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Spin gap and magnetic coherence in a clean high-temperature superconductor

Abstract

A notable aspect of high-temperature superconductivity in the copper oxides is the unconventional nature of the underlying paired-electron state. A direct manifestation of the unconventional state is a pairing energy—that is, the energy required to remove one electron from the superconductor—that varies (between zero and a maximum value) as a function of momentum, or wavevector1,2: the pairing energy for conventional superconductors is wavevector-independent3,4. The wavefunction describing the superconducting state will include the pairing not only of charges, but also of the spins of the paired charges. Each pair is usually in the form of a spin singlet5, so there will also be a pairing energy associated with transforming the spin singlet into the higher-energy spin triplet form without necessarily unbinding the charges. Here we use inelastic neutron scattering to determine thewavevector-dependence of spin pairing in La2−xSrxCuO4, the simplest high-temperature superconductor. We find that the spin pairing energy (or ‘spin gap’) is wavevector independent, even though superconductivity significantly alters the wavevector dependence of the spin fluctuations at higher energies.

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Figure 1: The reciprocal-space regions over which measurements were made and the resulting data as a function of wavevector and energy transfer.
Figure 2: Constant-E scans at various energies through the incommensurate peaks at Tc and in the superconducting phase at 5 K.
Figure 3: Spectra at various wavevectors, and the Q-dependence of the inverse lifetime and susceptibility extracted by fitting such profiles.
Figure 4: The wavevector dependence of the spin gap in the superconducting state at 5 K.

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Acknowledgements

We thank K. N. Clausen for help and support during the experiments, and B.Batlogg, G. Boebinger, V. Emery, S. Kivelson, H. Mook, D. Morr, D. Pines, Z.-X. Shen, C.-C. Tsuei and J. Zaanen for discussions. Work done at the University of Toronto was sponsored by the Natural Sciences and Engineering Research Council and the Canadian Institute for Advanced Research, while work done at Oak Ridge was supported by the US DOE. T.E.M. was supported by the Alfred P. Sloan Foundation, and A.S. was supported by the TMR program.

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Correspondence to B. Lake.

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Lake, B., Aeppli, G., Mason, T. et al. Spin gap and magnetic coherence in a clean high-temperature superconductor. Nature 400, 43–46 (1999). https://doi.org/10.1038/21840

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