b > E g , the coexistence of heavy holes in the Ce-O/F layer and electrons in the As-Mn-As block that can form excitons as shown by the Bethe-Salpeter equation (BSE) calculation, a transition temperature ( T C ) that increases as the band gap narrows and interlayer electronic coupling is enhanced through a reduction in the Ce-As distance. Further defining features include a sign reversal of both Hall and Seebeck coefficients at T C and a significant exciton binding energy of E b = 50 meV, comparable to that observed in heterostructures of monolayer WSe 2 and moiré WS 2 /WSe 2 that exhibit the EI phase below 60 K 15, 16 . Other bulk EI candidates such as Ta 2 NiSe 5 4 and 1 T -TiSe 2 5 exhibit strong changes in the crystal lattice at the critical temperature which makes it difficult to fully elucidate details regarding the order parameter driving the emergence of the EI phase. Importantly, in-depth analysis of high-resolution neutron diffraction data clearly demonstrate that there is no change in crystal symmetry at T C for CeMnAsO 1 − x F x and have ruled out every possible q = 0 symmetry-breaking structural distortion which can occur. Hence there are no significant lattice distortions so that the transition is primarily electronic in nature. CeMnAsO 1 − x F x hence represents a model system in which to study the EI transition and its relation to other quantum phenomena. The discovery of a novel correlated interlayer EI phase in CeMnAsO 1 − x F x is important as EI phases are predicted to exhibit exotic properties such as superfluid energy transport alongside exotic quantum phases of excitons. We note the intrinsic real-space separation of holes and electrons enabled by the interlayer structure of CeMnAsO 1 − x F x will favour long-lived excitons which suitable for potential technological applications, and the ability to tune the exciton condensation through strain engineering strategies should enable T C to be further optimised towards practical operating temperatures , 3 . These findings motivate further exploration of Mn oxypnictides to elucidate further structure–property relationships of the excitonic insulator phase. EXPERIMENTAL SECTION Synthesis Polycrystalline samples of CeMnAsO 1- x F x ( x = 0.00, 0.035, 0.05, 0.075) were prepared in a two-step solid-state reaction. A CeAs precursor was first synthesised by reacting stoichiometric quantities of Ce pieces (Aldrich 99.9%) and As chips (Alfa Aesar 99.999%) in an evacuated quartz tube at 980 °C for 33 h. This precursor was then reacted with stoichiometric masses of Mn, MnO 2 , and MnF 2 powders before being ground in an agate mortar and pestle under an inert atmosphere. The powder was then pressed into pellets using a 10 mm die set, placed in a Ta crucible, and sealed in an evacuated quartz tube before being sintered at 1150 °C for 48 h. Diffraction Laboratory X-ray powder diffraction (XRD) patterns were collected on a PANalytical Empyrean powder diffractometer equipped with a Cu Kα tube at ambient temperature. Data were recorded in the range 10° < 2θ < 110° with a step size of 0.013°. Neutron powder diffraction (NPD) measurements were recorded on the high resolution D2B diffractometer at the Institut Laue-Langevin (ILL, Grenoble) at a constant wavelength of 1.594 Å. Data were recorded at 1.5-10 K and 300 K for ~ 0.6 g samples of CeMnAsO 1- x F x in an 8 mm vanadium can, with a collection time of 4 h for each sample at each temperature. Due to issues with the cryostat the low temperature recordings were not all at 1.5 K, each different doping level was recorded at a slightly different temperature (all temperatures were ≤ 10 K). High intensity neutron diffraction data were recorded on the D20 beamline between 1.5 and 380 K for x = 0.035 and 0.075 and between 300 – 380 K for x =0.00 and 0.05. The ramp rates for data recording were 19 seconds per 0.1°. Rietveld refinements 28 were performed using the GSAS/EXPGUI package 29 to determine both the nuclear and magnetic structures of the series at the two temperatures investigated. Data were excluded in the range 39–41° 2θ in all the refinements performed due to peaks from the cryostat, a consequence of the small sample mass. The backgrounds were fitted using a shifted Chebyschev function, and the peak shapes were modelled with a pseudo-Voigt function. Physical Property Measurements The temperature dependence of the Hall resistivity, electronic resistivity and Seebeck effect of selected CeMnAsO 1- x F x phases were recorded using a Quantum Design physical property measurement system (PPMS) between 4 and 300 K. First-principles calculations The electronic structure was calculated using plane-wave DFT within the projector-augmented wave scheme as implemented in VASP. Following our previous work, 19 a cutoff energy of 600 eV and reciprocal space sampling of 6 × 6 × 3 was used 30 , 31, 32 . Initial DFT calculation was performed with PBE+ U functional within the collinear spin representation, where Hubbard U was set to 5.0 and 1.0 eV for Ce-f and Mn-d orbitals, respectively 33, 34 .G 0 W 0 was performed on top of the single-particle wave functions. The plane-wave cutoff energy was 400 eV used both for wave functions and the response function. 12 occupied and 12 unoccupied states were selected and were used to calculate BSE calculation with Tamm-Dancoff approximation , 35 . Declarations Acknowledgements For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising from this submission.This research was supported by EPSRC (Grant no. EP/L002493/1), the Carnegie Trust for the Universities of Scotland (PhD scholarship for S.S.) and the ILL (PhD studentship for G.B.L.). We also acknowledge STFC-GB for provision of beamtime at the ILL. ASSOCIATED CONTENT Supporting information consists of figures showing the electronic properties and tables and figures of crystallographic data. AUTHOR INFORMATION Correspondence and requests for materials should be addressed to A.C.M. (email: [email protected] ). References Mott, N. F. 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Supplementary Files ACMSupplementaryInformation4.docx Evidence of a Bulk Interlayer Excitonic Insulator Phase in the Oxypnictide CeMnAsO1-xFx Cite Share Download PDF Status: Posted Version 1 posted 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. