Raman Scattering Studies of High-Tc Superconductors

Doktorsavhandling, 1995

This thesis describes Raman scattering investigations of cuprate high-Tc superconductors. The objective is to gain further knowledge on the physical properties that are relevant for the appearance of high-Tc superconductivity. Most studies have concerned even symmetry k ~ 0 phonons, and how these phonons are affected by the crystal structure, chemical composition, temperature, pressure and the interaction with other elementary excitations. These studies are complemented by investigations of the Raman active electronic excitation spectrum. Particular attention is given to the effects of various atomic substitutions. The variation of the phonon spectra of Tl2(Ba1-ySry)2Can-1CunO2n+1 (n=1,2) and YBa2(Cu1-yMy)3O7 (M=Co, Fe, Al) with composition y have revealed internal charge redistribution effects that can be linked to the rapid suppression of superconductivity for these substitutions. In contrast, in the case of Pr substitution for Y in (Y1-xPrx)Ba4Cu6+nO14+n (n=1,2) the decrease of Tc is not accompanied by any particular peculiarity of the phonons that can be interpreted as charge transfer or valence changes. Instead, the variation in phonon parameters with x can be understood from the change in internal bond-lengths, as obtained by refinement of neutron diffraction data. For pure YBa2Cu4O8 it is found that the increase in Tc under pressure correlates with the pressure dependence of a certain phonon frequency. This indicates that the Tc enhancement is due to a pressure induced charge transfer effect. The temperature dependencies of the phonon frequencies and line-widths in YBa4Cu6+nO14+n (n=1,2) and YBa2(Cu1-yZny)4O8 exhibit discontinuities both at Tc and around 150 K. The phonon renormalization at ~150 K can be correlated to the opening of a spin-gap that occurs exclusively in superconductors with short-range anti-ferromagnetic correlations. The effects around Tc can, however, not be explained within traditional theories of superconductivity induced phonon self-energy effects. The temperature dependence of low-frequency electronic excitations has been investigated in Co and Zn substituted YBa2Cu3O7 thin films and single crystals. For A1g symmetry the electronic excitation spectrum redistributes below Tc, in a manner that is consistent with the opening of a superconducting gap. The characteristic energy of the gap-like feature decreases in energy with increasing substitution and scales approximately with Tc, indicating a superconducting gap energy of 2.DELTA./kBTc ~ 5.0. In contrast, there is no such simple scaling for the B1g symmetry scattering, which indicates that the electronic excitation spectrum in this symmetry is not only due to quasi-particle excitations across a superconducting gap.