Two-dimensional oxygen crystal with a honeycomb lattice on an ultraflat Cu(111)

Two-dimensional oxygen crystal with a honeycomb lattice on an ultraflat Cu(111) | 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 Physical Sciences - Article Two-dimensional oxygen crystal with a honeycomb lattice on an ultraflat Cu(111) Jungdae Kim, Se–Young Jeong, Binod Regmi, Su Jae Kim, Nguyen Huu Lam, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4940255/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Two-dimensional (2D) materials have served as key platforms for exploring novel phenomena and innovative applications, driven by their reduced dimensionality. The existing library of 2D materials includes carbon-based and transition metal-based systems. However, oxygen-based 2D materials are missing. Oxygen, the most abundant element on Earth with a higher electronegativity than carbon, holds the potential to introduce unprecedented functionalities in 2D materials. Here we report the experimental realization of a 2D oxygen crystal with a highly ordered honeycomb structure, termed “oxylene”, formed on an ultraflat Cu(111) surface with exceptional resistance to oxidation. Scanning tunneling microscopy and density functional theory studies reveal that oxylene consists of oxygen atom clusters occupying both face-centered cubic (fcc) and hexagonal closed-packed (hcp) sites with a 4×4 periodicity on the Cu(111) surface. Furthermore, oxylene exhibits distinct magnetic ordering due to the magnetic octupole moments of oxygen atoms. Symmetry analysis suggests that oxylene exhibits linear responses that are observed in the noncollinear antiferromagnetic Weyl semimetal Mn 3 Sn. Our discovery of oxylene provides intriguing opportunities for exploiting its unique electric and magnetic properties, as well as harnessing oxygen’s high reactivity in 2D form. Physical sciences/Physics/Condensed-matter physics/Surfaces, interfaces and thin films Physical sciences/Materials science/Nanoscale materials/Magnetic properties and materials Physical sciences/Nanoscience and technology/Nanoscale materials/Two-dimensional materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Full Text Three-dimensional (3D) solid oxygen exists in several distinct phases under various temperature and pressure conditions 1,2 . 3D solid oxygen is one of the few simple molecular solids and exhibits the antiferromagnetic ordering in low-temperature phases due to the unpaired electrons in the O 2 molecules 3,4 . However, a 2D oxygen crystal has not yet been discovered. 2D materials composed of naturally abundant elements, such as graphene, have served as model systems for investigating exotic physical phenomena and have exhibited the potential for practical applications by exploiting their unique electric and magnetic properties 5-12 . In this respect, the discovery of a 2D oxygen crystal could significantly impact both scientific research and technological development. In this work, we experimentally realize a 2D oxygen crystal, termed “oxylene”, on an ultraflat Cu(111) surface. In a previous study, we demonstrated that an ultraflat Cu(111) surface with monatomic steps exhibits exceptional resistance to oxidation 13,14 (Extended Data Fig. 1). We find that this extreme oxidation resistance is decisive for the formation of a 2D oxygen crystal. At low temperature (80 K), oxygen atoms on the oxidation-resistant ultraflat Cu surface migrate to the most energetically favourable positions, eventually organizing into a periodic honeycomb lattice atop the Cu surface, forming a 2D oxygen crystal instead of copper oxide. Using scanning tunnelling microscopy (STM), density functional theory (DFT) calculations, and symmetry analysis based on multipole theory, we elucidate the lattice structure, as well as the electronic, and magnetic properties of oxylene. The honeycomb structure of oxylene exhibits insulating characteristics along with distinct magnetic properties, induced by strong correlations. Symmetry analyses indicate that oxylene is likely to exhibit a range of linear responses, including the magneto-optical Kerr effect, the magnetoelectric effect, the piezomagnetic effect, and the piezoelectric effect. Formation of oxylene on a flat Cu(111) surface The STM topography image (Fig. 1a) reveals the network of oxygen atoms on the ultraflat surface of a 100-nm-thick single-crystal Cu(111) thin film (SCCF). The image was captured after spraying 100 langmuirs (L) of oxygen onto an SCCF surface at 80 K. The highly ordered honeycomb structure of oxylene (Fig. 1a) is consistently observable across the entire 2-inch wafer when magnified at any point, as depicted in the STM image obtained at the 10 nm scale (Fig. 1b). The two different colours, yellow and brown in Fig. 1b, represent a one-atomic-layer thickness difference, and the straight lines forming 60° highlight the crystal quality and surface state of the SCCF used in this study. Given the oxygen atom’s diameter of ~1.5 Å, the distance between the bright spots (~5.9 Å; Fig. 1a) suggests that these spots are too far apart for direct contact between two individual oxygen atoms. Consequently, a more intricate structure is necessary to elucidate the high crystallinity of oxylene. Figure 1c schematically presents the structure model of oxylene, derived from the STM and DFT results. The bright triangular spots observed in Fig. 1a correspond to clusters of three oxygen atoms, forming a honeycomb lattice. Upon enlargement, three other oxygen atoms arrange around this trio, creating a hexagonal pattern resembling a waffle shape (Fig. 1d). These surrounding oxygen atoms are situated closer to the Cu surface than the central trio, which are elevated to a higher position. A more detailed structure will be provided below based on STM and DFT results. Clean surface of SCCF(111) and changes in the copper surface upon exposure to oxygen We here describe the exceptional resistance of SCCF(111) surface to oxidation. Figure 2 presents the morphology changes of an ultraflat SCCF(111) surface after exposure to 100 L of oxygen at room temperature (RT). Large-scale STM images of clean SCCF(111) surfaces show an atomically flat surface with triangular terraces aligned along the crystal axes, as shown in Fig. 2a, b (see also Extended Data Fig. 2). The atomic-resolution image shows a triangular lattice on the Cu(111) surface (Fig. 1c). The line profile in Fig. 2d, taken along the blue line in Fig. 2b, indicates a predominance of monatomic steps with a height of 2.1 Å for SCCF(111). Upon exposure to 100 L of oxygen at RT, the terrace structure of SCCF(111) undergoes noticeable changes while maintaining the monatomic step nature of the flat surface. In contrast to the triangular terraces shown in Fig. 2a and 2b, those in Fig. 2e and 2f display irregular terrace shapes, indicating that oxygen atoms alter the terrace structure at RT. However, as shown in Fig. 2g, taken from the box in Fig. 2f, the Cu(111) lattice remains clean and ultraflat in the region away from the terrace