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-7419291","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":512885419,"identity":"205c80ee-588d-442e-ba4c-2cd1df71ca53","order_by":0,"name":"Abbie Mclaughlin","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABBElEQVRIiWNgGAWjYPACCyDmYTyQwGDDYMCQABGTwK8FJM3DANSSRqoWBobDhLXwNzA/YLpRIyFvcH7tgQMPd5yXN2fPPcDwo4YhcWYDDuMPsBkw5xyTMNxw413CgcQztw139rxLYOw5xpA4G5eTDvAwMOc2SDBuuHHG4EBi220gI8eAgbeBIXEeDh3yUC32UC3n7EFaGP/i0WIA1ZK44XwPSMuBRJAWZpAtuBxmeJjN4DDQL8kzb/CAtCQn7+x5Y3BY5piEMS7vyx1vfvg4p8bGtu/8GcOHP9vsbLez5xg+fFNjIzvjAA5rmBkYIFISCUhhQjAiwYAfl6GjYBSMglEw4gEAIjBh+dQDCSUAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-9960-723X","institution":"University of Aberdeen","correspondingAuthor":true,"prefix":"","firstName":"Abbie","middleName":"","lastName":"Mclaughlin","suffix":""},{"id":512885420,"identity":"9f86d6ea-a890-4b78-9efd-aac10d2b8bb0","order_by":1,"name":"Struan Simpson","email":"","orcid":"","institution":"University of Warwick","correspondingAuthor":false,"prefix":"","firstName":"Struan","middleName":"","lastName":"Simpson","suffix":""},{"id":512885421,"identity":"120dd643-f97d-4cc5-afaf-9492feb67fdd","order_by":2,"name":"Gaynor Lawrence","email":"","orcid":"","institution":"University of Aberdeen","correspondingAuthor":false,"prefix":"","firstName":"Gaynor","middleName":"","lastName":"Lawrence","suffix":""},{"id":512885422,"identity":"91ecd4ae-dbfc-41be-963a-22b92c44083e","order_by":3,"name":"Clemens Ritter","email":"","orcid":"https://orcid.org/0000-0003-3674-3378","institution":"Institute Laue Langevin","correspondingAuthor":false,"prefix":"","firstName":"Clemens","middleName":"","lastName":"Ritter","suffix":""},{"id":512885423,"identity":"49417e82-a00d-4ad4-abcf-fe4db2fa95b2","order_by":4,"name":"Angel Arevalo-Lopez","email":"","orcid":"https://orcid.org/0000-0002-8745-4990","institution":"Laboratory of Catalysis and Solid State Chemistry","correspondingAuthor":false,"prefix":"","firstName":"Angel","middleName":"","lastName":"Arevalo-Lopez","suffix":""},{"id":512885424,"identity":"36c7d4cc-0194-4885-b560-46234a50541f","order_by":5,"name":"Kazuki Morita","email":"","orcid":"https://orcid.org/0000-0002-2558-6963","institution":"University of Pennsylvania","correspondingAuthor":false,"prefix":"","firstName":"Kazuki","middleName":"","lastName":"Morita","suffix":""},{"id":512885425,"identity":"2b587d96-1a62-48a9-a056-87a82dd7cf74","order_by":6,"name":"Quinn Gibson","email":"","orcid":"https://orcid.org/0000-0001-7020-199X","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Quinn","middleName":"","lastName":"Gibson","suffix":""},{"id":512885426,"identity":"3a2107a7-3cc0-4bbd-a257-ea34ad014fe3","order_by":7,"name":"Eve Wildman","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Eve","middleName":"","lastName":"Wildman","suffix":""}],"badges":[],"createdAt":"2025-08-20 16:20:42","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-7419291/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7419291/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":91549746,"identity":"cedd79bb-5c01-4d7d-b4b5-d783b94b48b7","added_by":"auto","created_at":"2025-09-17 15:31:06","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":50620,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eComparison of the phase diagram of CeMnAsO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e1-\u003c/strong\u003e\u003c/sub\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003ex\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cstrong\u003eF\u003c/strong\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003ex \u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cstrong\u003ewith that of an excitonic insulator\u003c/strong\u003e. (a) Electronic phase diagram of an excitonic insulator as a function of band gap, \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e, assuming a constant exciton binding energy. (b) The electronic phase diagram of CeMnAsO\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex \u003c/em\u003e\u003c/sub\u003ewhich exhibits a semiconductor-insulator phase transition with a clear dependence on the doping value, \u003cem\u003ex\u003c/em\u003e.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7419291/v1/c6202b8a05f4c3c2cdd1f13a.png"},{"id":91548809,"identity":"033e3698-4ab3-4f97-ade0-991f6fd54dc7","added_by":"auto","created_at":"2025-09-17 15:23:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":108983,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCalculated electronic properties of CeMnAsO.\u003c/strong\u003e a) The electronic band structure of CeMnAsO calculated by the G\u003csub\u003e0\u003c/sub\u003eW\u003csub\u003e0\u003c/sub\u003e method. The conduction and the valence band are coloured in yellow and blue, respectively. The energy was shifted so that 0.0 eV corresponds to the middle of the band gap. b) Exciton states calculated by the BSE and joint density of states calculated by the G\u003csub\u003e0\u003c/sub\u003eW\u003csub\u003e0\u003c/sub\u003e method. The inset shows the crystal structure of CeMnAsO\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e,\u003csub\u003e \u003c/sub\u003ewhere\u003csub\u003e \u003c/sub\u003eCeO\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e layers are situated between tetrahedral blocks of As-Mn-As.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7419291/v1/cf5e2ede9d9f10991b4efa1f.png"},{"id":91549747,"identity":"20b49390-52a0-4309-a847-fbca6d02bf7a","added_by":"auto","created_at":"2025-09-17 15:31:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":136533,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExciton formation in CeMnAsO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e1-\u003c/strong\u003e\u003c/sub\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003ex\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cstrong\u003eF\u003c/strong\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003ex\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e (a) A schematic of the formation of the correlated interlayer excitonic insulator phase in CeMnAsO\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e. (b) Temperature variation of the Hall coefficient for Ce\u003csub\u003e0.97\u003c/sub\u003eMnAsO\u003csub\u003e0.95\u003c/sub\u003eF\u003csub\u003e0.05\u003c/sub\u003e which has a \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e of 50 K. Results show a sign reversal from \u003cem\u003eR\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e being negative to positive below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e. This suggests the remaining dominant carriers change from electrons to holes as the excitons condense.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7419291/v1/6552660450be5de6934ce1b4.png"},{"id":91548813,"identity":"53b5eb3f-e72b-45c2-a682-13b07a541ece","added_by":"auto","created_at":"2025-09-17 15:23:06","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":52071,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe linear coefficients of thermal expansion for \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003ea\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e (red), \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003ec\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e (green) and cell volume (black) for CeMnAsO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.965\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eF\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.035.\u003c/strong\u003e\u003c/sub\u003e\u003csup\u003e\u003cstrong\u003e \u003c/strong\u003e\u003c/sup\u003eCeMnAsO\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e phases exhibit a spin reorientation of the Mn\u003csup\u003e2+\u003c/sup\u003e moments below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eSR\u003c/sub\u003e = 34 K and so the data are shown for the temperature ranges 50 K - 350 K (above \u003cem\u003eT\u003c/em\u003e\u003csub\u003eSR\u003c/sub\u003e) (a) and 10 K - 30 K (below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eSR\u003c/sub\u003e) (Supplementary Section 3) (b). The cell volume data is divided by a factor of three for a more direct comparison.