Paramagnetic Effect in Superconductors. V. Resistance Transition of Tin Wires

Abstract

The resistance of tin wires between 0.15 and 3 mm in diameter has been measured as a function of the current in the transition region to superconductivity. The samples used were, with one exception, single crystals and had residual-resistance ratios ${r}_{0}=\frac{{R}_{0\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}}}{{R}_{273\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}}}$ of a few times ${10}^{\ensuremath{-}5}$. Most of the transition curves showed a steep rise and a break at the critical current. The critical resistance, defined as the resistance at the break of the curves, is for single crystals independent of the temperature between 1.9\ifmmode^\circ\else\textdegree\fi{}K and the critical temperature, if proper corrections for the temperature dependence and the field dependence of the resistivity are made. Extensive measurements of these dependences have been made for this purpose.Measurements on one polycrystalline sample and comparison with measurements by Rinderer showed a strong influence of the electronic mean free path on the ratio of the critical resistance to the normal resistance, this ratio being smaller for the purer samples with the longer mean free paths. The single crystals which were used here were probably still not quite good enough to represent samples free from all imperfections.The heat transfer from horizontal wires to the liquid helium was measured to assure that the temperature difference between sample and bath is not significant. It was found that the bubbles rising from a heater at the bottom of the Dewar simulate a forced convection which greatly increases the heat transfer coefficient and makes it independent of the temperature difference between sample and bath. It was observed that this temperature difference fluctuates at least by 10% of its value, and probably by a considerably larger amount.Measurements on a tin-coated manganin wire showed that a heating of the wire displaces the critical curve, but does not give rise to a hysteresis. Below 3.4\ifmmode^\circ\else\textdegree\fi{}K hysteresis occurred which was not connected to the heating and which can probably be explained in terms of Ginsburg's phenomenological theory of superconductivity.A device for maintaining the temperature of the helium bath constant within a millidegree is described.