Abstract
Cuprates and other high-temperature superconductors consist of two-dimensional layers that are crucial to their properties. The dynamics of the quantum spins in these layers lie at the heart of the mystery of the cuprates1,2,3,4,5,6,7. In bulk cuprates such as La2CuO4, the presence of a weak coupling between the two-dimensional layers stabilizes a three-dimensional magnetic order up to high temperatures. In a truly two-dimensional system however, thermal spin fluctuations melt long-range order at any finite temperature8. Here, we measure the spin response of isolated layers of La2CuO4 that are only one-unit-cell-thick. We show that coherent magnetic excitations, magnons, known from the bulk order, persist even in a single layer of La2CuO4, with no evidence for more complex correlations such as resonating valence bond correlations9,10,11. These magnons are, therefore, well described by spin-wave theory (SWT). On the other hand, we also observe a high-energy magnetic continuum in the isotropic magnetic response that is not well described by two-magnon SWT, or indeed any existing theories.
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Acknowledgements
We thank R. Konik, M. Haverkort, A. Boothroyd and G. Luke for fruitful discussions, X. Liu for assistance with the sample characterization and S. Hayden and R. Coldea for sharing their data in ref. 6. The experiment was performed at the ADRESS beamline of the Swiss Light Source using the SAXES instrument jointly built by the Paul Scherrer Institut, Switzerland and the Politecnico di Milano, Italy. We acknowledge V. Strocov for support at the ADRESS beamline and A. Suter and T. Prokscha for their assistance with the muon spin rotation measurements. Work at Brookhaven National Laboratory was supported by the Office of Basic Energy Sciences, Division of Materials Science and Engineering, US Department of Energy under Award No. DEAC02-98CH10886. M.P.M.D. and J.P.H. are supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Basic Energy Sciences. C.M., K.J.Z and T.S. acknowledge support from the Swiss National Science Foundation and its NCCR MaNEP.
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Experiment: M.P.M.D., J.P.H., R.S.S., C.M., K.J.Z. and T.S.; sample growth: I.B.; sample characterization: I.B., M.P.M.D., J.P., R.S.S. and E.M.; two-magnon calculations: B.D.P. and H.M.R.; data analysis and interpretation: M.P.M.D., J.P.H., J.v.d.B., T.S., C.M., K.J.Z. and H.M.R.; project planning: J.P.H., M.P.M.D., T.S. and I.B.; paper writing: M.P.M.D. and J.P.H., with contributions from all authors.
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Dean, M., Springell, R., Monney, C. et al. Spin excitations in a single La2CuO4 layer. Nature Mater 11, 850–854 (2012). https://doi.org/10.1038/nmat3409
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DOI: https://doi.org/10.1038/nmat3409
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