ANOMALOUS BEHAVIOR OF THE SPECIFIC-HEAT AND UPPER CRITICAL MAGNETIC-FIELD IN THE SUPERCONDUCTING SINGLE-CRYSTAL LA1.85SR0.15CUO4

The specific heat and resistive upper critical magnetic field of the s ingle crystal La1.85Sr0.15CuO4 are investigated in the temperature ran ge 2-50 K in magnetic fields up to 8 T for two directions of the magne tic field, parallel and normal to the ab crystalline plane. For both o rientations a nonlinear (close to square root) magnetic held dependenc e of the mixed-state specific heat and a positive curvature of the tem perature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of super conductivity. Within the framework of the thermodynamic relations it i s shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the fie ld dependence of the specific heat. The anomalies observed in these ph enomena are interrelated. (C) 1997 American Institute of Physics.