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7419291/v1/577ea83782f4fe808801c705.png"},{"id":91550561,"identity":"6b54337b-efc9-4eee-96c0-0e63c1d9605e","added_by":"auto","created_at":"2025-09-17 15:39:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":41392,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCoupling of the electronic and structural degrees of freedom in CeMnAsO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e1-x\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eF\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003ex\u003c/strong\u003e\u003c/sub\u003e\u003csup\u003e\u003cstrong\u003e \u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eand Ce\u003c/strong\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003ey\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cstrong\u003eMnAsO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.95\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eF\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.05\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e (a) Variation of the anisotropy ratio with the Ce-As distance showing that the anisotropy ratio is enhanced as the Ce-As distance decreases. (b) demonstrates that there is a direct relationship between the anisotropy ratio and \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e. Both plots show the data for CeMnAsO\u003csub\u003e1-x\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003ex\u003c/em\u003e = 0.03 – 0.075) and Ce\u003csub\u003e0.96\u003c/sub\u003eMnAsO\u003csub\u003e0.95\u003c/sub\u003eF\u003csub\u003e0.05\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7419291/v1/d1cea29cc92dd450efdb1cd9.png"},{"id":101752715,"identity":"d9bed7c4-3963-4a52-aa57-ddd103ef8264","added_by":"auto","created_at":"2026-02-03 10:29:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1090351,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7419291/v1/78a74d45-0755-4629-b340-95e05392faf4.pdf"},{"id":91549751,"identity":"176367ba-77df-4d41-b848-72fc93960db2","added_by":"auto","created_at":"2025-09-17 15:31:06","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":7931355,"visible":true,"origin":"","legend":"Evidence of a Bulk Interlayer Excitonic Insulator Phase in the Oxypnictide CeMnAsO1-xFx","description":"","filename":"ACMSupplementaryInformation4.docx","url":"https://assets-eu.researchsquare.com/files/rs-7419291/v1/133a60010a88410d5187266a.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eEvidence of a Bulk Interlayer Excitonic Insulator Phase in the Oxypnictide CeMnAsO\u003csub\u003e1-x\u003c/sub\u003eF\u003csub\u003ex\u003c/sub\u003e\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eElectron correlations can stabilise a plethora of emergent electronic states with fascinating properties. The excitonic insulator (EI) is one such interaction-driven many-body state which is predicted to arise in small-gap semiconductors and semimetals. In this scenario, the Coulomb interaction between holes and electrons is sufficiently strong to induce the spontaneous formation of excitons (bound electron-hole pairs) \u003ca class=\"FNLink\" href=\"#Fn1\" id=\"#FNLinkFn1\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e \u003ca class=\"FNLink\" href=\"#Fn2\" id=\"#FNLinkFn2\"\u003e\u003c/a\u003e, producing an exotic insulating ground state \u0026ndash; the excitonic insulator. The electronic phase diagram predicted for the EI phase is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). Below a critical temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e, a many-body gap 2Δ\u003csub\u003eE\u003c/sub\u003e opens which is related to the excitonic binding energy \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e. In a semiconductor, the excitonic insulating state is realised when \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e is greater than the charge band gap \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e. On the semiconductor side of the phase diagram, a Bose-Einstein (BE) condensate of tightly bound excitons is predicted whereas on the semimetal side of the phase diagram a BCS-like condensate of loosely bound excitons is expected below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e. The excitonic state is most stable and the maximum transition temperature is predicted when \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e is zero.\u003c/p\u003e\u003cp\u003eThe EI phase has attracted considerable interest due to its potential in developing novel electronic devices based on the manipulation of exciton flux, such as excitonic transistors and switches \u003ca class=\"FNLink\" href=\"#Fn3\" id=\"#FNLinkFn3\"\u003e\u003c/a\u003e. To date, several bulk chalcogenide materials have been proposed to demonstrate the EI phase including Ta\u003csub\u003e2\u003c/sub\u003eNiSe\u003csub\u003e5\u003c/sub\u003e \u003ca class=\"FNLink\" href=\"#Fn4\" id=\"#FNLinkFn4\"\u003e\u003c/a\u003e and 1\u003cem\u003eT\u003c/em\u003e-TiSe\u003csub\u003e2\u003c/sub\u003e \u003ca class=\"FNLink\" href=\"#Fn5\" id=\"#FNLinkFn5\"\u003e\u003c/a\u003e. However, the nature of the excitonic order parameter governing the EI transitions in these candidate systems can be controversial owing to the confounding influence of symmetry-breaking lattice distortions. For example, a second-order band Jahn-Teller effect have been suggested to explain the electronic and structural properties of 1\u003cem\u003eT\u003c/em\u003e-TiSe\u003csub\u003e2\u003c/sub\u003e \u003ca class=\"FNLink\" href=\"#Fn6\" id=\"#FNLinkFn6\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e \u003ca class=\"FNLink\" href=\"#Fn7\" id=\"#FNLinkFn7\"\u003e\u003c/a\u003e. In Ta\u003csub\u003e2\u003c/sub\u003eNiSe\u003csub\u003e5\u003c/sub\u003e, the semiconductor-insulator transition is accompanied by a structural phase transition from orthorhombic to monoclinic symmetry, leading to controversy over whether the order parameter has predominantly structural or electronic character \u003ca class=\"FNLink\" href=\"#Fn8\" id=\"#FNLinkFn8\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e \u003ca class=\"FNLink\" href=\"#Fn9\" id=\"#FNLinkFn9\"\u003e\u003c/a\u003e. Recent research has focussed on WTe\u003csub\u003e2\u003c/sub\u003e, which is a layered semimetal. The properties of WTe\u003csub\u003e2\u003c/sub\u003e change remarkably when exfoliated to a monolayer, transforming into a two-dimensional topological insulator \u003ca class=\"FNLink\" href=\"#Fn10\" id=\"#FNLinkFn10\"\u003e\u003c/a\u003e.. There is now evidence that the insulating behaviour observed below 100 K in monolayer WTe\u003csub\u003e2\u003c/sub\u003e is a result of an excitonic insulator state \u003ca class=\"FNLink\" href=\"#Fn11\" id=\"#FNLinkFn11\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e \u003ca class=\"FNLink\" href=\"#Fn12\" id=\"#FNLinkFn12\"\u003e\u003c/a\u003e in the clean limit without a structural phase transition.