Low-energy electrodynamics of infinite-layer nickelates: evidence for d-wave superconductivity in the dirty limit

TL;DR

This study uses terahertz spectroscopy to reveal that infinite-layer nickelates exhibit d-wave superconductivity in the dirty limit, with weak coupling and notable fluctuations near Tc, resembling electron-doped cuprates.

Contribution

It provides the first direct evidence of d-wave pairing symmetry and electrodynamics in nickelates, expanding understanding of their unconventional superconductivity.

Findings

Terahertz responses align with d-wave superconductivity in the dirty limit.

Superconducting gap-to-Tc ratio is 3.4, indicating weak coupling.

Superconducting fluctuations are significant near Tc but do not extend deep into the normal state.

Abstract

The discovery of superconductivity in infinite-layer nickelates establishes a new category of unconventional superconductors that share structural and electronic similarities with cuprates. Despite exciting advances, such as the establishment of a cuprate-like phase diagram and the observation of charge order and short-range antiferromagnetic fluctuation, the key issues of superconducting pairing symmetry, gap amplitude, and superconducting fluctuation remain elusive. In this work, we utilize static and ultrafast terahertz spectroscopy to address these outstanding problems. We demonstrate that the equilibrium terahertz conductivity and nonequilibrium terahertz responses of an optimally Sr-doped nickelate film ($T_c$ = 17 K) are in line with the electrodynamics of $d$-wave superconductivity in the dirty limit. The gap-to-$T_c$ ratio 2$\Delta/k_\mathrm{B}T_\mathrm{c}$ is extracted to be 3.4, indicating the superconductivity falls in the weak-coupling regime. In addition, we observed significant superconducting fluctuation near $T_\mathrm{c}$, while it does not extend into the deep normal state as optimally hole-doped cuprates. Our result highlights a new $d$-wave system which closely resembles the electron-doped cuprates, expanding the family of unconventional superconductivity in oxides.