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Two Ising-like magnetic excitations in a single-layer cuprate superconductor

Abstract

There exists increasing evidence that the phase diagram of the high-transition temperature (Tc) cuprate superconductors is controlled by a quantum critical point. According to one distinct theoretical proposal, on decreasing the hole-carrier concentration a transition occurs to an ordered state with two circulating orbital currents per CuO2 square. Below the ‘pseudogap’ temperature T*(T*>Tc), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds which should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa2CuO4+δ the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points towards a unifying picture of magnetism in the cuprates that will probably require a multi-band description.

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Figure 1: Observation of two excitation branches.
Figure 2: Doping and momentum dependence of intensity.
Figure 3: Temperature dependence.
Figure 4: Magnetic origin verified by spin-polarized measurements.
Figure 5: Mixing between pseudogap excitations and antiferromagnetic spin fluctuations.

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Acknowledgements

We thank C. M. Varma, A. R. Bishop and B. Keimer for stimulating discussions. This research was supported by the US Department of Energy, Office of Basic Energy Sciences. X.Z. acknowledges support by the National Natural Science Foundation, China. Y. Li acknowledges support from the Alexander von Humboldt Foundation.

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M.G., P.B. and Y.L. planned the project. Y.L., G.Y., M.K.C., V.B. and Yangmu L. performed the neutron scattering experiments. Y.L., N.B. and X.Z. characterized and prepared the samples. P.S., R.A.M., K.H., Y.S. and P.B. were local contacts for the neutron scattering experiments. Y.L. and M.G. analysed the data and wrote the manuscript with input from all coauthors.

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Correspondence to M. Greven.

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Li, Y., Yu, G., Chan, M. et al. Two Ising-like magnetic excitations in a single-layer cuprate superconductor. Nature Phys 8, 404–410 (2012). https://doi.org/10.1038/nphys2271

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