Evidence for highly damped Higgs mode in infinite-layer nickelates

TL;DR

This study uses terahertz spectroscopy to reveal that the Higgs mode in infinite-layer nickelate superconductors is heavily damped, indicating a likely $d$-wave pairing symmetry and providing insights into their unconventional superconductivity.

Contribution

It demonstrates the absence of long-lived Higgs modes in nickelates and links this to $d$-wave pairing, advancing understanding of their superconducting properties.

Findings

Higgs modes are heavily damped by quasiparticles in nickelates.

The nonlinear terahertz response is dominated by quasiparticle excitations.

Short-range superconducting fluctuations are observed near $T_c$.

Abstract

The dynamics of Higgs mode in superconductors, manifested as coherent oscillations of the superconducting order parameter amplitude, provides vital insights into the nature of the superconducting gap structure and symmetry. Here we utilize two-dimensional terahertz coherent spectroscopy to investigate Higgs dynamics of a newly discovered infinite-layer nickelate superconductor. While we observe distinct nonlinear terahertz responses from the superconducting state, well-defined long-lived Higgs modes, as commonly observed in $s$-wave superconductors, are entirely absent in the nickelate film. Instead, we find the coherent nonlinear terahertz response is dominated by the quasiparticle excitations. These observations strongly indicate that the Higgs mode in infinite-layer nickelates is heavily damped by the quasiparticle excitations at arbitrarily low energies, which is a characteristic of $d$-wave pairing symmetry. Additionally, by examining the temperature dependence of the nonlinear terahertz response, we discover short-range superconducting fluctuations in the vicinity of $T_\mathrm{c}$. Our findings provide proof of a new $d$-wave system and establish a foundation for investigating the unconventional superconductivity in nickelates.