First- and Second-Order Superconducting Phase Transitions of Tin Whiskers in a Magnetic Field

Using a sensitive inductance technique, first- and second-order phase transitions in tin (type I) whiskers have been observed in both parallel and perpendicular magnetic fields. For T much less than T, the samples exhibit considerable superheating and supercooling from which values of the Ginzburg Landau parameter kappa are calculated. However, these values are not as low as those obtained for single spheres (diameters 10-30 mu), probably due to mean free path and size effects. Close to the transition temperature it is observed that the hysteresis vanishes indicating the onset of a second-order phase. transition. Good qualitative agreement between these results and theoretical predictions based on the Ginzburg Landau theory are obtained. Size effects on the ratio H-C3/H-C2 = H-SC/H-C2 are clearly illustrated.