Flow resistance and electric oscillations in the flux‐flow state of Nb‐25 at.% Zr wires

摘要

The current‐voltage (Is‐Vs) characteristics of Nb‐25 at.% Zr wires with three kinds of strong pinning forces were measured in the region of magnetic field 15–62.4 kOe and temperature 1.95–5.5°K, using a measuring method which controlled the catastrophic transition from the superconducting to the normal state by connecting a shunt resistor of a few micro‐ohms with the sample. The Is‐Vs characteristics of the sample with strong pinning force showed negative resistance. The negative resistance appears even in the region where Joule heat decreases with the increase of Vs, and it is strongly dependent on the strength of the pinning force rather than the temperature rise by Joule heat. This negative resistance is interpreted as being a negative flux‐flow resistance, which appears when the pinning force is strong, as shown from the local model—a modification of the London model by pinning force. The abrupt change of Vs in the Is‐Vs characteristics occurs when the absolute value of negative flux‐flow resistance is smaller than the shunt resistance. The oscillations of Is and Vs appear in the whole region of negative flux‐flow resistance, and the fundamental frequency of Is varies only by strength of the pinning force. The behavior of oscillations depends strongly on the pinning force, which, however, cannot be explained with usual theories.