The Higgs Mode in Disordered Superconductors Close to a Quantum Phase Transition

TL;DR

This paper provides experimental evidence of the Higgs mode in disordered two-dimensional superconductors near the superconductor-insulator transition, using terahertz spectroscopy and tunneling measurements.

Contribution

It demonstrates the visibility of the Higgs mode in disordered superconductors close to a quantum critical point, confirming recent theoretical predictions.

Findings

Discrepancy between $2\Delta$ and electromagnetic absorption threshold near SIT

Excess absorption below $2\Delta$ indicates Higgs mode presence

Higgs mode becomes observable in 2D superconductors close to criticality

Abstract

The concept of mass-generation via the Higgs mechanism was strongly inspired by earlier works on the Meissner-Ochsenfeld effect in superconductors. In quantum field theory, the excitations of longitudinal components of the Higgs field manifest as massive Higgs bosons. The analogous Higgs mode in superconductors has not yet been observed due to its rapid decay into particle-hole pairs. Following recent theories, however, the Higgs mode should decrease below the pairing gap $2\Delta$ and become visible in two-dimensional systems close to the superconductor-insulator transition (SIT). For experimental verification, we measured the complex terahertz transmission and tunneling density of states (DOS) of various thin films of superconducting NbN and InO close to criticality. Comparing both techniques reveals a growing discrepancy between the finite $2\Delta$ and the threshold energy for electromagnetic absorption which vanishes critically towards the SIT. We identify the excess absorption below $2\Delta$ as a strong evidence of the Higgs mode in two dimensional quantum critical superconductors.