Resistance measurements of high Tc superconductors using a novel `bathysphere´ cryostat

A novel cryostat has been developed for variable-temperature testing of high-temperature superconductors. The cryostat is a bathysphere consisting of an overturned stainless steel Dewar suspended in liquid helium. A sample-heater-thermometer assembly is located at the top of the encapsulated (and thermally insulated) vapor space inside of the Dewar. The sample can be rapidly cycled from 300 K to 4 K at an average rate of 1 K/min with a thermal hysteresis of less than 0.1 K. Helium vapor flows through a plug in the bottom of the bathysphere so that pressure of the vapor is roughly ambient. This provides ample heat transfer to and from the sample to maintain thermal equilibrium in the vapor space. Results for resistance versus temperature of some high-temperature superconductors in a magnetic field are presented. Various definitions for thermodynamic and practical T_c s derived from transport resistance measurements are suggested and discussed. These definitions are based on T_c midpoint, various relative resistance criteria, or absolute resistivity criteria