edges, with no evidence of copper oxide formation. Although oxygen atoms primarily decorate the step edges, the monoatomic step edges still prevent oxygen from penetrating, as depicted in the STM images (Fig. 2e,f) and the corresponding line profile (Fig. 2h, Extended Data Fig. 3). This observation aligns with a previous report of the exceptional oxidation resistance of ultraflat SCCF(111) surfaces, even at RT 6 . In the following section, we demonstrate that lowering the temperature to 80 K drastically alters the arrangement of oxygen atoms, leading to the formation of oxylene. Crystallographic structure of oxylene Upon exposure to 100 L of oxygen at a low temperature (80 K), a crystalline 2D oxygen structure emerges on the ultraflat SCCF(111) surface, exhibiting a honeycomb oxylene structure reminiscent of graphene, as shown in Fig. 3a (see also Fig. 1a and 1b). The unit cell of oxylene in Fig. 3a was determined by comparing it with the Cu(111) lattice (Fig. 3c) at the same scale. Fast Fourier transform (FFT) images of oxylene (Fig. 3b) and Cu(111) (Fig. 3d) reveal that their lattices (red hexagon of oxylene and blue hexagon of Cu(111)) are rotated by 30° relative to each other. The composite image (Fig. 3e) combining oxylene (Fig. 3a) and Cu(111) (Fig. 3c) clearly indicates that a single bright spot in oxylene covers the same area as six Cu atoms on the Cu(111) surface, indicating that each bright spot in oxylene corresponds to a cluster of oxygen atoms. Line profiles (Fig. 3f) along the red and blue lines in Fig. 3a and 3c confirm that the unit cell of oxylene exhibits a periodicity relative to the Cu(111) lattice. This uniform and stable structure on the macroscopic scale of the Cu(111) surface suggests that oxygen atoms are densely packed, nearly saturating the Cu(111) lattice (Extended Data Fig. 4). The structural model for oxylene was constructed from an exhaustive list of candidate structures that meet the essential requirements based on experimental observations using first-principles DFT calculations 15-17 . Our oxylene model (Fig. 3g and 3h) shows that there are 12 oxygen atoms in the unit cell: six are adsorbed at the fcc sites (blue triangle), whereas the other six at the hcp sites (green triangle). Within each group, three outer oxygen atoms are closely integrated with the Cu surface atoms (lower yellow O), whereas the three inner atoms are positioned higher (upper red O). Owing to the elevated positions, the STM images predominantly capture the tunnelling currents from the upper O atoms, rendering the honeycomb lattice structure (Fig. 3a). Electronic and magnetic properties of oxylene The 2D crystalline oxylene exhibits intriguing electronic and magnetic properties. Topography images of oxylene were obtained with tungsten (W) (Fig. 4a) and ferromagnetic nickel (Ni) tips (Fig. 4b) (see also Extended Data Fig. 5). In Fig. 4a, the honeycomb structure of oxylene shows two bright spots, but in Fig. 4b, where the Ni tip is sensitive to the sample's spin structure, the brightness (intensity) of one of these spots is noticeably suppressed, indicating the presence of magnetic structures in oxylene. Line profiles across the bright spots of oxylene (Fig. 4c and 4d), taken along the arrows in Figs. 4a and 4b, confirm that the oxylene lattice exhibits different magnetic properties at the hcp and fcc sites. Fig. 4e shows the perpendicular (to the Cu(111) surface) component of the magnetization density of oxylene, obtained from DFT calculations, which is consistent with the STM image obtained using the Ni tip (Fig. 4b). DFT results, combined with a theoretical analysis of magnetic multipoles 18 in oxylene (Supplementary Note 1), indicate that the magnetic moments around the upper O atoms are -like magnetic octupoles at the hcp site and -like magnetic octupoles at the fcc site, whereas those around the lower O atoms are -like magnetic octupoles (see Fig. 4f, 4g, 4h, and Extended Data Fig. 6). The net moments of the atomic magnetic octupoles at the upper O atoms are nearly compensated (see Extended Data Fig. 7), whereas the lower O atoms exhibit substantial net moments ( at the hcp sites and at the fcc sites; see Extended Data Fig. 7). The magnetic moments of the O atoms at the fcc sites are oriented in the film plane and cancel each other at the centre of the group, whereas those of the O atoms at the hcp sites are oriented out of the plane and show a finite value at the centre of the group. As a result, the integrated magnetic moment at the hcp sites (green dotted circle in Fig. 4e) has a net magnetic moment of , whereas those at the fcc sites (blue dotted circle in Fig. 4e) have less than . The significant difference in integrated magnetic moments between the hcp and fcc sites is consistent with the observed intensity changes in spin-sensitive STM images obtained with the Ni tip (Fig. 4b and Extended Data Fig. 5). This consistency allows us to experimentally identify the presence of magnetic multipoles in solids by real-space imaging. We next discuss the electric property of oxylene. Conventional band calculations suggest that oxylene is metallic. However, dI/dV spectroscopy results reveal that oxylene has a bandgap of approximately 0.65 eV (Fig. 4i). Band structure calculations using the DFT+U method 19 (with for O) confirm the insulating behaviour of oxylene (red line in Fig. 4j). These findings highlight the significant role of electron correlation in the electric and magnetic properties of oxylene. Based on these observations, oxylene could potentially be classified as a Mott insulator. We next discuss the macroscopic order parameter and linear responses that are allowed by symmetry in oxylene. The unit cell of oxylene comprises one hcp site and one fcc site, each hosting multiple O atoms with different atomic magnetic octupoles. Consequently, it is necessary to define a cluster multipole as the macroscopic order parameter that encompasses all atomic multipoles within the unit cell. Following established procedure 18,20 and focusing on the upper O layer, which primarily responds to external stimuli from the top surface, we find that the cluster multipole of oxylene is a cluster magnetic octupole (Supplementary Note 1). Symmetry-wise, the cluster magnetic octupole for the magnetic point group () of oxylene is equivalent to the magnetization in ferromagnetic states. In Supplementary Note 2, we demonstrate that the symmetry-allowed linear responses of oxylene include the anomalous Hall/Nernst effect, magneto-optical Kerr effect, magnetoelectric effect, magnetic spin Hall effect, and piezo-magnetic/electric effects. However, given the insulating characteristic of oxylene (Fig. 4i), it may not be feasible to experimentally observe the anomalous Hall/Nernst effect and magnetic spin Hall effect among the listed linear responses. It is noteworthy that most of the linear responses listed above have been experimentally identified in the noncollinear antiferromagnet Mn 3 Sn 21-26 , a magnetic Weyl semimetal that also exhibits macroscopic