The Pinning Potential and High-Frequency Studies of Type-II Superconductors

The electrical impedance of type‐II superconductors as a function of frequency is discussed in terms of a ``pinning potential.'' Many flux‐pinning properties of the mixed state at any frequency can be treated in terms of such a potential and a simple phenomenological extension of the Bardeen‐Stephen flux‐flow theory. The form of the potential can be determined by various dc and high‐frequency experiments. There is a critical frequency above which the impedance for subcritical currents becomes the impedance of the ``ideal'' mixed state. Much below this frequency the impedance is zero. This critical frequency is given by $${\omega }_0{\text{=2πCρ}}_n{\mathrm{\alpha /\phi }}_0^{\text{1/2}}{H}_{\text{c2}}{H}^{\text{1/2}}$$ . At microwave frequencies, almost all effects of flux pinning are absent, and the properties of the ``ideal'' mixed state are observed even in the most strongly pinned material. Such measurements have been used to determine the effective mass of the flux tube.