Investigations of Resonances in the RF Absorption of Superconducting Niobium

Abstract

The influence of a static magnetic field H _dc on the surface impedance of a superconductor in the Meissner state is very complex¹ and has eluded a uniform theoretical explanation for many years. In the last few years attempts have been put forward in terms of magnetic-field-induced surface states² and a special inelastic scattering at the surface.³ The latter is defined “ad hoc,” i.e., has a less firm basis than the magnetic-field-induced surface states. The magnetic-field-induced surface states result from the BCS theory in the case of a magnetic field parallel to the surface⁴; they change the BCS density of states by introducing states into the energy gap. These surface states with energies inside the energy gap can explain several observed aspects of the behavior in the surface resistance R(H). ² However, some features of the experimental results remain unexplained by this model, such as the, peak observed by Garfunkel⁵ or the enhanced decrease of R (H) with lowered frequency and temperature.