Superheating,supercooling, surface superconductivity and Ginzburg-Landau parameters of pure type-I superconductors and their alloys

We have measured the low-frequency differential susceptibility of several pure type 1 superconductors, viz. Sn, In, Tl, Pb and Hg, as a function of increasing or decreasing applied magnetic fields. Special emphasis was placed on the observation of metastable (superheated and supercooled) states. Experimental values of the Ginzburg-Landau parameter? at the transition temperatureT _c and of suitably defined temperature dependent parameters for superheating,? _sh(t), and for supercooling,? _sc(t), have been obtained and compared with theoretical estimates. The rapid drop in? _sh close toT _c observed in materials with??1 such as Sn, In, and Tl is interpreted as a manifestation of nonlocal electrodynamics. Supercooling of the normal state down to the bulk nucleation fieldH _c2, rather thanH _c3, has been achieved close toT _c in Cu-plated samples of Sn and In. The parameters? and?(t) have also been determined for a series of SnIn alloys spanning the range between type I and type II behavior. Supercooling of the superconducting surface sheath below the thermodynamic critical fieldH _c has been observed in these alloys as well as in pure Pb. The measured values of? are compared with estimates based on normal state parameters extracted from independent low temperature experiments. Corrections for energy gap anisotropy and electron-phonon retardation are typically below 10%. The strong temperature dependence of? _sc(t), observed in all the materials studied, is not explained by present theories, even in the concentrated alloys where anisotropy effects should be washed out.