Alternating Current Resistance of Nonideal Superconductors

Abstract

A direct electrical measurement is made of the resistance of nonideal superconductors at power and audio frequencies, at 4.2°K and in zero applied field. The resistance of short samples of several Nb‐Zr and Mo‐Re alloys is found to be independent of the composition and of impressed current. The resistivity is found to depend on f^0.5 above 1000 cps and on f^1.7 for frequencies f below 500 cps. The resistance is also independent of wire diameter for some sizes near 0.25 mm. Three explanations are considered to account for resistance at low frequencies. Relaxation time effects are ruled out by the observed frequency dependence at high frequencies. Inductive coupling between the magnetic fields of the normal and the superconducting currents leads to a resistance effect six orders of magnitude lower than the observed ones, and an f^2.0 dependence. Trapping of flux within the superconductor leads to an upper bound for the resistance that agrees with the experimental results in being independent of the wire diameter and the current, and giving the approximately correct resistance values, although an f^1.0 dependence. The measurements suggest that the ac resistance may result from eddy currents induced in normal regions near the surface of heavily cold‐worked superconductors.