Discriminating Gap Symmetries of Superconducting La$_3$Ni$_2$O$_7$

TL;DR

This paper models the superconducting gap symmetry in La$_3$Ni$_2$O$_7$ using phenomenological functions and predicts observable differences to help identify the pairing symmetry in this high-T$_c$ nickelate superconductor.

Contribution

It introduces a theoretical framework to distinguish between $s_$ and $d$-wave pairing symmetries in La$_3$Ni$_2$O$_7$ through experimentally accessible observables.

Findings

Distinct tunneling density of states for different symmetries

Unique Raman spectroscopy signatures for each gap symmetry

Predicted superfluid density behaviors corresponding to pairing types

Abstract

The discovery of high-T$_c$ superconductor in Ruddlesden-Popper nickelate materials represented by La$_3$Ni$_2$O$_7$ has opened new directions in the quest for unconventional superconductivity. A central unresolved issue concerns the pairing symmetry of the superconducting order. In this paper, we model the superconducting order of La$_3$Ni$_2$O$_7$ using the established Fermi surface structure together with phenomenological pairing functions belonging to $s_\pm$ and $d$-wave symmetry classes, which are the leading possibilities in the current debate. We compute several experimentally accessible observables-including tunneling density of states, point contact spectroscopy, superfluid density, and Raman spectroscopy-each of which exhibits distinct characteristics for different gap symmetries. These quantities provide a concrete and experimentally testable route for identifying the pairing symmetry of La$_3$Ni$_2$O$_7$ and for clarifying the microscopic nature of nickelate superconductivity.