IR-Excited Raman Spectroscopy on HTc Superconductors

Abstract

The Raman scattering study of high critical temperature superconductors, shortly after their discovery, has played a significant role in characterizing materials and in investigating such low energy excitations as phonons, electronic or magnetic transitions and their interactions. In spite of some limitation of Raman scattering (the first order transitions are limited to the Γ point of the Brillouin zone) compared with other spectroscopic techniques, like neutron scattering, the high accuracy and resolution that can be achieved allow detailed study of the lineshape of the phonon bands, their interaction with low energy electronic excitations, and also the detection of some minor changes that can be determined by doping or temperature changes. The strong intensity dependence of the Raman scattering on laser excitation frequency can provide information on the nature and the interband transitions and/or, because of theresonantly enhanced scattering, on the microstructure of the sample. In this sense it is very important to study the Raman scattering in as wide range of excitation energy as possible. In this paper we report the Raman scattering in Ba2u3exciting at 1.16 eV and compare the results, obtained at different oxygen content, with different oxygen isotopes and different annealing procedures with those obtained by visible excitation. In particular we will discuss the peculiar behaviour of the apex O(4) oxygen atom in this system, showing the existence of an anharmonic potential associated to this atom, the evidence for the existence of electronic levels, related to the O(4), at the Fermi energy and the relevance of these observations on the formation of the superconducting state.