\u003c/p\u003e\u003cp\u003eA new approach to realizing the EI phase has been to construct charge transfer interlayer excitons by separating the electrons and holes in distinct layers. Exciton condensation has now been reported under thermal non equilibrium conditions in two-dimensional atomic double layers \u003ca class=\"FNLink\" href=\"#Fn13\" id=\"#FNLinkFn13\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e \u003ca class=\"FNLink\" href=\"#Fn14\" id=\"#FNLinkFn14\"\u003e\u003c/a\u003e. For example, a correlated interlayer excitonic insulator has been reported in heterostructures of monolayer WSe\u003csub\u003e2\u003c/sub\u003e and moir\u0026eacute; WS\u003csub\u003e2\u003c/sub\u003e/WSe\u003csub\u003e2\u003c/sub\u003e \u003ca class=\"FNLink\" href=\"#Fn15\" id=\"#FNLinkFn15\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e \u003ca class=\"FNLink\" href=\"#Fn16\" id=\"#FNLinkFn16\"\u003e\u003c/a\u003e. Here, interlayer excitons form when electrons were added to the Mott insulator in the WS\u003csub\u003e2\u003c/sub\u003e/WSe\u003csub\u003e2\u003c/sub\u003e bilayer and holes were added to the WSe\u003csub\u003e2\u003c/sub\u003e monolayer under an out-of-plane electric field. Excitonic condensation was observed below 60 K \u003csup\u003e16\u003c/sup\u003e. More recently, a charge transfer excitonic insulator was reported in the organic-inorganic superlattice Co(Cp)\u003csub\u003e2\u003c/sub\u003e\u0026ndash;SnSe\u003csub\u003e2\u003c/sub\u003e \u003ca class=\"FNLink\" href=\"#Fn17\" id=\"#FNLinkFn17\"\u003e\u003c/a\u003e through self-assembly intercalation of Co(Cp)\u003csub\u003e2\u003c/sub\u003e molecules into the inorganic chalcogenide SnSe\u003csub\u003e2\u003c/sub\u003e. Given that excitonic insulators are predicted to host exotic quantum phenomena including unconventional superconductivity, \u003ca class=\"FNLink\" href=\"#Fn18\" id=\"#FNLinkFn18\"\u003e\u003c/a\u003e it is important to identify further tuneable chemical systems in which the transition to the EI state can be decoupled from any structural symmetry-breaking to enable a deeper understanding of the EI phase.\u003c/p\u003e\u003cp\u003eRecently, we reported a novel quantum insulating phase in the layered oxypnictide CeMnAsO\u003csub\u003e1\u0026minus;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e (0.035\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.075) and in Ce non-stoichiometric phases Ce\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003eMnAsO\u003csub\u003e0.95\u003c/sub\u003eF\u003csub\u003e0.05\u003c/sub\u003e (\u003cem\u003ey\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.96, 0.97) \u003ca class=\"FNLink\" href=\"#Fn19\" id=\"#FNLinkFn19\"\u003e\u003c/a\u003e. Above \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e, the samples are semiconducting but at the transition, their resistivities abruptly increase by more than two orders of magnitude over a narrow temperature interval and rapidly become immeasurably large. It was tentatively suggested that the semiconductor-insulator transition could be a result of many-body localisation (MBL). However, it is not apparent yet if MBL is possible in greater than one-dimensional systems \u003ca class=\"FNLink\" href=\"#Fn20\" id=\"#FNLinkFn20\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e \u003ca class=\"FNLink\" href=\"#Fn21\" id=\"#FNLinkFn21\"\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e \u003ca class=\"FNLink\" href=\"#Fn22\" id=\"#FNLinkFn22\"\u003e\u003c/a\u003e and so more investigation of the exotic quantum insulating state is needed.\u003c/p\u003e\u003cp\u003eThe results from electrical measurements, neutron diffraction analysis and first principal calculations shown here strongly suggest that the semiconductor-insulator transition in CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e is a result of a transition to an excitonic insulator phase where holes in the CeO/F layer bind with electrons in the Mott insulating As-Mn-As block below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) inset). The remaining charge carriers are deeply localised so that the resistivity increases by orders of magnitude below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e. High-resolution neutron diffraction studies show that there is no change in crystal symmetry at \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e in CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e which is important to facilitate deeper study of the EI state. \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e can be tuned by both band gap engineering and reducing the Ce-As distance so that CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e is an excellent chemical platform for further investigating the exotic quantum phases of excitons.\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cp\u003eThe electronic phase diagram for an excitonic insulator is shown in Figure 1(a). On the semiconducting side of the phase diagram, a transition from a semiconductor to an excitonic insulator is observed when the bandgap, \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e, is reduced to a critical level (so that \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e is greater \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e). The transition temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e) from the semiconductor to the EI phase is observed to increase as \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e is reduced reaching a maximum at \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/sub\u003e= 0 eV. The electronic phase diagram of CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003eshown in Figure 1(b). On the semiconducting side, the observed phase boundaries closely match theoretical predictions for an excitonic insulator (Fig 1(a)) which is possible if the F\u003csup\u003e‑\u003c/sup\u003e doping value \u003cem\u003ex\u003c/em\u003e has a monotonic relationship with (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e-\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e). CeMnAsO is a Mott insulator; substituting F\u003csup\u003e-\u003c/sup\u003e for O\u003csup\u003e2-\u003c/sup\u003e in CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e reduces the electronic bandgap, \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e, by almost an order of magnitude from 0.31(5) eV to 0.04(2) eV\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003csup\u003e19\u003c/sup\u003e. Upon cooling, CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e phases with \u003cem\u003ex\u003c/em\u003e \u0026sup3;\u0026nbsp;0.035 undergo a transition into a quantum insulating phase below a critical temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e. The emergence of an insulating phase, only when \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e is tuned below a critical threshold by carrier doping, alongside a \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e that increases from 18 K (\u003cem\u003ex\u003c/em\u003e = 0.035) to 82 K (\u003cem\u003ex\u003c/em\u003e = 0.075) as the system approaches the semi metallic regime (Figure 1b)\u0026nbsp;is consistent with the theoretical expectations for EI formation\u0026nbsp;(Fig. 1(a)). This suggests that the quantum insulating phase observed in\u0026nbsp;CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e could be the result of a transition to an excitonic insulator