Doping-Dependent Nonlinear Meissner Effect and Spontaneous Currents in High-Tc Superconductors

TL;DR

This study investigates the nonlinear electromagnetic response of high-Tc superconductors near the critical temperature, revealing doping-dependent behaviors and evidence of spontaneous time-reversal symmetry breaking.

Contribution

It provides the first systematic doping-dependent analysis of nonlinear Meissner effects and identifies spontaneous TRSB currents in high-Tc superconductors.

Findings

TRS nonlinear response scales with de-pairing current density.

Spontaneous TRSB currents onset at Tc and decrease with under-doping.

Doping influences the magnitude of TRSB and TRS nonlinearities.

Abstract

We measure the local harmonic generation from superconducting thin films at microwave frequencies to investigate the intrinsic nonlinear Meissner effect near Tc in zero magnetic field. Both second and third harmonic generation are measured to identify time-reversal symmetry breaking (TRSB) and time-reversal symmetric (TRS) nonlinearities. We perform a systematic doping-dependent study of the nonlinear response and find that the TRS characteristic nonlinearity current density scale follows the doping dependence of the de-pairing critical current density. We also extract a spontaneous TRSB characteristic current density scale that onsets at Tc, grows with decreasing temperature, and systematically decreases in magnitude (at fixed T/Tc) with under-doping. The origin of this current scale could be Josephson circulating currents or the spontaneous magnetization associated with a TRSB order parameter.