Peculiarities of nucleation and propagation of the normal phase in HTSC materials with an YBCO film layer

Abstract

The peculiarities of nucleation and the evolution mechanisms of the normal phase as an alternating current passes through a strip YBCO-based HTSC wire in liquid nitrogen are studied experimentally. It is found that depending on the properties and structure of the substrate of the HTSC strip, the evolution of the normal phase is governed by different mechanisms. For a high thermal conductivity along the conductor and intense heat removal to the coolant, the entire sample passes from the superconductive to the resistive state. Upon an increase in the current, its resistance increases and approaches the resistance of the substrate. For materials with a high-resistivity substrate (i.e., with a low thermal conductivity), the nucleation of the normal phase occurs in regions with the smallest superconducting parameters. In these regions, the resistive state is unstable, part of the sample passes after a certain time to the normal state, and the remaining part returns to the superconductive state. A normal domain is formed. The domain size increases linearly with the voltage. It is found that the region of (and the reason for) domain nucleation depends on the extent to which the current exceeds the critical value. If the excess is small, the resistive state exists for a few periods of the alternating current. In this case, the sample temperature increases from period to period, and a normal domain appears in the region of the conductor with a lower superconducting transition temperature. If, however, the resistive state exists during a time interval shorter than a quarter of a period, a normal domain appears in the region with a lower critical current. It is found that upon a jumpwise variation of the applied voltage, several domains can be formed, and for a certain value of the voltage jump, the domain passes to an unstable state and starts moving with a velocity of about 1 mm/s from the sample periphery to the center. The potential and temperature profiles of a normal domain are obtained experimentally. It is found that the maximal temperature increases monotonically with the applied voltage, and the temperature gradient at the domain boundary along the strip is about 100 K/mm. It is shown that the stabilized parameters of the domain and I–V branches occurring during its nucleation are successfully described by the heat balance equation for the stationary case.