Phase Transition of Thin-Film Superconducting Cylinders in a Magnetic Field. I. Parallel-Field Measurements

The superconducting transition temperature was measured as a function of magnetic field for thin-film hollow cylinders in an accurately parallel magnetic field. Quantitative results were obtained for one indium and nine aluminum cylinders with film thicknesses $d$ varying from 2900 to 340 Å and with diameters from 1.4 to 4.8 μ. The results agree well with Tinkham's analysis based on the Ginzburg-Landau theory even in the very-thin-film region where the penetration depth $\lambda \gg d$ and the transition region is greatly broadened. Values of $\lambda$ obtained from the amplitude of periodic variations of $T_c$ with $H$ and from the background monotonic variation of $T_c$ both agree with values derived from the low-temperature critical field and theoretical estimates. A search for higher-order electron correlations which would give an admixture of higher periodicity of flux quantization yielded a negative result. Evidence was found for the effect of fluctuations on the magnetic field dependence of the phase boundary.