Pair-breaking effect on mesoscopic persistent currents

TL;DR

This paper investigates how superconducting fluctuations and pair-breaking effects influence mesoscopic persistent currents in normal metallic rings, revealing that pair-breaking can suppress superconductivity without significantly reducing persistent currents.

Contribution

It demonstrates that pair-breaking effects can drastically lower the superconducting transition temperature while leaving persistent currents largely unaffected, explaining experimental observations in copper and gold.

Findings

Persistent currents remain large despite suppressed superconductivity.

Superconducting transition temperature can be minimized by pair-breaking without affecting current.

Measured currents suggest very low intrinsic $T_c^0$ in copper and gold.

Abstract

We consider the contribution of superconducting fluctuations to the mesoscopic persistent current (PC) of an ensemble of normal metallic rings, made of a superconducting material whose low bare transition temperature $T^{0}_{c}$ is much smaller than the Thouless energy $E_{c}$. The effect of pair breaking is introduced via the example of magnetic impurities. We find that over a rather broad range of pair-breaking strength $\hbar/\tau_{s}$, such that $T_c^0 \lesssim \hbar/\tau_s \lesssim E_c$, the superconducting transition temperature is normalized down to minute values or zero while the PC is hardly affected. This may provide an explanation for the magnitude of the average PC's in copper and gold, as well as a way to determine their $T^0_c$'s. The dependence of the current and the dominant superconducting fluctuations on $E_c\tau_s$ and on the ratio between $E_c$ and the temperature is analyzed. The measured PC's in copper (gold) correspond to $T^0_c$ of a few (a fraction of) mK.