Optical phonons and electron-phonon interactions in high-Tc materials as probed by Raman scattering

Abstract

Raman scattering has played two roles in high Tc superconductivity studies: characterization and fundamental physical properties. After one has separated intrinsic spectra from those due to impurity phases, the physical properties of the intrinsic superconducting material may then be studied. In single crystals of YBa2Cu3O7−δ, the most interesting Raman-active mode is one at 330 cm−1 due to O atoms in the “planes.” This mode has nearly pure B1g tetragonal symmetry, even in the orthorhombic phase, and it shows strong softening below Tc. In detail, its Raman spectra reveal a Fano antiresonance with a continuum [Cooper et al., Phys. Rev. B 37, 5920–5923 (1988)] of the same symmetry. An Ag mode at 118 cm−1 assigned primarily to Ba vibrations shows an even stronger antiresonance, but little softening. The continuum responsible for its antiresonance has Ag symmetry and a somewhat different shape from that of the B1g continuum, but both Raman continua extend all the way to zero Raman frequency shift, implying linear low-frequency response. For T≪Tc, the linear response is still present with five times reduced intensity, implying some form of gaplessness to the superconducting state. Efforts to rule out spurious surface effects will be described, and comparisons with results on materials such as Bi2Sr2Ca1Cu2O8 + δ will be made. [Work supported by NSF under DMR 8715103 and DMR 8612860.]