cluster magnetic octupole order 21 . Despite the difference in their magnetic point groups ( for oxylene and for Mn 3 Sn), the cluster magnetic octupole in oxylene allows for the exhibition of similar linear responses to those observed in Mn 3 Sn. The ultraflat surface of SCCF, with alternating hcp and fcc sites, provides a unique environment for the emergence of 2D oxygen crystal, oxylene. In oxylene, oxygen atoms are divided into two types at different elevations at hcp and fcc sites, exhibiting a unique arrangement with intriguing magnetic configurations and electron density distributions. This arrangement effectively precludes the formation of three-dimensional structures beyond a 2D monolayer, as the stable sites such as fcc/hcp offered by the Cu(111) surface are unavailable on the oxylene layer. Consequently, oxylene extends uniformly across the flat surface, leading to the formation of an unprecedented 2D structure. This discovery not only highlights the potential of ultraflat surfaces in 2D material synthesis but also opens new avenues for exploring two-dimensional materials with intriguing electric and magnetic properties. Online content Any methods, additional references, Nature Research reporting summaries, source data, statements of data availability and associated accession codes are available at https://doi.org/ Declarations Data availability statements The data that support the findings of this study are available from the corresponding authors upon reasonable request. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. NRF-RS-2024-00334854, NRF-2019R1A6A1A11053838, NRF-2021R1A5A1032937, NRF-2020R1A2C3013302) and by the Samsung Science and Technology Foundation under Project Number SRFC-MA2202-02. Computer time allocation has been provided by the US DOE INCITE program (DE-AC02-06CH11357) and National Science Foundation ACCESS program (NSF-2138296). Author contributions S.-Y.J., S.-G.K., and J.K. conceived and coordinated this work. G.D. and J.K. experimentally realized the oxylene. G.D., N.H.L., and J.K. conducted the STM analysis. S.-Y.J., S.J.K., and Y.L. prepared the SCCF samples. S.-Y.J., G.D., S.-G.K. and J.K. established the structure model of oxylene. B.R., and S.-G.K. performed first-principles calculations. Y.-M.K. conducted the TEM analysis. H.-C.K., H.-W.K. and K.-J.L. theoretically analysed magnetic multipoles and symmetry-allowed linear responses. S.-Y.J., S.-G.K., K.-J.L. and J.K. wrote the manuscript and all authors contributed to the discussion and analysis of the results. Competing interests The authors declare that there are no competing interests. Additional information Additional information Supplementary information The online version contains supplementary material available at https://doi.org/. Correspondence and requests for materials should be addressed to Peer review information Reprints and permissions information is available at http://www.nature.com/reprints References Desgreniers, S., Vohra, Y. K. & Ruoff, A. L. Optical response of very high density solid oxygen to 132 GPa. J. Phys. Chem. 94 , 1117–1122 (1990). Jordan, T. H., Streib, W. D., Smith, H. W. & Lipscomb, W. N. Single-crystal studies of β-F 2 and of γ-O 2 . Acta Crystallogr. 17 , 777-778 (1964). Meier, R. J. Magnetic interactions in solid oxygen. Phys. Lett. 95A , 115-117 (1983). Meier, R., Colpa, J. & Sigg, H. Far-infrared spectroscopy of solid α-oxygen in magnetic fields up to 15 tesla. J. Phys. C: Solid State Phys. 17 , 4501 (1984). Novoselov, K. S. et al. Electric Field Effect in Atomically Thin Carbon Films. Science 306 , 666-669 (2004). 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Piezomagnetic switching of the anomalous Hall effect in an antiferromagnet at room temperature. Nat. Phys. 18 , 1086 (2022). Kim, J. et al. Compact low temperature scanning tunneling microscope with in-situ sample preparation capability. Rev. Sci. Instrum. 86 , 093707 (2015). Kohn, W. & Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 140 , A1133-A1138 (1965). Methods Growth and STM observation of oxylene on a flat SCCF(111) surface Growth of oxylene crystals . The SCCF was cleaned by repeating Ar sputtering (0.5 kV) for 10 minutes and annealing at 450°C for 1 hour under ultrahigh vacuum (UHV) conditions at 1.2 × 10 -10 Torr. Exposure to oxygen was conducted in the STM chamber at a high-purity oxygen (99.998% purity) pressure of 1.0 × 10 -6 Torr while maintaining the sample temperature at 80 K for 100 seconds. Scanning tunnelling microscopy/spectroscopy measurements. STM/STS experiments were conducted via a custom-built low-temperature STM system under ultrahigh vacuum (base pressure ~ 1.2 × 10 -10 Torr) 27 . STM images were acquired using nonmagnetic tungsten (W) and ferromagnetic nickel (Ni) tips. The W and Ni tips were prepared by electrochemical etching in NaOH and KCl solutions. After being loaded into the UHV STM chamber, the tips were cleaned by electron beam heating. Topography images were acquired in constant current mode, with bias voltages applied to the sample. All the STM topography and spectroscopy measurements were performed at 78 K. Growth of an SCCF with an ultraflat surface . SCCF(111), known for its atomically flat surface, was chosen as the host material for growing oxylene. SCCF(111) growth was achieved via the atomic sputtering epitaxy (ASE) technique, which is a refined version of the general sputtering method. ASE allows the deposition of atoms one by one on the substrate, completely preventing cluster formation. Al 2 O 3 was used as the substrate for SCCF(111) growth, which has a lattice mismatch with Cu(111) of approximately 7%. However, the extended atomic distance mismatch (EADM) over a long periodicity is approximately 0.1%, enabling growth without grain boundaries or vacancies. The RMS surface roughness of the copper thin film grown in this study is approximately 0.2 nm, corresponding to the distance of one atomic layer. The ASE system incorporates three major improvements over conventional sputtering equipment. First, a single-crystal target was used instead of a polycrystalline target. Second, some of the conducting wires in the wiring network were replaced with single-crystal copper wires. Finally, a mechanical noise reduction system was installed to minimize mechanical vibrations from the surroundings. A double-sided polished (001) Al 2 O 3 wafer, 430 μm thick, was used as the substrate. The deposition temperature and RF (13.56 MHz) power were approximately 170°C and 30 W, respectively. The target-to-substrate distance was set at 95 mm. The base pressure was maintained at less than 2 × 10 −7 Torr, and the working pressure was 5.4 × 10 −3 Torr with an Ar gas (99.9999% (6 N)) flow of 50 sccm. The relationship between the deposition time and thin film thickness (or the average growth rate) was determined from the average deposition time of a 200-nm-thick film grown under optimal conditions. Theoretical calculations. All total-energy calculations and geometry optimizations were performed based on