Electronic Theory for Scanning Tunneling Microscopy Spectra in Infinite-Layer Nickelate Superconductors

TL;DR

This paper presents a theoretical model explaining the diverse STM spectra observed in nickelate superconductors by considering a two-band $d$-wave pairing scenario and realistic tunneling simulations, clarifying the gap symmetry nature.

Contribution

It introduces a two-band model with $d_{x^2-y^2}$-wave gaps and realistic tunneling simulations to explain STM spectra in nickelates, providing a unified explanation for experimental observations.

Findings

V, U, and mixed spectral line-shapes depend on intra-unit cell position.

Ni and R-bands contribute to V- and U-shaped spectra respectively.

Position-dependent tunneling weights determine spectral line-shapes.

Abstract

Recent scanning tunneling microscopy (STM) observation of U-shaped and V-shaped spectra (and their mixture) in superconducting Nd$_{1-x}$Sr$_x$NiO$_2$ thin films has been interpreted as presence of two distinct gap symmetries in this nickelate superconductor [Gu et al., Nat. Comm. 11, 6027 (2020)]. Here, using a two-band model of nickelates capturing dominant contributions from Ni-$3d_{x^2-y^2}$ and rare-earth (R)-$5d_{3z^2 - r^2}$ orbitals, we show that the experimental observation can be simply explained within a pairing scenario characterized by a conventional $d_{x^2-y^2}$-wave gap structure with lowest harmonic on the Ni-band and a $d_{x^2-y^2}$-wave gap with higher-harmonics on the R-band. We perform realistic simulations of STM spectra employing first-principles Wannier functions to properly account for the tunneling processes and obtain V, U, and mixed spectral line-shapes depending on the position of the STM tip within the unit cell. The V- and U-shaped spectra are contributed from Ni and R-bands, respectively, and Wannier functions, in essence, provide position-dependent weighing factors, determining the spectral line-shape at a given intra-unit cell position. We propose a phase-sensitive experiment to distinguish between the proposed $d$-wave gap structure and time-reversal symmetry breaking $d+is$ gap which yields very similar intra-unit cell spectra.