Mixed superconducting state without applied magnetic field

TL;DR

This paper demonstrates a method to reversibly create and eliminate a normal state line within a superconductor using a vector potential from a current-carrying coil, without applying an external magnetic field.

Contribution

It introduces a novel, reversible technique to control superconductivity locally via vector potential, avoiding magnetic field application.

Findings

Normal state lines can be turned on and off externally in zero magnetic field.

The method is reversible and reproducible across different cooldowns.

Potential applications include magnetic-field-free control of superconducting states.

Abstract

A superconducting (SC) mixed state occurs in type-II superconductors where the upper critical field Hc2 is higher than the thermodynamic critical field Hc. When an applied field is in between these fields, the free energy depends weakly on the order parameter which therefore can be small (SC state) or zero (normal state) at different parts of the sample. In this paper we demonstrate how a normal state along a line traversing a superconductor can be turned on and off externally in zero field. The concept is based on a long, current-carrying excitation coil, piercing a ringshaped superconductor. The ring experiences zero field, but the vector potential produced by the coil generates a circular current that destroys superconductivity along a radial line starting at preexisting nucleation points in the sample. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil, and different cooldowns yield the same normal lines. We suggest potential applications of this magnetic-field-free mixed state.