first-principles spin-polarized density functional theory 28 as implemented by Kresse and Joubert 17 using the projector augmented-wave method 16 . The exchange‒correlation functional was modelled using the generalized gradient approximation in the Perdew–Burke–Ernzerhof form 15 . All calculations were spin-polarized, and the positions of the atoms and the size and shape of the unit cell were fully relaxed to obtain the optimized lattice structure. All the atoms of the bulk Cu were fully relaxed until the force on the atom was less than and the change in total energy was less than to construct the Cu(111) substrate. The electron wavefunctions were expanded via a plane-wave basis set with a cut-off energy of 400 eV for both the bulk and slab calculations. The Cu(111) substrate had four monatomic layers of Cu atoms, with the bottom two layers maintained in their bulk positions. We maintained a 20 Å vacuum to prevent interactions between the periodic images. To account for the strong electron‒electron interactions and capture the Mott transition, we used the GGA+U method 19 with an onsite energy of U = 10 eV for oxygen. Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformationOxylene.docx Supplementary information ExtendeddataFig.docx Cite Share Download PDF Status: Under Review 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. 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University","correspondingAuthor":false,"prefix":"","firstName":"Seong-Gon","middleName":"","lastName":"Kim","suffix":""},{"id":346757939,"identity":"11e54336-4123-4e55-972b-864cfa9fcd82","order_by":11,"name":"Ganbat Duvjir","email":"","orcid":"","institution":"Department of Semiconductor Physics and Engineering, University of Ulsan","correspondingAuthor":false,"prefix":"","firstName":"Ganbat","middleName":"","lastName":"Duvjir","suffix":""}],"badges":[],"createdAt":"2024-08-19 17:35:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4940255/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4940255/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":64207676,"identity":"49f59cb0-dad2-4193-aff7-fc204b1a95c7","added_by":"auto","created_at":"2024-09-10 06:08:59","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2553241,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e2D oxygen crystal, oxylene, grown on a ultraflat Cu(111) surface. a, b, \u003c/strong\u003eSTM image of a 2D oxygen crystal (\u003cstrong\u003ea\u003c/strong\u003e) showing a structure isomorphic to that of graphene, and enlarged image of the boxed area in the STM image (\u003cstrong\u003eb\u003c/strong\u003e). The entire sample is uniformly covered with oxylene. \u003cstrong\u003ec,\u003c/strong\u003e Schematic image of oxylene showing that a bright spot in the STM image in (\u003cstrong\u003ea\u003c/strong\u003e) corresponds to a triangular arrangement of oxygen atoms. \u003cstrong\u003ed,\u003c/strong\u003e Enlarged image providing a more detailed view of the oxylene structure, revealing three additional oxygen atoms situated on the Cu surface around the triangle of oxygen atoms in (\u003cstrong\u003ec\u003c/strong\u003e). The central triangular oxygen atoms are elevated above the three outer oxygen atoms.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4940255/v1/b0b50dd8506330680645da09.png"},{"id":64207682,"identity":"6634928b-9933-45c1-95e9-844b1e2b4b2b","added_by":"auto","created_at":"2024-09-10 06:09:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":905163,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eComparison of the surface of SCCF(111) before and after oxygen exposure at room temperature. a, b,\u003c/strong\u003e Large-scale STM images of ultraflat Cu(111) films. \u003cstrong\u003ec,\u003c/strong\u003e Atomic-resolution image of the SCCF(111) surface (\u003cstrong\u003eb\u003c/strong\u003e). \u003cstrong\u003ed, \u003c/strong\u003eLine profile taken along the blue line in (\u003cstrong\u003eb\u003c/strong\u003e). \u003cstrong\u003ee, f, \u003c/strong\u003eLarge-scale STM images of Cu(111) after exposure to 100 L of oxygen at RT. \u003cstrong\u003eg,\u003c/strong\u003eAtomic-resolution image taken from the black box in (\u003cstrong\u003ef\u003c/strong\u003e). \u003cstrong\u003eh,\u003c/strong\u003e Line profile measured along the red line in (f).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4940255/v1/1c6b339df7aeb50db813b9f5.png"},{"id":64207683,"identity":"4a1b7546-6d45-48b5-9723-849d96b6c848","added_by":"auto","created_at":"2024-09-10 06:09:00","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1689597,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDetailed STM images and structural model of oxylene. a, b,\u003c/strong\u003eSTM image of oxylene (\u003cstrong\u003ea\u003c/strong\u003e), and corresponding fast Fourier transform(FFT) image (\u003cstrong\u003eb\u003c/strong\u003e). \u003cstrong\u003ec, d,\u003c/strong\u003eSTM image of pristine Cu(111) (\u003cstrong\u003ec\u003c/strong\u003e), and corresponding FFT image (\u003cstrong\u003ed\u003c/strong\u003e). The black and blue diamonds in \u003cstrong\u003ea\u003c/strong\u003e and \u003cstrong\u003ec\u003c/strong\u003e represent the unit cellsof oxylene and Cu(111), respectively. In the FFT images, the lattice peaks of oxylene and Cu(111) are marked with red and blue hexagons, respectively. \u003cstrong\u003ee,\u003c/strong\u003eComposite image combining oxylene (\u003cstrong\u003ea\u003c/strong\u003e) and Cu(111) (\u003cstrong\u003ec\u003c/strong\u003e). \u003cstrong\u003ef,\u003c/strong\u003e Lineprofiles taken along the red and blue lines in (\u003cstrong\u003ea\u003c/strong\u003e) and (\u003cstrong\u003ec\u003c/strong\u003e). \u003cstrong\u003eg, h,\u003c/strong\u003eTop (\u003cstrong\u003eg\u003c/strong\u003e) and side (\u003cstrong\u003eh\u003c/strong\u003e) views of the structuralmodel for oxylene on the SCCF(111) surface. In the side-view model (\u003cstrong\u003eh\u003c/strong\u003e), the inner three oxygen atoms (red) are positioned 1.45 Å above the three outer oxygen atoms (yellow). Clusters of six oxygen atoms at the fcc and hcp sites are represented by blue and green triangles, respectively.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4940255/v1/20b8d5d0702a2f20c0aaa3cc.png"},{"id":64207681,"identity":"bed6a432-9623-4515-af8e-1bc80baae41d","added_by":"auto","created_at":"2024-09-10 06:08:59","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1335122,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMagnetic ordering and band structure of oxylene. a, b,\u003c/strong\u003e STM images of oxylene obtained with tungsten (W) (a) and ferromagnetic nickel (Ni) (b) tips. c, d, Line profiles taken along the pink line in (a) and the green line in (b). e, Contour plot of the perpendicular component of the magnetization density of oxylene evaluated on the horizontal slice plane at the upper O atoms. f, Isosurface at ±0.115 μ\u003csub\u003eB\u003c/sub\u003e/Å\u003csup\u003e3\u003c/sup\u003e of the perpendicular component of the magnetization density in (e). Magenta (yellow) lobes have positive (negative) values. The contour plot is evaluated on the horizontal slice plane 0.15 Å below the upper O atoms. The view is tilted from the vertical direction to exhibit the 3D character of octupole of the atomic magnetic moments. g, Enlarged view of the boxed area in (f). h, Contour plot of the perpendicular component of the magnetization density of oxylene on the vertical slice plane along the dotted line in (e). i, j, dI/dV measurement and projected density of states (PDOS) of oxylene.