Raman Scattering Studies of High-Tc Superconductors

Abstract: This thesis describes Raman scattering investigations of cuprate high-T_c superconductors. The objective is to gain further knowledge on the physical properties that are relevant for the appearance of high-T_c 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(Ba_1-ySr_y)₂Ca_n-1Cu_nO_2n+1 (n=1,2) and YBa₂(Cu_1-yM_y)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 (Y_1-xPr_x)Ba₄Cu_6+nO_14+n (n=1,2) the decrease of T_c 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 T_c under pressure correlates with the pressure dependence of a certain phonon frequency. This indicates that the T_c enhancement is due to a pressure induced charge transfer effect.

The temperature dependencies of the phonon frequencies and line-widths in YBa₄Cu_6+nO_14+n (n=1,2) and YBa₂(Cu_1-yZn_y)₄O₈ exhibit discontinuities both at T_c 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 T_c 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 A_1g symmetry the electronic excitation spectrum redistributes below T_c, 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 T_c, indicating a superconducting gap energy of 2.DELTA./k_BT_c ~ 5.0. In contrast, there is no such simple scaling for the B_1g symmetry scattering, which indicates that the electronic excitation spectrum in this symmetry is not only due to quasi-particle excitations across a superconducting gap.