Theory of potential impurity scattering in pressurized superconducting La$_3$Ni$_2$O$_7$

TL;DR

This paper investigates how non-magnetic impurities affect the superconducting symmetry in pressurized La$_3$Ni$_2$O$_7$, proposing impurity effects as a way to distinguish between $d$-wave and $s_{\pm}$-wave states.

Contribution

It introduces a theoretical analysis of impurity effects on superconducting symmetry in La$_3$Ni$_2$O$_7$, providing a method to experimentally identify the order parameter symmetry.

Findings

$d$-wave superconductivity is suppressed by impurities.

$s_{\pm}$-wave state shows robustness against impurities.

Transition from $s_{\pm}$-wave to $s_{++}$-wave occurs with increasing impurity concentration.

Abstract

Recently discovered high-T$_c$ superconductivity in pressurized bilayer nickelate La$_3$Ni$_2$O$_7$ (La-327) is believed to be driven by the non-phononic repulsive interaction. Depending on the strength of the interlayer repulsion, the symmetry of the superconducting order parameter is expected to be either $d$-wave or sign-changing bonding-antibonding $s_{\pm}$-wave. Unfortunately, due to the need of high pressure to reach superconducting phase, conventional spectroscopic probes to validate the symmetry of the order parameter are hard to use. Here, we study the effect of the point-like non-magnetic impurities on the superconducting state of La-327 and show that $s_{\pm}$-wave and $d$-wave symmetries show a very different behavior as a function of impurity concentration, which can be studied experimentally by irradiating the La-327 samples by electrons prior applying the pressure. While $d-$wave superconducting state will be conventionally suppressed, the $s_{\pm}$-wave state shows more subtle behavior, depending on the asymmetry between bonding and antibonding subspaces. For the electronic structure, predicted to realize in La-327, the $s_{\pm}-$wave state will be robust against complete suppression and the transition temperature, $T_c$ demonstrates a transition from convex to concave behavior, indicating a crossover from $s_{\pm}$-wave to $s_{++}$-wave symmetry as a function of impurity concentration. We further analyze the sensitivity of the obtained results with respect to the potential electronic structure modification.