Spin-Paramagnetic Effects in Superconducting Aluminum Films

We have measured the resistive transition of very thin superconducting Al films in an applied magnetic field H. Values of the critical field Hc were obtained as a function of temperature T, film thickness, and field direction. Qualitative and quantitative comparisons are made here with the theory of high-field superconductors including the effects of Pauli paramagnetism, spin-orbit interaction, and thermodynamic fluctuations. In particular, the predicted first-order transition to the normal state at low temperature and large H was observed. From theoretical analysis of the Hc-vs-T data, values of the spin-orbit scattering rate b of about 0.2 were obtained. The thickness of the films studied were in the range 35-1000 {}, and critical fields up to 55 kOe were observed.