Nanoscale defects as probes of time reversal symmetry breaking

TL;DR

This paper explores how nanoscale NV centers can detect time-reversal symmetry breaking in low-dimensional conductors, providing a novel method to probe electromagnetic fluctuations and characterize exotic quantum states.

Contribution

It introduces a new approach using NV center relaxation rates to directly measure TRSB phenomena and related properties like Hall viscosity and pairing symmetry in superconductors.

Findings

NV centers' relaxation rates differentiate TRSB states from non-TRSB states.

The method can potentially identify fractional quantum Hall states and pairing angular momentum.

Predicted relaxation rate enhancements near TRSB superconductors below T_c.

Abstract

Nanoscale defects such as Nitrogen Vacancy (NV) centers can serve as sensitive and non-invasive probes of electromagnetic fields and fluctuations from materials, which in turn can be used to characterize these systems. Here we specifically discuss how NV centers can probe time-reversal symmetry breaking (TRSB) phenomena in low-dimensional conductors. We argue that the difference in relaxation rates $\Gamma_{\pm \hat{z}}$ of NV centers starting from $m = \pm 1$ spin states to the ground state with $m = 0$ directly probes TRSB. The effect arises from the difference in the fluctuation spectrum of left and right-polarized electromagnetic fields emanating from such materials. In the quantum Hall setting, the NV center experiences (nearly zero) large additional contribution to its relaxation due to the presence of the material when its magnetic dipole (anti-) aligns with the external field. More generally, the difference in the relaxation rates is sensitive to the imaginary part of the wave-vector dependent Hall conductivity. We argue that this can be used to determine the Hall viscosity, which can potentially distinguish candidate fractional quantum Hall states and be used to infer the pairing angular momentum in TRSB superconductors. For the latter, we consider specifically the case of TRSB in stacked twisted Bismuth strontium calcium copper oxide (BSCCO) flakes, which have recently been investigated experimentally and are suggested to exhibit TRSB. We show that the average relaxation rate $\left[\Gamma_{+\hat{z}} + \Gamma_{-\hat{z}}\right]$ near such a system exhibits a Hebel-Slichter like enhancement below $T_c$. The difference $\Gamma_{+\hat{z}} - \Gamma_{-\hat{z}}$ also inherits this peak but is only non-zero for $T < T_c$ and only in a chiral d-wave superconductor. We provide concrete estimates for observing this effect.