Superconducting gap node spectroscopy using nonlinear electrodynamics

TL;DR

This paper introduces a bulk-sensitive nonlinear electrodynamics method to identify the nodal structure of the energy gap in unconventional superconductors, aiding the determination of their order parameter symmetry.

Contribution

It develops a systematic approach for node spectroscopy using nonlinear electrodynamics, including effects of anisotropy and mixed symmetries, to distinguish different superconducting order parameters.

Findings

Analytic expressions for nonlinear supercurrent response including anisotropy.

Method to differentiate order parameter symmetries, including mixed types.

Detection of gap nodes or small-gap regions via magnetic moment behavior.

Abstract

We present a method to determine the nodal structure of the energy gap of unconventional superconductors such as high $T_c$ materials. We show how nonlinear electrodynamics phenomena in the Meissner regime, arising from the presence of lines on the Fermi surface where the superconducting energy gap is very small or zero, can be used to perform ``node spectroscopy'', that is, as a sensitive bulk probe to locate the angular position of those lines. In calculating the nonlinear supercurrent response, we include the effects of orthorhombic distortion and $a-b$ plane anisotropy. Analytic results presented demonstrate a systematic way to experimentally distinguish order parameters of different symmetries, including cases with mixed symmetry (for example, $d+s$ and $s+id$). We consider, as suggested by various experiments, order parameters with predominantly $d$-wave character, and describe how to determine the possible presence of other symmetries. The nonlinear magnetic moment displays a distinct behavior if nodes in the gap are absent but regions with small, finite, values of the energy gap exist.