below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo investigate if interlayer excitons are supported in\u0026nbsp;CeMnAsO,\u0026nbsp;first-principles calculations were performed. Using density functional theory (DFT) Kohn-Sham single-particle wave function as a starting point, we utilised the G\u003csub\u003e0\u003c/sub\u003eW\u003csub\u003e0\u003c/sub\u003e method to calculate the band structure. The obtained band gap was 1.0 eV, which was larger than experimental electronic band gap, so we applied scissors correction to match the experimental band gap of 0.31 eV. The resulting band structure is presented in Figure 2 (a). Although the S-point had a slightly narrower band gap,\u0026nbsp;the band structure indicates that a \u0026Gamma; \u0026ndash; \u0026Gamma; exciton can be hosted. \u0026nbsp;To characterise excitons, based on the G\u003csub\u003e0\u003c/sub\u003eW\u003csub\u003e0\u003c/sub\u003e method, we performed the Bethe-Salpeter equation (BSE) calculation. The optical response function and joint density of states are presented in Figure 2 (b). Two exciton peaks are clearly discernible around 0.27 and 0.31 eV, which correspond to exciton binding energies \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e = 0.05 and 0.01 eV, respectively, confirming the presence of interlayer excitons for the first time in\u0026nbsp;CeMnAsO.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe crystal structure of the\u0026nbsp;CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u0026nbsp;\u003c/em\u003ephases is shown in the inset of Figure 2(b). DFT calculations have previously shown that the electronic structure of\u0026nbsp;CeMnAsO\u003csub\u003e0.94\u003c/sub\u003eF\u003csub\u003e0.06\u0026nbsp;\u003c/sub\u003eis highly unusual\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003csup\u003e19\u003c/sup\u003e. Upon electron doping CeMnAsO, by substitution of F\u003csup\u003e-\u003c/sup\u003e for O\u003csup\u003e2-\u003c/sup\u003e, the Ce 4f band is destabilised in the vicinity of F\u003csup\u003e-\u003c/sup\u003e, so that upon F\u003csup\u003e-\u0026nbsp;\u003c/sup\u003edoping some of the Ce\u003csup\u003e3+\u003c/sup\u003e oxidises to Ce\u003csup\u003e4+\u003c/sup\u003e, hence creating holes in the 4\u003cem\u003ef\u003c/em\u003e band. At the same time there is reduction of the Mn 3d band by the electrons added by substitution of F\u003csup\u003e-\u003c/sup\u003e for O\u003csup\u003e2-\u003c/sup\u003e and the simultaneous oxidation of Ce\u003csup\u003e3+\u003c/sup\u003e \u003csup\u003e19\u003c/sup\u003e. Hence CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u0026nbsp;\u003c/em\u003ephases intrinsically host spatially separated charge carriers with holes in the CeO/F layer and electrons in the As-Mn-As block.\u003c/p\u003e\n\u003cp\u003eThe emergence of excitons in CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u0026nbsp;\u003c/em\u003ecould then arise in a similar fashion to that recently reported in heterostructures of monolayer WSe\u003csub\u003e2\u003c/sub\u003e and moir\u0026eacute; WS\u003csub\u003e2\u003c/sub\u003e/WSe\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003csup\u003e15\u003c/sup\u003e\u003csup\u003e,\u0026nbsp;\u003c/sup\u003e\u003csup\u003e16\u003c/sup\u003e where interlayer excitons form when electrons are added to the Mott insulator in the WS\u003csub\u003e2\u003c/sub\u003e/WSe\u003csub\u003e2\u003c/sub\u003e bilayer and holes are added to the WSe\u003csub\u003e2\u003c/sub\u003e monolayer. Analogously, we propose heavy holes from the CeO/F layer bind together with the electrons from the Mott insulating As-Mn-As block to form a correlated interlayer EI state through the Coulomb interaction (Figure 3 (a)). The holes and electrons are then separated in real space which stabilises the electron-hole pair, resulting in an excitonic insulator below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e when \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e is reduced to a critical value \u003csup\u003e4\u003c/sup\u003e. In further corroboration, the calculated exciton binding energy of CeMnAsO (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e = 50 meV) is comparable to the binding energy reported for the strongly correlated excitonic insulator consisting of atomic double layers of MoSe\u003csub\u003e2\u003c/sub\u003e and WSe\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e15\u003c/sup\u003e\u003csup\u003e,\u0026nbsp;\u003c/sup\u003e\u003csup\u003e16\u003c/sup\u003e (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e \u0026asymp; 25 meV) which has a \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e of 60 K. \u0026nbsp;The calculated exciton binding energy of CeMnAsO is also lower than the calculated bandgap (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e = 0.05 eV; \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e = 0.31 eV) which explains why the EI phase is only realised as the band gap is reduced by an order of magnitude upon F\u003csup\u003e-\u003c/sup\u003e doping CeMnAsO\u003csub\u003e1-x\u003c/sub\u003eF\u003csub\u003ex\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eFurther verification of the interlayer EI state below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e is provided by Hall coefficient measurements on\u0026nbsp;Ce\u003csub\u003e0.97\u003c/sub\u003eMnAsO\u003csub\u003e0.95\u003c/sub\u003eF\u003csub\u003e0.05\u003c/sub\u003e (Figure 3 (b), Supplementary Section 1) where the presence of both electron and hole carriers are observed. Electrons dominate the Hall response above \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e, consistent with the presence of heavy/immobile holes in the 4\u003cem\u003ef\u0026nbsp;\u003c/em\u003eband and more mobile electrons in the 3\u003cem\u003ed\u003c/em\u003e band above \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC.\u003c/sub\u003e Below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e, when the electrons and holes are fully condensed into bound excitons, the Hall coefficient will then be sensitive to any small imbalance in hole and electron concentration. In\u0026nbsp;Ce\u003csub\u003e0.97\u003c/sub\u003eMnAsO\u003csub\u003e0.95\u003c/sub\u003eF\u003csub\u003e0.05\u003c/sub\u003e, a crossover from a negative to a positive Hall coefficient is observed below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e (Figure 3 (b)), from the residual 4\u003cem\u003ef\u003c/em\u003e hole carriers that remain unbound following exciton formation. This reflects an extremely low density of free charge carriers (holes) in the condensed excitonic phase. Above \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e the charge carriers are localised as a result of Anderson localisation, and variable range hopping is observed. Below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e the exciton condensate and disorder most likely block all thermalisation channels which would explain the gradual decoupling of electrons and phonons in CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e and significant reduction in Hall mobility\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003csup\u003e19\u003c/sup\u003e. The residual charge carriers are then deeply localised (potentially many body localised) so that the resistivity rapidly increases and become immeasurable below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e. The results from Seebeck coefficient measurements on two different CeMnAsO\u003csub\u003e0.965\u003c/sub\u003eF\u003csub\u003e0.035\u003c/sub\u003e\u003csup\u003e\u0026nbsp;\u003c/sup\u003esamples are also consistent with this picture as a sign reversal and a colossal Seebeck coefficient are observed below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e \u003csup\u003e19\u003c/sup\u003e (Supplementary Section 1).