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4940255/v1/a8bb5fc909e073ca914a80be.png"},{"id":64208084,"identity":"fc077ed1-7054-47b3-b998-2dad02d3b034","added_by":"auto","created_at":"2024-09-10 06:17:02","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":9275891,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4940255/v1/a1fe4896-80cf-4ba0-aee2-8fe0ede06479.pdf"},{"id":64207674,"identity":"315b2f81-a058-4b02-9a43-a2852323c1b2","added_by":"auto","created_at":"2024-09-10 06:08:58","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":103672,"visible":true,"origin":"","legend":"Supplementary information","description":"","filename":"SupplementaryInformationOxylene.docx","url":"https://assets-eu.researchsquare.com/files/rs-4940255/v1/99cb333a755dcf373b76fe0c.docx"},{"id":64207680,"identity":"18eb3aa5-1dc4-46a0-8ff2-fbabaa35da94","added_by":"auto","created_at":"2024-09-10 06:08:59","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":6374311,"visible":true,"origin":"","legend":"","description":"","filename":"ExtendeddataFig.docx","url":"https://assets-eu.researchsquare.com/files/rs-4940255/v1/1a71f606fc76d9c800048332.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Two-dimensional oxygen crystal with a honeycomb lattice on an ultraflat Cu(111)","fulltext":[{"header":"Full Text","content":"\u003cp\u003eThree-dimensional (3D) solid oxygen exists in several distinct phases under various temperature and pressure conditions\u003csup\u003e1,2\u003c/sup\u003e. 3D solid oxygen is one of the few simple molecular solids and exhibits the antiferromagnetic ordering in low-temperature phases due to the unpaired electrons in the O\u003csub\u003e2\u003c/sub\u003e molecules\u003csup\u003e3,4\u003c/sup\u003e. However, a 2D oxygen crystal has not yet been discovered. 2D materials composed of naturally abundant elements, such as graphene, have served as model systems for investigating exotic physical phenomena and have exhibited the potential for practical applications by exploiting their unique electric and magnetic properties\u003csup\u003e5-12\u003c/sup\u003e.\u0026nbsp;In this respect, the discovery of a 2D oxygen crystal could significantly impact both scientific research and technological development.\u003c/p\u003e\n\u003cp\u003eIn this work, we experimentally realize a 2D oxygen crystal, termed \u0026ldquo;oxylene\u0026rdquo;, on an ultraflat Cu(111) surface. In a previous study, we demonstrated that an ultraflat Cu(111) surface with monatomic steps exhibits exceptional resistance to oxidation\u003csup\u003e13,14\u003c/sup\u003e (Extended Data Fig. 1). We find that this extreme oxidation resistance is decisive for the formation of a 2D oxygen crystal. At low temperature (80 K), oxygen atoms on the oxidation-resistant ultraflat Cu surface migrate to the most energetically favourable positions, eventually organizing into a periodic honeycomb lattice atop the Cu surface, forming a 2D oxygen crystal instead of copper oxide. Using scanning tunnelling microscopy (STM), density functional theory (DFT) calculations, and symmetry analysis based on multipole theory, we elucidate the lattice structure, as well as the electronic, and magnetic properties of oxylene. The honeycomb structure of oxylene exhibits insulating characteristics along with distinct magnetic properties, induced by strong correlations. Symmetry analyses indicate that oxylene is likely to exhibit a range of linear responses, including the magneto-optical Kerr effect, the magnetoelectric effect, the piezomagnetic effect, and the piezoelectric effect.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFormation of oxylene on a flat Cu(111) surface\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe STM topography image (Fig. 1a) reveals the network of\u0026nbsp;oxygen atoms on the ultraflat surface of a 100-nm-thick single-crystal Cu(111) thin film (SCCF). The image was captured after spraying 100 langmuirs (L) of oxygen onto an SCCF surface at 80 K.\u0026nbsp;The highly ordered honeycomb structure of oxylene (Fig. 1a) is consistently observable across the entire 2-inch wafer when magnified at any point, as depicted in the STM image obtained\u0026nbsp;at the\u0026nbsp;10 nm scale (Fig. 1b). The two different\u0026nbsp;colours, yellow and brown in Fig. 1b, represent a one-atomic-layer thickness difference, and the straight lines forming 60\u0026deg;\u0026nbsp;highlight the crystal quality and surface state of\u0026nbsp;the\u0026nbsp;SCCF used in this study. Given the oxygen atom\u0026rsquo;s diameter of ~1.5 \u0026Aring;, the distance between the bright spots (~5.9 \u0026Aring;; Fig. 1a) suggests that these spots are too far apart for direct contact between two individual oxygen atoms. Consequently, a more intricate structure is necessary to elucidate the high crystallinity of oxylene. Figure 1c schematically presents the structure model of oxylene, derived from the STM and DFT results.\u0026nbsp;The bright\u0026nbsp;triangular spots observed in Fig. 1a correspond to clusters of three oxygen atoms, forming a honeycomb lattice. Upon enlargement, three other oxygen atoms arrange around this trio, creating a hexagonal pattern resembling a waffle shape (Fig. 1d). These surrounding oxygen atoms are situated closer to the Cu surface than the central trio, which are elevated to a higher position. A more detailed structure will be provided below based on STM and DFT results.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eClean surface of SCCF(111) and changes in\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ethe\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ecopper surface upon exposure to oxygen\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe here describe the exceptional resistance of SCCF(111) surface to oxidation. Figure 2 presents the morphology changes of an ultraflat SCCF(111) surface after exposure to 100 L\u0026nbsp;of\u0026nbsp;oxygen at room temperature (RT). Large-scale STM images of clean SCCF(111) surfaces show an atomically flat surface with triangular terraces aligned along the crystal axes, as shown in Fig. 2a, b (see also Extended Data Fig.\u0026nbsp;2). The atomic-resolution image shows a triangular lattice\u0026nbsp;on the\u0026nbsp;Cu(111) surface (Fig. 1c).\u0026nbsp;The line\u0026nbsp;profile in Fig. 2d, taken along the blue line in Fig. 2b, indicates a predominance of monatomic steps with\u0026nbsp;a\u0026nbsp;height of 2.1 \u0026Aring; for SCCF(111). Upon exposure to 100 L\u0026nbsp;of\u0026nbsp;oxygen at RT, the terrace structure of SCCF(111) undergoes noticeable changes while maintaining\u0026nbsp;the\u0026nbsp;monatomic step nature of the flat surface.