Thermal-expansion, specific-heat, and pressure dependence of T c in YBa 2 (Cu 1-xM x) 3O 7-δ: M=Fe and Zn

High-resolution thermal-expansion and specific-heat measurements of YBa ₂(Cu _{1- x}M _x) ₃O _7-δ (M=Fe and Zn; x=0-5%) at the superconducting transition are presented. Thermal expansivity α( T) provides an alternative, to the specific heat, method for examining the bulk thermodynamics of the superconducting phase transition. We show that if, | d T_c/d p>0.1 K/kbar, expansivity is a more sensitive probe of the superconducting transition than the specific heat because of a relative smaller background. The expansivity jump δα of the Fe doped samples increases by a factor of 4 with increasing x, in stark contrast to the specific heat jump δ C₀, which decreases and is unobservable for x=5%. For Zn, a jump in α at T_c was only observed for the x=1% sample. Calculated d T_c/d p values show a large increase (factor of 20) for Fe and little change for Zn doping. These values agree well with actual pressure data, indicating the the jumps in α are electronic in nature and are not due to a structural transition. The large d T_c/d p values of the Fe doped samples are discussed in terms of a disturbed oxygen coordination of Fe occupying chain sites. Anomalies in α( T) at 40-90 K and ≈ 250 K, indicative of structural changes, are also discussed.