Abstract
The surfaces of strongly correlated materials can exhibit exotic properties that differ substantially from the bulk. Investigation of these surface effects requires both high quality surfaces, and an experimental technique capable of disentangling surface and underneath layers or bulk. Herein, we overcome these two challenges, using newly developed grazing-incident-angle resonant soft x-ray and magnetic scattering techniques to study ultrathin films of Spin(S) = 1/2 La2CuO4, the parent compound of high-critical-temperature (high-Tc) superconductor. We observe emergent surface magnetic ordering and surface-like electronic dipole layers at room temperature. As a function of temperature, intrinsic holes from Cu2+ and O2- ions are mitigated from the surface into subsurface layers and back into the surface. The intrinsic hole and oxygen ion mitigation have different rates, yielding strong temperature hysteresis. Supported with theoretical calculations, these surface orderings consist of a mixture of Cu2+ 3d9 (S = 1/2) and Cu1+ 3d10-like (S = 0) with strong magneto-electronic coupling through the many-body upper Hubbard band. Our results highlight intrinsic holes and oxygen ion mitigations, yielding surface-like electronic dipole and magnetic orderings and the potential of grazing-incident-angle resonant soft X-ray and magnetic scattering to elucidate, layer-by-layer, magnetic and electronic structures.”
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Acknowledgements
The authors thank Jason Chee Wai Lim for technical support. This work was supported by the Ministry of Education of Singapore (MOE) AcRF Tier-2 (T2EP50220-0041 and T2EP50122-0028), MOE AcRF Tier-1 (R-144-000-439-114), NRF-NUS Resilience and Growth Postdoctoral Fellowships (R-144-000-455-281 and R-144-000-459-281), and NUS Core Support (C-380-003-003-001). The authors also thank the Singapore Synchrotron Light Source (SSLS) for providing the facility necessary for conducting the research. SSLS is a National Research Infrastructure under the Singapore National Research Foundation.
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B.L.O., E.S.T., and A.R. grew samples and performed in-situ RHEED and AFM measurements. A.J., C.D., and A.R. performed RSXMS, RSXS, and XAS measurements. B.L.O., E.S.T., M.S., and A.R. performed XRD measurements. A.J., E.S.T., and A.R. analyzed data comprehensively and wrote the manuscript with inputs from C.D., B.L.O., A.D.F., K.J., M.A.N., S.P.J., X.C., T.J.W., and M.B.H.B. A.R. proposed the idea, designed GA-RSMXS, GA-RSXS, and integrated UHV-MBE-PLD with in-situ RHEED and led the project. All authors have approved the final version of the manuscript.
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Jain, A., Diao, C., Ong, B.L. et al. Emergent surface magnetic ordering and surface electronic dipole layers in a two-dimensional spin=1/2 La2CuO4 film. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69457-2
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DOI: https://doi.org/10.1038/s41467-026-69457-2
