AC conductivity of a niobium thin film in a swept magnetic field

We report the results of the measurement the ac conductivity of a Nb superconducting thin film in a swept dc magnetic field. In the mixed state the swept dc field creates vortices at the film surface which pass through the film and form the observed ac conductivity. Vortex rate generation does not depend on the value of the dc field and there is a large plateau-like region of dc magnetic fields where the dissipation is approximately constant. A proposed phenomenological model describes quite well the main features of the ac response in these fields including its dependency on the sweep rate, ac amplitude, frequency, and value of the second and third harmonics.Comment: 7 pages and 10 figures; Journal of Physics: Condensed Matter, 201

Isotherm and heat of adsorption in porous solids with defective pores-adsorption of argon and nitrogen at 77K in Saran activated carbon

The isotherm and isosteric heat of a porous solid are studied in terms of the local isotherms and isosteric heats of individual pores with defective walls, rather than graphitic walls as commonly assumed in the literature. We point out the incorrect formulas that have been used in the literature, and present a correct formula to calculate the isosteric heat for a porous solid. The correct formula is illustrated with a direct Monte Carlo ( MC) simulation of systems of two pores of different sizes, and finally we apply our theory to experimental data of argon and nitrogen adsorption at 77K on S600H and S84 Saran charcoals to derive their pore size distributions ( PSD). We show that the PSD derived from the fitting either the isotherm only or the heat curve only may not be reliable. It is necessary to utilize both the isotherm and heat curves in the derivation of a more reliable PSD. We also show that it is essential to use defected walls of carbon pores to model adsorption in pores as the model using graphitic walls can not describe isotherm and heat of adsorption adequately