\u0026nbsp;In contrast to the triangular terraces shown in Fig. 2a and 2b, those in Fig. 2e and 2f display irregular terrace shapes, indicating that oxygen atoms alter the terrace structure at RT. However, as shown in Fig. 2g, taken from the box in Fig. 2f, the Cu(111) lattice remains clean and ultraflat in the region away from the terrace edges, with no evidence of copper oxide formation.\u0026nbsp;Although oxygen atoms primarily decorate the step edges, the monoatomic step edges still prevent oxygen from penetrating, as depicted in the STM images (Fig. 2e,f) and the corresponding line profile (Fig. 2h, Extended Data Fig. 3). This observation aligns with a previous report of the exceptional oxidation resistance of ultraflat SCCF(111) surfaces, even at RT\u003csup\u003e6\u003c/sup\u003e. In the following section, we demonstrate that lowering the temperature to 80 K drastically alters the arrangement of oxygen atoms, leading to the formation of oxylene.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eCrystallographic structure of oxylene\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUpon exposure to 100 L\u0026nbsp;of\u0026nbsp;oxygen at a low temperature (80 K), a crystalline 2D oxygen structure emerges on the ultraflat SCCF(111) surface, exhibiting\u0026nbsp;a honeycomb\u0026nbsp;oxylene structure reminiscent of graphene, as shown in Fig. 3a (see also Fig. 1a and 1b). The unit cell of oxylene in Fig. 3a\u0026nbsp;was\u0026nbsp;determined by comparing it with\u0026nbsp;the\u0026nbsp;Cu(111) lattice (Fig. 3c) at the same scale. Fast Fourier\u0026nbsp;transform\u0026nbsp;(FFT) images of oxylene (Fig. 3b) and Cu(111) (Fig. 3d) reveal that their lattices (red hexagon of oxylene and blue hexagon of Cu(111)) are rotated by 30\u0026deg;\u0026nbsp;relative to each other. The composite image (Fig. 3e) combining oxylene (Fig. 3a) and Cu(111) (Fig. 3c) clearly indicates that a single bright spot in oxylene covers the same area as six Cu atoms on the Cu(111) surface, indicating that each bright spot in oxylene corresponds to a cluster of oxygen atoms.\u0026nbsp;Line profiles (Fig. 3f) along the red and blue lines in Fig. 3a and 3c confirm that the unit cell of oxylene exhibits a\u0026nbsp;\u0026nbsp;periodicity relative to the Cu(111) lattice. This uniform and stable structure on the macroscopic scale of the Cu(111) surface suggests that oxygen atoms are densely packed, nearly saturating the Cu(111) lattice (Extended Data Fig. 4). The\u0026nbsp;structural\u0026nbsp;model for oxylene was constructed from an exhaustive list of candidate structures that meet the essential requirements based on experimental observations using first-principles DFT calculations\u003csup\u003e15-17\u003c/sup\u003e. Our oxylene model (Fig. 3g and 3h) shows that there are 12 oxygen atoms in the\u0026nbsp;\u0026nbsp;unit cell: six are adsorbed at the fcc sites (blue triangle), whereas\u0026nbsp;the other six at the hcp sites (green triangle). Within each group, three outer oxygen atoms are closely integrated with the Cu surface atoms (lower yellow O), whereas\u0026nbsp;the three inner atoms are positioned higher (upper red O).\u0026nbsp;Owing\u0026nbsp;to the elevated positions, the STM images predominantly capture the tunnelling currents from the upper O\u0026nbsp;atoms, rendering the honeycomb lattice structure (Fig.\u0026nbsp;3a).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eElectronic and magnetic properties of oxylene\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe 2D crystalline oxylene exhibits intriguing electronic and magnetic properties. Topography images of oxylene were obtained with tungsten (W) (Fig. 4a) and ferromagnetic nickel (Ni) tips (Fig. 4b) (see also Extended Data Fig. 5). In Fig. 4a, the honeycomb structure of oxylene shows two bright spots, but in Fig. 4b, where the Ni tip is sensitive to the sample\u0026apos;s spin structure, the brightness (intensity) of one of these spots is noticeably suppressed, indicating the presence of magnetic structures in oxylene. Line profiles across the bright spots of oxylene (Fig. 4c and 4d), taken along the arrows in Figs. 4a and 4b, confirm that the oxylene lattice exhibits different magnetic properties at\u0026nbsp;the\u0026nbsp;hcp and fcc sites. Fig. 4e shows the perpendicular (to the Cu(111) surface) component of the magnetization density\u0026nbsp;of oxylene, obtained from DFT calculations, which\u0026nbsp;is consistent\u0026nbsp;with the STM image obtained using the Ni tip (Fig. 4b). DFT results, combined with a theoretical analysis of magnetic multipoles\u003csup\u003e18\u003c/sup\u003e in oxylene (Supplementary Note 1), indicate\u0026nbsp;that the magnetic moments around the upper O\u0026nbsp;atoms are\u0026nbsp;-like magnetic octupoles at the hcp site and\u0026nbsp;-like magnetic octupoles at the fcc site, whereas\u0026nbsp;those\u0026nbsp;around\u0026nbsp;the lower O\u0026nbsp;atoms are\u0026nbsp;-like magnetic octupoles (see Fig. 4f, 4g, 4h, and\u0026nbsp;Extended Data Fig.\u0026nbsp;6). The net moments of the atomic magnetic octupoles at the upper O atoms are nearly compensated (see\u0026nbsp;Extended Data Fig.\u0026nbsp;7), whereas\u0026nbsp;the lower O\u0026nbsp;atoms exhibit substantial net moments (\u0026nbsp;at the hcp sites and\u0026nbsp;\u0026nbsp;at the fcc sites;\u0026nbsp;see Extended Data Fig. 7). The magnetic moments of\u0026nbsp;the\u0026nbsp;O\u0026nbsp;atoms at the fcc sites are oriented in the film plane and cancel each other at the centre of the group, whereas\u0026nbsp;those of\u0026nbsp;the\u0026nbsp;O\u0026nbsp;atoms at the hcp sites are oriented out of the plane and show a finite value at the centre of the group. As a result, the integrated magnetic moment at the hcp sites (green dotted circle in Fig. 4e)\u0026nbsp;has a\u0026nbsp;net magnetic moment of\u0026nbsp;, whereas\u0026nbsp;those at the fcc sites\u0026nbsp;(blue dotted circle in Fig. 4e) have less than\u0026nbsp;. The significant difference\u0026nbsp;in\u0026nbsp;integrated magnetic moments between the hcp and fcc sites is consistent with the observed\u0026nbsp;intensity changes in spin-sensitive STM images obtained with\u0026nbsp;the\u0026nbsp;Ni tip (Fig. 4b and Extended Data Fig. 5). This consistency allows us to experimentally identify the presence of magnetic multipoles in solids by real-space imaging.\u003c/p\u003e\n\u003cp\u003eWe next discuss the electric property of oxylene. Conventional band calculations suggest that oxylene is metallic. However, dI/dV spectroscopy results reveal that oxylene has a bandgap of approximately 0.65 eV (Fig.\u0026nbsp;4i). Band structure calculations using the DFT+U method\u003csup\u003e19\u003c/sup\u003e (with\u0026nbsp;\u0026nbsp;for O) confirm the insulating behaviour of oxylene (red line in Fig. 4j). These findings highlight the significant role of electron correlation in the electric and magnetic properties of oxylene. Based on these observations, oxylene could potentially be classified as a Mott insulator.