Effects of Self-field and Low Magnetic Fields on the Normal-Superconducting Phase Transition

TL;DR

This paper demonstrates that even minimal magnetic fields, like Earth's, can significantly affect measurements of the superconducting phase transition, emphasizing the need for proper shielding to obtain accurate critical parameters.

Contribution

It reveals that unshielded magnetic fields distort critical parameter measurements and shows that the self-field from current flow does not influence the phase transition data.

Findings

Earth's magnetic field alters current-voltage curves in superconductors.

Proper magnetic shielding is essential for accurate critical parameter determination.

Self-field effects are negligible on the phase transition measurements.

Abstract

Researchers have studied the normal-superconducting phase transition in the high-$T_c$ cuprates in a magnetic field (the vortex-glass or Bose-glass transition) and in zero field. Often, transport measurements in "zero field" are taken in the Earth's ambient field or in the remnant field of a magnet. We show that fields as small as the Earth's field will alter the shape of the current vs. voltage curves and will result in inaccurate values for the critical temperature $T_c$ and the critical exponents $\nu$ and $z$, and can even destroy the phase transition. This indicates that without proper screening of the magnetic field it is impossible to determine the true zero-field critical parameters, making correct scaling and other data analysis impossible. We also show, theoretically and experimentally, that the self-field generated by the current flowing in the sample has no effect on the current vs. voltage isotherms.