\u003c/p\u003e\n\u003cp\u003eTo further explore structure property relationships of the potential EI phase in CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e, high resolution neutron diffraction experiments have been performed on the solid solution CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e (\u003cem\u003ex\u003c/em\u003e = 0 \u0026ndash; 0.075). Rietveld fits,\u0026nbsp;cell parameters, agreement factors, and atomic parameters obtained from the refinements are provided in Section 2 of the Supplementary Information.\u0026nbsp;Selected bond lengths and angles are displayed in Supplementary Figures 2-4 and Supplementary Tables 1-4. Results show that \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e increases as the interlayer electronic coupling between the CeO/F layer and Mott insulating As-Mn-As block is enhanced, giving further evidence of the interlayer EI state as described below.\u003c/p\u003e\n\u003cp\u003eThe Ce\u0026ndash;O/F bond length expands from 2.3636(10) \u0026Aring; to 2.3670(12) \u0026Aring; as \u003cem\u003ex\u003c/em\u003e increases from 0 \u0026ndash; 0.075 so that the interlayer Ce-As distance reduces upon increasing \u003cem\u003ex\u003c/em\u003e in CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e (Supplementary Figures 1 and 2) and the CeO/F layer moves closer to the semiconducting layer (As-Mn-As block). The same increase in the Ce-O/F bond length is observed in the superconducting CeFeAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e series\u003csup\u003e\u0026nbsp;23\u003c/sup\u003e and has been reported to facilitate electron transfer from the Ce-O/F layer to the As-Fe-As block and hence enhances electronic coupling between\u0026nbsp;the layers\u0026nbsp;\u003csup\u003e23\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe temperature variation of the lattice parameters were determined from Rietveld fits to the \u003cem\u003eP\u003c/em\u003e4/\u003cem\u003enmm\u003c/em\u003e structural model from neutron diffraction data for the CeFeAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e series. The linear coefficients of thermal expansion were calculated by fitting the temperature dependence of the lattice parameters to a third order polynomial, separately from 50 K - 300 K and 10 K - 30 K (above and below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eSR\u003c/sub\u003e, Supplementary Section 3) and are shown\u0026nbsp;for CeMnAsO\u003csub\u003e0.965\u003c/sub\u003eF\u003csub\u003e0.035\u003c/sub\u003e in Figure 4. \u003cem\u003e\u0026alpha;\u003c/em\u003e(\u003cem\u003ea\u003c/em\u003e) is approximately zero below 30 K whereas \u003cem\u003ec\u003c/em\u003e appears to plateau at a constant (finite) value. Below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eSR\u003c/sub\u003e the elastic properties are anisotropic because of the magnetostriction observed in \u003cem\u003ec\u003c/em\u003e (Supplementary Figure 8) as a result of the interlayer magnetic coupling between the Mn\u003csup\u003e2+\u003c/sup\u003e spins and Ce\u003csup\u003e3+\u003c/sup\u003e spins\u003csup\u003e\u0026nbsp;24\u003c/sup\u003e. Unusually, at 300 K (well above \u003cem\u003eT\u003c/em\u003e\u003csub\u003eSR\u003c/sub\u003e), the anisotropy ratio \u003cem\u003ea\u003c/em\u003e(\u003cem\u003ec\u003c/em\u003e)/\u003cem\u003ea\u003c/em\u003e(\u003cem\u003ea\u003c/em\u003e) significantly increases from 1.08(3) to 1.20(3) as \u003cem\u003ex\u003c/em\u003e is changed from 0.00 to 0.075 in CeMnAsO\u003csub\u003e0.965\u003c/sub\u003eF\u003csub\u003e0.035\u003c/sub\u003e. Figure 5(a) shows that the anisotropy in the thermal expansion increases as the Ce-As distance, and therefore the inter-layer distance, reduces. Reducing the Ce-As distance enhances charge transfer and electronic coupling between the Ce(O/F) and MnAs layers. The resulting change in anisotropy ratio with chemical doping reflects this stronger interlayer coupling, making it a quantitative proxy for the interlayer coupling strength between the CeO/F layer and the As\u0026ndash;Mn\u0026ndash;As block.\u003c/p\u003e\n\u003cp\u003eWe had previously shown that\u0026nbsp;\u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e can also be tuned by varying the Ce non-stoichiometry in Ce\u003cem\u003e\u003csub\u003ey\u003c/sub\u003e\u003c/em\u003eMnAsO\u003csub\u003e0.95\u003c/sub\u003eF\u003csub\u003e0.05\u003c/sub\u003e where \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e = 34 K and 104 K for \u003cem\u003ey\u003c/em\u003e = 0.00 and 0.96 respectively\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003csup\u003e19\u003c/sup\u003e.\u0026nbsp;Figure 5 (b) shows\u0026nbsp;that there is a clear correlation between the anisotropy ratio and\u0026nbsp;\u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e across all CeMnAsO\u003csub\u003e1-x\u003c/sub\u003eF\u003csub\u003ex\u003c/sub\u003e and\u0026nbsp;Ce\u003csub\u003e0.96\u003c/sub\u003eMnAsO\u003csub\u003e0.95\u003c/sub\u003eF\u003csub\u003e0.05\u003c/sub\u003e phases reported here,\u0026nbsp;so that\u0026nbsp;\u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e monotonically increases as\u0026nbsp;the\u0026nbsp;interlayer electronic coupling\u0026nbsp;is\u0026nbsp;enhanced.\u0026nbsp;Hence it would appear that the significant enhancement of\u0026nbsp;\u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e with either F\u003csup\u003e-\u003c/sup\u003e doping or Ce non stoichiometry is related to the increased interlayer electronic coupling, consistent with the hypothesis that the transition to a novel insulating phase in CeMnAsO\u003csub\u003e1-x\u003c/sub\u003eF\u003csub\u003ex\u003c/sub\u003e below\u0026nbsp;\u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e is a result of interlayer excitonic condensation. Reducing the Ce-As distance and increasing inter-layer coupling will result in greater spatial overlap between the electrons and holes and hence enhance the inter-layer Coulomb interaction, leading to a greater exciton binding energy and higher transition temperatures, as observed \u003csup\u003e25\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e \u003csup\u003e26\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDFT calculations show that CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u0026nbsp;\u003c/em\u003ehas a direct band gap\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003csup\u003e19\u003c/sup\u003e and so a change in crystal symmetry would not be anticipated at the EI transition.