\u003c/p\u003e\n\u003cp\u003eWe next discuss the macroscopic order parameter and linear responses that are allowed by symmetry in oxylene. The unit cell of oxylene comprises one hcp site and one fcc site, each hosting multiple O atoms with different atomic magnetic octupoles. Consequently, it is necessary to define a cluster multipole as the macroscopic order parameter that encompasses all atomic multipoles within the unit cell. Following established procedure\u003csup\u003e18,20\u003c/sup\u003e and focusing on the upper O layer, which primarily responds to external stimuli from the top surface, we find that the cluster multipole of oxylene is a cluster magnetic octupole (Supplementary Note 1). Symmetry-wise, the cluster magnetic octupole for the magnetic point group () of oxylene is equivalent to the magnetization in ferromagnetic states. In Supplementary Note 2, we demonstrate that the symmetry-allowed linear responses of oxylene include the anomalous Hall/Nernst effect, magneto-optical Kerr effect, magnetoelectric effect, magnetic spin Hall effect, and piezo-magnetic/electric effects. However, given the insulating characteristic of oxylene (Fig. 4i), it may not be feasible to experimentally observe the anomalous Hall/Nernst effect and magnetic spin Hall effect among the listed linear responses. It is noteworthy that most of the linear responses listed above have been experimentally identified in the noncollinear antiferromagnet Mn\u003csub\u003e3\u003c/sub\u003eSn\u003csup\u003e21-26\u003c/sup\u003e, a magnetic Weyl semimetal that also exhibits macroscopic cluster magnetic octupole order\u003csup\u003e21\u003c/sup\u003e. Despite the difference in their magnetic point groups (\u0026nbsp;for oxylene and\u0026nbsp;\u0026nbsp;for Mn\u003csub\u003e3\u003c/sub\u003eSn), the cluster magnetic octupole in oxylene allows for the exhibition of similar linear responses to those observed in Mn\u003csub\u003e3\u003c/sub\u003eSn.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe ultraflat surface of SCCF, with alternating hcp and fcc sites, provides a unique environment for the emergence of 2D oxygen crystal, oxylene. In oxylene, oxygen atoms are divided into two types at different elevations at hcp and fcc sites, exhibiting a unique arrangement with intriguing magnetic configurations and electron density distributions. This arrangement effectively precludes the formation of three-dimensional structures beyond a 2D monolayer, as the stable sites such as fcc/hcp offered by the Cu(111) surface are unavailable on the oxylene layer. Consequently, oxylene extends uniformly across the flat surface, leading to the formation of an unprecedented 2D structure. This discovery not only highlights the potential of ultraflat surfaces in 2D material synthesis but also opens new avenues for exploring two-dimensional materials with intriguing electric and magnetic properties.\u0026nbsp;\u003c/p\u003e"},{"header":"Online content","content":"\u003cp\u003eAny methods, additional references, Nature Research reporting summaries, source data, statements of data availability and associated accession codes are available at https://doi.org/\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability statements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding authors upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by\u0026nbsp;the\u0026nbsp;Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of\u0026nbsp;Science, ICT \u0026amp; Future Planning (No.\u0026nbsp;NRF-RS-2024-00334854, NRF-2019R1A6A1A11053838, NRF-2021R1A5A1032937, NRF-2020R1A2C3013302) and by\u0026nbsp;the Samsung Science and Technology Foundation under Project Number SRFC-MA2202-02. Computer time allocation has been provided by the US DOE INCITE program (DE-AC02-06CH11357) and National Science Foundation ACCESS program (NSF-2138296). \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eS.-Y.J., S.-G.K., and J.K. conceived and coordinated this work. G.D. and J.K. experimentally realized the oxylene. G.D., N.H.L., and J.K. conducted the STM analysis. S.-Y.J., S.J.K., and Y.L. prepared the SCCF samples. S.-Y.J., G.D., S.-G.K. and J.K. established the structure model of oxylene. B.R., and S.-G.K. performed first-principles calculations. Y.-M.K. conducted the TEM analysis. H.-C.K., H.-W.K. and K.-J.L. theoretically analysed magnetic multipoles and symmetry-allowed linear responses. S.-Y.J., S.-G.K., K.-J.L. and J.K. wrote the manuscript and all authors contributed to the discussion and analysis of the results.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that there are no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u003c/strong\u003e The online version contains supplementary material available at https://doi.org/.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrespondence and requests for materials\u0026nbsp;\u003c/strong\u003eshould be addressed to\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePeer review information\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReprints and permissions information\u003c/strong\u003e is available at http://www.nature.com/reprints\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eDesgreniers, S., Vohra, Y. 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Rev.\u003c/em\u003e \u003cstrong\u003e140\u003c/strong\u003e, A1133-A1138 (1965). \u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eGrowth and STM observation of oxylene on\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ea\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eflat SCCF(111) surface\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGrowth of oxylene\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ecrystals\u003c/strong\u003e\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003eThe SCCF was cleaned by repeating Ar sputtering (0.5 kV) for 10 minutes and annealing at 450°C for 1 hour under\u0026nbsp;ultrahigh\u0026nbsp;vacuum (UHV) conditions at 1.2 × 10\u003csup\u003e-10\u003c/sup\u003e Torr. Exposure to oxygen was conducted in the STM chamber at a high-purity oxygen (99.998% purity) pressure of 1.0 × 10\u003csup\u003e-6\u003c/sup\u003e Torr while maintaining the sample temperature at 80 K for 100 seconds.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eScanning\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003etunnelling microscopy/spectroscopy\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;measurements.\u003c/strong\u003e STM/STS experiments were conducted\u0026nbsp;via\u0026nbsp;a\u0026nbsp;custom-built low-temperature STM system\u0026nbsp;under\u0026nbsp;ultrahigh vacuum (base pressure ~ 1.2 × 10\u003csup\u003e-10\u003c/sup\u003e Torr)\u003csup\u003e\u0026nbsp;27\u003c/sup\u003e. STM images were acquired using\u0026nbsp;nonmagnetic\u0026nbsp;tungsten (W) and ferromagnetic nickel (Ni) tips. The W and Ni tips were prepared by electrochemical etching in NaOH and KCl solutions. After being loaded into the UHV STM chamber, the tips were cleaned by electron beam heating. Topography images were acquired in constant current mode, with bias voltages applied to the sample. All\u0026nbsp;the\u0026nbsp;STM topography and spectroscopy measurements were\u0026nbsp;performed\u0026nbsp;at 78 K.