Critical magnetic fields of superconducting tungsten

Abstract

The thermodynamic critical field curve of a superconducting tungsten specimen with resistivity ratio 7,500 has been measured as a function of temperature down to 3 m K (1 m K - 10-3 oK). From the raw data the critical field H0 at T = 0 is found to be 1.14·1 ± .007 G, and the zero field transition temperature Tc is 15.39 + .17 mOK. The normal-state electronic specific heat parameter y, as derived from the critical field data, is 0.87 + .04 mJ/mole °K2. However, the magnetic field at the specimen is slightly altered by the nearby paramagnetic refrigerant- thermometer. An estimated correction applied to the data gives the new values Ho = 1.148 +/- .010 G, Tc = 15.37 +/- .17 moK, and gamma = 0.90 ± .06 mJ/mole °K2, which are considered to be the most probable values of these parameters for the tungsten specimen studied here. Deviation of the measured critical field from a parabolic dependence on temperature is in good agreement with the weak-coupling BCS theory. The very small calculated Ginzburg-Landau parameter (x:: 2 X 10^-3 ) and extremely large calculated coherence length (~o ~ 32u) are favorable for observation of a large supercooling effect in tungsten at temperatures well below Tc. Assuming that the ideal critical supercooling field was observed on an electropolished specimen, the experimental value for u is (2.6 ± 1.6) X 10^-3. A large and reproducible, though less than ideal, superheating effect was also observed with this same electropolished specimen.