\u0026nbsp;We find no evidence of any subtle peak splitting or superstructure reflections within the high resolution of our diffraction experiments to suggest there is any change in crystal symmetry upon cooling the samples from 300 K, with the \u0026nbsp;~\u0026nbsp;10 K neutron diffraction data showing an excellent Rietveld fit to the aristotypical\u0026nbsp;\u003cem\u003eP\u003c/em\u003e4/\u003cem\u003enmm\u003c/em\u003e structural model (Supplementary Figure 1). One alternative possibility is that since the electronic band structure enables a \u0026Gamma; \u0026ndash; \u0026Gamma; exciton to form, the EI transition may be accompanied by a \u0026Gamma;-point symmetry-breaking distortion which cannot be distinguished reliably by Rietveld refinement alone. However, group-theoretical analysis reveals the majority of \u0026Gamma;-point distortions of the aristotype structure should change the crystal symmetry (Supplementary Table 5), thus any hypothetical structural symmetry-breaking at the EI transition would be accompanied by some symmetry-breaking strain as well. Analysis of the peak shapes in our high-resolution powder neutron diffraction data shows there is\u0026nbsp;no significant increase in microstrain both across the series as well as upon cooling below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e in each sample (Supplementary Table 6). This shows there is no significant deviation from the tetragonal lattice parameters across the EI transition, demonstrating that the exciton condensation is not strongly coupled to any change in crystal symmetry.\u0026nbsp;\u003c/p\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eThe combined experimental and theoretical evidence presented strongly supports the identification of CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e as a tuneable correlated interlayer excitonic insulator that emerges from a Mott insulator upon reducing the electronic band gap, \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e, to a critical level by carrier doping. Key signatures of the EI phase include a transition from a Mott insulator to an exotic insulating state when \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e \u0026gt;\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e, the coexistence of heavy holes in the Ce-O/F layer and electrons in the As-Mn-As block that can form excitons as shown by the Bethe-Salpeter equation (BSE) calculation, a transition temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e) that increases as the band gap narrows and interlayer electronic coupling is enhanced through a reduction in the Ce-As distance. Further defining features include a sign reversal of both Hall and Seebeck coefficients at \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e and a significant exciton binding energy of \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e 50 meV, comparable to that observed in heterostructures of monolayer WSe\u003csub\u003e2\u003c/sub\u003e and moir\u0026eacute; WS\u003csub\u003e2\u003c/sub\u003e/WSe\u003csub\u003e2\u003c/sub\u003e that exhibit the EI phase below 60 K \u003csup\u003e15, 16\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eOther bulk EI candidates such as Ta\u003csub\u003e2\u003c/sub\u003eNiSe\u003csub\u003e5\u003c/sub\u003e \u003csup\u003e4\u003c/sup\u003e and 1\u003cem\u003eT\u003c/em\u003e-TiSe\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e5\u003c/sup\u003e exhibit strong changes in the crystal lattice at the critical temperature which makes it difficult to fully elucidate details regarding the order parameter driving the emergence of the EI phase. Importantly, in-depth analysis of high-resolution neutron diffraction data clearly demonstrate that there is no change in crystal symmetry at \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e for CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e and have ruled out every possible \u003cem\u003eq\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0 symmetry-breaking structural distortion which can occur. Hence there are no significant lattice distortions so that the transition is primarily electronic in nature. CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e hence represents a model system in which to study the EI transition and its relation to other quantum phenomena. The discovery of a novel correlated interlayer EI phase in CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e is important as EI phases are predicted to exhibit exotic properties such as superfluid energy transport alongside exotic quantum phases of excitons. We note the intrinsic real-space separation of holes and electrons enabled by the interlayer structure of CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e will favour long-lived excitons which suitable for potential technological applications, and the ability to tune the exciton condensation through strain engineering strategies should enable \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e to be further optimised towards practical operating temperatures \u003ca class=\"FNLink\" href=\"#Fn27\" id=\"#FNLinkFn27\"\u003e\u003c/a\u003e\u003csup\u003e, 3\u003c/sup\u003e. These findings motivate further exploration of Mn oxypnictides to elucidate further structure\u0026ndash;property relationships of the excitonic insulator phase.\u003c/p\u003e"},{"header":"EXPERIMENTAL SECTION","content":"\u003cp\u003e\u003cstrong\u003eSynthesis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePolycrystalline samples of CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u0026nbsp;\u003c/em\u003e(\u003cem\u003ex\u003c/em\u003e = 0.00, 0.035, 0.05, 0.075) were prepared in a two-step solid-state reaction. A CeAs precursor was first synthesised by reacting stoichiometric quantities of Ce pieces (Aldrich 99.9%) and As chips (Alfa Aesar 99.999%) in an evacuated quartz tube at 980 °C for 33 h. This precursor was then reacted with stoichiometric masses of Mn, MnO\u003csub\u003e2\u003c/sub\u003e, and MnF\u003csub\u003e2\u003c/sub\u003e powders before being ground in an agate mortar and pestle under an inert atmosphere. The powder was then pressed into pellets using a 10 mm die set, placed in a Ta crucible, and sealed in an evacuated quartz tube before being sintered at 1150 °C for 48 h.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDiffraction\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eLaboratory X-ray powder diffraction (XRD) patterns were collected on a PANalytical Empyrean powder diffractometer equipped with a Cu Kα\u0026nbsp;tube at ambient temperature. Data were recorded in the range 10° \u0026lt; 2θ\u0026nbsp;\u0026lt; 110° with a step size of 0.013°.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNeutron powder diffraction (NPD) measurements were recorded on the high resolution D2B diffractometer at the Institut Laue-Langevin (ILL, Grenoble) at a constant wavelength of 1.594 Å. Data were recorded at 1.5-10 K and 300 K for ~ 0.6 g samples of CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u0026nbsp;\u003c/em\u003ein an 8 mm vanadium can, with a collection time of 4 h for each sample at each temperature. Due to issues with the cryostat the low temperature recordings were not all at 1.5 K, each different doping level was recorded at a slightly different temperature (all temperatures were ≤ 10 K). High intensity neutron diffraction data were recorded on the D20 beamline between 1.5 and 380 K for\u0026nbsp;x = 0.035 and 0.075 and between 300 – 380 K for x =0.00 and 0.05. The ramp rates for data recording were 19 seconds per 0.1°. Rietveld refinements \u003csup\u003e28\u0026nbsp;\u003c/sup\u003ewere performed using the GSAS/EXPGUI package \u003csup\u003e29\u003c/sup\u003e to determine both the nuclear and magnetic structures of the series at the two temperatures investigated. Data were excluded in the range 39–41° 2θ in all the refinements performed due to peaks from the cryostat, a consequence of the small sample mass. The backgrounds were fitted using a shifted Chebyschev function, and the peak shapes were modelled with a pseudo-Voigt function.