Precise Critical-Field Measurements of Superconducting Sn Films in the London Limit

Precise measurements of the critical field of superconducting tin films were made as a function of temperature and thickness. Particular attention was paid to measurements near the transition temperature where the London limit holds. It was found that near the transition temperature the critical field could be expressed in the form $H_c={\left(\Delta t\right)}^{\frac{1}{2}}\gamma (1+\epsilon \Delta t)$, where $\gamma$ and $\epsilon$ are independent of temperature. In terms of the Ginzburg-Landau theory modified to include the lower temperatures, the constant $\gamma$ determines the penetration depth and the constant $\epsilon$ is different for different modifications of the field-independent free energy.