Superconducting Transition Width in Pure Gallium Single Crystals

The superconducting transition in 99.9999% pure gallium single crystals was investigated. The change in mutual inductance at 23 Hz of a pair of coils containing the sample was used to monitor the transition. In long, thin single crystals, the transition was 90% complete in a temperature interval of 2×${10}^{-5\mathrm{}}$ °K, a smaller superconducting transition width than is usually reported in the literature. This transition width corresponds to a superconducting penetration depth at 0°K of ${\lambda }_0\le 3.8\times {10}^{-5\mathrm{}}$ cm, in agreement with the nuclear-resonance data of Hammond and Knight. Several factors were investigated for possible effects on the transition width. For pure single crystals, the dominant contribution to the transition width was correlated to the relative lengths of the sample and the primary coil. Samples shorter than the primary coil showed substantially broadened transitions, probably caused by a complex nucleation of the superconducting state at the blunt ends of the specimens in the weak (${10}^{-2\mathrm{}}$-${10}^{-3\mathrm{}}$-G) primary-coil magnetic field. All effects of damage on the transition width were removed by 40 h of room-temperature annealing, and the transition temperature from specimen to specimen of cut and annealed specimens was the same to within a temperature interval at least as small as the transition width. These results support the suggestion by Gregory, Sheahen, and Cochran that the critical-field curve of gallium may be used as a reliable secondary temperature standard below $T_c=1.083\mathrm{}$°K.