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePhysical Property Measurements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe temperature dependence of the Hall resistivity, electronic resistivity and Seebeck effect of selected CeMnAsO\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003cem\u003e\u003csub\u003ex\u003c/sub\u003e\u0026nbsp;\u003c/em\u003ephases were recorded using a Quantum Design physical property measurement system (PPMS) between 4 and 300 K.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFirst-principles calculations\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe electronic structure was calculated using plane-wave DFT within the projector-augmented wave scheme as implemented in VASP. Following our previous work,\u0026nbsp;\u003csup\u003e19\u003c/sup\u003ea cutoff energy of 600 eV and reciprocal space sampling of 6 × 6 × 3 was used \u003csup\u003e30\u003c/sup\u003e\u003csup\u003e, 31, 32\u003c/sup\u003e.\u0026nbsp;Initial\u0026nbsp;DFT calculation was performed with PBE+\u003cem\u003eU\u003c/em\u003e functional\u0026nbsp;within the collinear spin representation, where Hubbard \u003cem\u003eU\u003c/em\u003e was set to 5.0 and 1.0 eV for Ce-f and Mn-d orbitals, respectively\u003csup\u003e\u0026nbsp;33, 34\u003c/sup\u003e.G\u003csub\u003e0\u003c/sub\u003eW\u003csub\u003e0\u003c/sub\u003e was performed on top of the single-particle wave functions. The plane-wave cutoff energy was 400 eV used both for wave functions and the response function. 12 occupied and 12 unoccupied states were selected and were used to calculate BSE calculation with Tamm-Dancoff approximation\u003csup\u003e, 35\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFor the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising from this submission.This research was supported by EPSRC (Grant no. EP/L002493/1), the Carnegie Trust for the Universities of Scotland (PhD scholarship for S.S.) and the ILL (PhD studentship for G.B.L.). We also acknowledge STFC-GB for provision of beamtime at the ILL.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eASSOCIATED CONTENT\u003c/p\u003e\n\u003cp\u003eSupporting information consists of figures showing the electronic properties and tables and figures of crystallographic data.\u003c/p\u003e\n\u003cp\u003eAUTHOR INFORMATION\u003c/p\u003e\n\u003cp\u003eCorrespondence and requests for materials should be addressed to A.C.M. (email: [email protected]).\u0026nbsp;\u003c/p\u003e"},{"header":" References","content":"\u003col\u003e\n \u003cli\u003eMott, N. F. The transition to the metallic state. Phil. Mag. 6, 287\u0026ndash;309 (1961).\u003c/li\u003e\n \u003cli\u003eKnox, R in Solid State Physics, edited by F. Seitz and D. Turnbull, Academic Press, New York, 196, Suppl. 5 p. 100.\u003c/li\u003e\n \u003cli\u003eUnuchek, D. et al. Room-temperature electrical control of exciton flux in a van der Waals heterostructure. Nature 560, 340\u0026ndash;344 (2018).\u003c/li\u003e\n \u003cli\u003eLu, Y. 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Peierls (Springer, 1991), pp. 157\u0026ndash;174.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7419291/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7419291/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eExcitonic insulators are a remarkable class of insulators that can exhibit a condensate of electron-hole pairs below a critical temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e and are predicted to give rise to exotic quantum phenomena. Thus far, the excitonic insulator phase has been predominantly explored in chalcogenide-based systems. However, the nature of the excitonic order parameter governing the EI transitions in these candidate systems is still controversial as there are few candidate materials that exhibit an EI phase without an accompanying symmetry-breaking lattice distortion. The discovery of new chemical systems that can host the EI phase is hence very important. Here we report evidence of a tuneable, bulk correlated interlayer excitonic insulator in the oxypnictide CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003ex\u003c/em\u003e \u0026ge; 0.035) with a maximum \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e of 104 K. Crucially, no symmetry-breaking lattice distortion or charge density wave is observed across the transition which will enable further study of the EI phase diagram. The proposed excitonic insulator (EI) phase emerges when the electronic band gap is tuned below a critical threshold through chemical doping and is marked by a significant upturn in the resistivity. First-principles calculations reveal the formation of bound excitons between spatially separated electrons and holes in distinct layers within the crystal structure with a binding energy, \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e \u003cem\u003e=\u003c/em\u003e 50 meV. A reversal in the Hall and Seebeck coefficients is observed below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e as holes from the CeO/F layer bind with electrons in the Mott insulating As-Mn-As block. Neutron diffraction shows that \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e can be further controlled by reducing the interlayer distance and enhancing the electronic coupling between layers. This work identifies CeMnAsO\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eF\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e as a promising chemical platform for exploring novel quantum phases arising from excitons and expands the range of materials considered to host the EI phase.\u003c/p\u003e","manuscriptTitle":"Evidence of a Bulk Interlayer Excitonic Insulator Phase in the Oxypnictide CeMnAsO1-xFx","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-17 15:23:02","doi":"10.21203/rs.3.rs-7419291/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a8a13293-5994-44c3-a08c-cb9f058ba050","owner":[],"postedDate":"September 17th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":54890058,"name":"Physical sciences/Materials science/Condensed-matter physics/Electronic properties and materials"},{"id":54890059,"name":"Physical sciences/Physics/Condensed-matter physics/Electronic properties and materials"}],"tags":[],"updatedAt":"2026-02-02T07:12:20+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-17 15:23:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7419291","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7419291","identity":"rs-7419291","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}