Electron spin resonance and infrared studies of Cu2+sire symmetry in the high-Tcsuperconductor YBa2Cu3O7-x

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Abstract
The author reports detailed electron spin resonance (ESR) studies (300-77 K) of the high-temperature superconductor YBa2Cu3O7-x for samples (type A) have a broad transition (Tc0-Tconset=13.5 K) as well as for samples (type B) with a sharp transition (Tc0-Tconset=3 K). The room-temperature axial symmetry around the Cu2+ ion in both cases is maintained down to Tc0+or-2 K (Tc0 equals 86.5 K for type A samples and 96 K for type B samples) below which the symmetry changes gradually to orthorhombic at 77 K. In type B samples the orthorhombicity in the g tensor is larger than in type A samples. When the sample undergoes as many as five repeated cooling and heating cycles (within a week for case II), the superconducting transition becomes broader with Tconset=96 K and Tc0 is correlated with the superconducting properties. The infrared spectrum of case II is significantly different from that of case I. The EST linewidth in both cases (types A and B) is almost temperature independent; this is a rather surprising result for the ESR of Cu complexes. The nearly linear variation in ESR signal intensity IS with temperature in type A samples shows the possible existence of Curie susceptibility whereas in type B samples the weak dependence of IS on T may indicate possible Pauli paramagnetism.