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGrowth of an SCCF with an ultraflat surface\u003c/strong\u003e\u003cstrong\u003e.\u003c/strong\u003e SCCF(111), known for its atomically flat surface, was chosen as the host material for growing oxylene. SCCF(111) growth was achieved\u0026nbsp;via\u0026nbsp;the atomic sputtering epitaxy (ASE) technique, which is a refined version of the general sputtering method. ASE allows the deposition of atoms one by one on the substrate, completely preventing cluster formation. Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e was used as the substrate for SCCF(111) growth, which has a lattice mismatch with Cu(111) of approximately 7%. However, the extended atomic distance mismatch (EADM) over a long periodicity is\u0026nbsp;approximately\u0026nbsp;0.1%, enabling growth without grain boundaries or vacancies. The RMS surface roughness of the copper thin film grown in this study is\u0026nbsp;approximately\u0026nbsp;0.2 nm, corresponding to the distance of one atomic layer. The ASE system incorporates three major improvements over conventional sputtering equipment.\u0026nbsp;First, a single-crystal target was used instead of a polycrystalline\u0026nbsp;target. Second, some\u0026nbsp;of the conducting wires in the wiring network\u0026nbsp;were\u0026nbsp;replaced with single-crystal copper wires.\u0026nbsp;Finally, a mechanical noise reduction system was installed to minimize mechanical vibrations from the surroundings.\u003c/p\u003e\n\u003cp\u003eA double-sided polished (001) Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e wafer, 430 μm thick, was used as the substrate. The deposition temperature and RF (13.56 MHz) power were approximately 170°C and 30 W, respectively. The target-to-substrate distance was set at 95 mm. The base pressure was maintained at less than 2 × 10\u003csup\u003e−7\u003c/sup\u003e Torr, and the working pressure was 5.4 × 10\u003csup\u003e−3\u003c/sup\u003e Torr with an Ar gas (99.9999% (6 N)) flow of 50 sccm. The relationship between the deposition time and thin film thickness (or the average growth rate) was determined from the average deposition time of a 200-nm-thick film grown under optimal conditions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTheoretical calculations.\u003c/strong\u003e All total-energy calculations and geometry optimizations were performed based on first-principles spin-polarized density functional theory\u003csup\u003e28\u003c/sup\u003e as implemented by Kresse and Joubert\u003csup\u003e17\u003c/sup\u003e using the projector augmented-wave method\u003csup\u003e16\u003c/sup\u003e. The\u0026nbsp;exchange‒correlation\u0026nbsp;functional was modelled using the generalized gradient approximation in the Perdew–Burke–Ernzerhof form\u003csup\u003e15\u003c/sup\u003e. All calculations were spin-polarized, and the positions of\u0026nbsp;the\u0026nbsp;atoms and the size and shape of the unit cell were fully relaxed to obtain the optimized lattice structure. All\u0026nbsp;the\u0026nbsp;atoms of\u0026nbsp;the\u0026nbsp;bulk\u0026nbsp;Cu\u0026nbsp;were fully relaxed until the force on the atom was less than\u0026nbsp;\u0026nbsp;and the change in total energy was less than\u0026nbsp;\u0026nbsp;to construct\u0026nbsp;the\u0026nbsp;Cu(111) substrate.\u0026nbsp;The electron wavefunctions were expanded\u0026nbsp;via\u0026nbsp;a plane-wave basis set with a cut-off energy of 400 eV for both\u0026nbsp;the\u0026nbsp;bulk and slab calculations. The Cu(111) substrate had four monatomic layers of Cu atoms, with the bottom two layers maintained in their bulk positions. We maintained a 20 Å vacuum to prevent interactions between the periodic images. To account for the strong\u0026nbsp;electron‒electron\u0026nbsp;interactions and capture the Mott transition, we used the GGA+U method\u003csup\u003e19\u003c/sup\u003e with an onsite energy of U = 10 eV for oxygen.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4940255/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4940255/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eTwo-dimensional (2D) materials have served as key platforms for exploring novel phenomena and innovative applications, driven by their reduced dimensionality. The existing library of 2D materials includes carbon-based and transition metal-based systems. However, oxygen-based 2D materials are missing. Oxygen, the most abundant element on Earth with a higher electronegativity than carbon, holds the potential to introduce unprecedented functionalities in 2D materials. Here we report the experimental realization of a 2D oxygen crystal with a highly ordered honeycomb structure, termed “oxylene”, formed on an ultraflat Cu(111) surface with exceptional resistance to oxidation. Scanning tunneling microscopy and density functional theory studies reveal that oxylene consists of oxygen atom clusters occupying both face-centered cubic (fcc) and hexagonal closed-packed (hcp) sites with a 4×4 periodicity on the Cu(111) surface. Furthermore, oxylene exhibits distinct magnetic ordering due to the magnetic octupole moments of oxygen atoms. Symmetry analysis suggests that oxylene exhibits linear responses that are observed in the noncollinear antiferromagnetic Weyl semimetal Mn\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eSn. Our discovery of oxylene provides intriguing opportunities for exploiting its unique electric and magnetic properties, as well as harnessing oxygen’s high reactivity in 2D form.\u003c/strong\u003e\u003c/p\u003e","manuscriptTitle":"Two-dimensional oxygen crystal with a honeycomb lattice on an ultraflat Cu(111)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-10 06:08:53","doi":"10.21203/rs.3.rs-4940255/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-materials","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nmat","sideBox":"Learn more about [Nature Materials](http://www.nature.com/nmat/)","snPcode":"","submissionUrl":"","title":"Nature Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"43487e09-4578-47de-a4a3-0ca77f83af20","owner":[],"postedDate":"September 10th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":36773130,"name":"Physical sciences/Physics/Condensed-matter physics/Surfaces, interfaces and thin films"},{"id":36773131,"name":"Physical sciences/Materials science/Nanoscale materials/Magnetic properties and materials"},{"id":36773132,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials/Two-dimensional materials"}],"tags":[],"updatedAt":"2024-09-10T06:08:54+00:00","versionOfRecord":[],"versionCreatedAt":"2024-09-10 06:08:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4940255","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4940255","identity":"rs-4940255","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}