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Asymmetry of collective excitations in electron- and hole-doped cuprate superconductors

Abstract

High-temperature superconductivity emerges on doping holes or electrons into antiferromagnetic copper oxides. The large energy scale of magnetic excitations, for example, compared with phonon energies, is thought to drive superconductivity with high transition temperatures (Tc). Comparing high-energy magnetic excitations of hole- and electron-doped superconductors provides an opportunity to test this hypothesis. Here, we use resonant inelastic X-ray scattering at the Cu L3-edge to reveal collective excitations in the electron-doped cuprate Nd2−xCexCuO4. Surprisingly, magnetic excitations harden significantly across the antiferromagnetic high-temperature superconductivity phase boundary despite short-ranged antiferromagnetic correlations, in contrast to the hole-doped cuprates. Furthermore, we find an unexpected branch of collective modes in superconducting compounds, absent in hole-doped cuprates. These modes emanate from the zone centre and possess a higher temperature scale than Tc, signalling a distinct quantum phase. Despite their differences, the persistence of magnetic excitations and the existence of a distinct quantum phase are apparently universal in both hole- and electron-doped cuprates.

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Figure 1: Magnons in antiferromagnetic NCCO (x = 0.04).
Figure 2: Paramagnons and unexpected collective modes in superconducting NCCO (x = 0.147).
Figure 3: Hardening of magnetic excitations.
Figure 4: Doping and temperature dependence of the collective modes.
Figure 5: Distinct doping evolution of the collective excitations in electron- and hole-doped cuprates.

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Acknowledgements

The authors appreciate Y. Lu’s support in characterizing the doping levels of the measured samples. This work was supported by the US Department of Energy, Office of Basic Energy Science, Division of Materials Science and Engineering under contract no. DE-AC02-76SF00515. The work at University of Minnesota was supported by the NSF and the NSF MRSEC program. S.G. acknowledges support from the Swiss NSF (Contract No. P2EZP2_148737). The authors appreciate the experimental support from the ADRESS beamline of the Swiss Light Source (SLS) at the Paul Scherrer Institut, Switzerland and the beamline BL05A1 at the National Synchrotron Radiation Research Center (NSRRC), Taiwan.

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Contributions

W.S.L. conceived and designed the experiments with suggestions from Z.X.S., M.G., T.P.D. and T.S.; W.S.L., J.J.L., W.T., S.G., S.W.H., Y.B.H., V.N.S. and T.S. performed the measurement at the SLS; W.S.L., H.Y.H., R.P.W., W.B.W., C.T.C. and D.J.H. performed the measurement at the NSRRC. E.M.M., G.Y. and M.G. synthesized and prepared the single-crystals used for the measurements. E.A.N., B.M. and T.P.D. performed the theoretical calculations. W.S.L. wrote the manuscript with contributions from all authors.

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Correspondence to W. S. Lee, Z. X. Shen or T. P. Devereaux.

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Lee, W., Lee, J., Nowadnick, E. et al. Asymmetry of collective excitations in electron- and hole-doped cuprate superconductors. Nature Phys 10, 883–889 (2014). https://doi.org/10.1038/nphys3117

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