Depairing Effect of a Magnetic Field: A Study of Microwave Absorption in the Surface-Sheath Regime of Pure Lead

The absorption of microwave radiation in superconductors in the presence of high magnetic fields—in particular, in the surface-sheath regime of low-$\kappa$ materials—is investigated. It is found that the surface resistance of pure lead increases very sharply just above the thermodynamical critical field $H_c$. This is interpreted in terms of the depairing effect of the magnetic field, which reduces the optical energy gap at the surface of the superconductor by an amount linear in the vector potential. To understand the fact that this reduction of the optical gap is observable, it is proposed that the absorption of microwave photons is simultaneous with diffuse scattering of the quasiparticles at the surface.