Model High-Temperature Superconductor HgBa2CuO4+d Studied by
Neutron, Muon and Photon Spectroscopies
Yuan Li, Peking University
Due to its simple crystal structure and high superconducting critical temperature,
HgBa2CuO4+d (Hg1201) is considered a model cuprate superconductor ideal for
fundamental research purposes. The production of high-quality single crystals of
Hg1201 has enabled a series of spectroscopic experiments leading to new findings
possibly related to the superconducting mechanism. In this talk, I will present some of
the results under the following topics:
a) Spin-polarized neutron scattering experiments have revealed a magnetic
diffraction signal that indicates a time-reversal-symmetry-broken state with preserved
translational symmetry below the pseudogap temperature T*. This signal has now
been observed in several hole-doped cuprates and therefore appears to be universal,
but static magnetic moments have not been observed by local probes including NMR
and muSR. The discrepancy may be attributed to the very different time scales of the
experimental probes.
b) Using inelastic neutron scattering, we have observed magnetic excitations
near the antiferromagnetic wave vector (pi,pi), including a resonant mode at energies
below twice the superconducting gap and a commensurate low-energy response
indicative of lack of spin-stripe fluctuations in Hg1201. In addition we have found
disperseless modes possibly related to the unusual magnetism in the pseudogap state.
c) Using Raman scattering, we have found that the high-energy two-magnon
excitations exhibit a pronounced feedback effect related to the formation of Cooper
pairs. The two-magnon signal is furthermore demonstrated to be strongly affected by
photon scattering resonant effects, the condition of which changes with doping and
reconciles a discrepancy between Raman and X-ray observation of spin excitations.
(E-mail address: yuan.li@pku.edu.cn)