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Abstract:
We report measurements, between helium and room temperature, of the thermal expansion α(T), sound velocitiesv L (T) andv T (v) and specific heatC(T) of YBa2Cu3O7 samples, all cut from the same sintered pellet. A linear term in α(T) is resolved at low temperatures. It is compared with a corresponding term in the low temperature specific heat and, using the bulk modulus from our sound propagation measurements, a Grüneisen parameter of usual size (1.2±0.4) is derived-in conformity with the possible existence of non-superconducting carriers well belowT c . Longtime drift effects in the length of the sample between 30 K and 60 K point to the vicinity of a structural instability. The Debye temperature derived fromC(T) amounts to about 450 K atT=T c =91 K and forT→0 approaches 350 K, in agreement with the value deduced fromv L (T) andv T (T). The superconducting transition is indicated by anomalies with idealized discontinuities Δα and ΔC. A thermodynamic relation between Δα and ΔC yields the relative pressure derivative ofT c ,t p =T c −1 (∂T c /∂p) p→0=+(0.7±0.2)·10−7 kbar−1.
