Chiral Higgs Mode in Nematic Superconductors

TL;DR

This paper predicts and analyzes two bosonic modes in nematic superconductors, revealing their electromagnetic signatures and the potential for a transition to a chiral ground state with broken symmetries.

Contribution

It introduces the chiral Higgs mode in nematic superconductors and explores its electromagnetic response and implications for phase transitions.

Findings

Identification of nematicity and chiral Higgs modes.

Softening of the chiral Higgs mode at critical doping.

Electromagnetic spectra reveal bosonic excitations and phase instability.

Abstract

Nematic superconductivity with spontaneously broken rotation symmetry has recently been reported in doped topological insulators, $M_x$Bi$_2$Se$_3$ ($M$=Cu, Sr, Nb). Here we show that the electromagnetic (EM) response of these compounds provides a spectroscopy for bosonic excitations that reflect the pairing channel and the broken symmetries of the ground state. Using quasiclassical Keldysh theory, we find two characteristic bosonic modes in nematic superconductors: the nematicity mode and the chiral Higgs mode. The former corresponds to the vibrations of the nematic order parameter associated with broken crystal symmetry, while the latter represents the excitation of chiral Cooper pairs. The chiral Higgs mode softens at a critical doping, signaling a dynamical instability of the nematic state towards a new chiral ground state with broken time reversal and mirror symmetry. Evolution of the bosonic spectrum is directly captured by EM power absorption spectra. We also discuss contributions to the bosonic spectrum from sub-dominant pairing channels to the EM response.