Thermal Phase Transitions in Superconducting Nb-Zr Alloys

Abstract

Experimental data are presented for the thermal propagation of a normal‐superconducting boundary along wires of Nb‐25%Zr, Nb‐37%Zr, and Nb‐51%Zr. The thermal propagation velocity was measured as a function of current at 4.2°K in constant longitudinal and transverse magnetic fields up to the upper critical field H_c2 as well as in a zero magnetic field for temperatures between 2.5° and 11°K. Previous models which describe thermal propagation phenomena neglected the temperature dependences of the specific heat, thermal conductivity, and electrical resistivity. A more exact model is proposed here which takes into account the temperature dependences of these quantities, and all the present experimental data are in good agreement with this model. From a measurement at 4.2°K of the dependence of the thermal velocity on magnetic field, the high‐field transition temperatures as a function of magnetic field are calculated for temperatures above 4.2°K. In conjunction with the theory, the linear dependence of the thermal velocity on current implies a weak current dependence for both the transition temperature and superconducting‐state specific heat in Nb‐Zr alloys up to 3×10⁵ A/cm². A method is discussed for using thermal propagation techniques to determine the superconducting‐state specific heat in the presence of known transport currents and magnetic fields.