THERMODYNAMIC CRITICAL FIELD OF La2-xSrxCuO4

Abstract

Thermodynamic critical fields, Hc, have been measured for the La2-xSrxCuO4 family of superconductors in order to determine the changes in free energy of the system as the number of carriers is reduced. Magnetization vs. magnetic field curves are thermodynamically reversible over large portions of the H-T plane, so the free energy is well defined in these regions. Magnetization vs. field data are then fit to theoretical models to determine the thermodynamic critical fields. As the Sr concentration is changed from x=0.10 to 0.23, the values of Hc(T=0) goes through a maximum at optimum doping in a manner similar to the Tc vs x curve. The ratio of Hc(T = 0)/Tc also peaks in the region of x somewhat larger than optimum doping. As the value of x increases from underdoped to optimum doping, the cross-over temperature in the Msc vs. T curve, T*, approaches the transition temperature. In the overdoped regime there is no crossing in these Msc vs. T curves.