Light heavy-fermions in Zn and Ni doped 123 Cu-O superconductors

Abstract The temperature dependence of the specific heat C(T) has been measured for YBa 2 (Cu 1−x M x ) 3 O 7 (YBCO) with M = Zn and Ni and 1.2 2 planes. The crystal structure remains orthorhombic for the concentrations studied, i.e., x T c with T c →0 at x Zn = 0.10, whereas Ni causes a less dramatic decrease in T c . The electronic contribution to C(T) increases monotonically with impurity doping with λ LT = C(T) T at T = 1.5 K reaching values of 100–150 mJ⧸mol K 2 for modest Zn and Ni doping. This γ LT dependency for Zn doped YBCO has been associated with a localization of d-holes within the CuO 2 planes caused by a disorder induced disruption of hybridization within the planes.