Three-Dimensional Electronic Structures in Superconducting Ruddlesden-Popper Bilayer Nickelate Films

TL;DR

This study uses ARPES to explore the three-dimensional electronic structures of superconducting bilayer nickelate films, revealing orbital-dependent dimensionality, a large superconducting gap, and strong electron correlations.

Contribution

It provides the first systematic 3D electronic band structure analysis of superconducting Ruddlesden-Popper bilayer nickelates, highlighting the importance of the $d_{z^2}$ orbital and correlations.

Findings

Orbital-dependent dimensionality with $d{x^2-y^2}$ bands being quasi-2D

Identification of a large superconducting gap $ ext{~}18$ meV on the $d_{z^2}$ band

Presence of waterfall-like spectral features indicating strong electron interactions

Abstract

Beyond the quasi-two-dimensional (2D) paradigm of cuprates, the role of the third dimension of the Ruddlesden-Popper bilayer nickelates is essential to decoding their superconducting mechanism. Here, using angle-resolved photoemission spectroscopy (ARPES) with varied photon energies, we systematically investigate the electronic band structures in three dimensions for superconducting (La,Pr,Sm)$_3$Ni$_2$O$_7$/SrLaAlO$_4$ thin films (superconducting onset temperature $T_c^{\text{onset}} \sim 48$ K) transferred via a cryogenic ultra-high vacuum suitcase. We reveal an orbital-dependent dimensionality: while the $d{x^2-y^2}$-dominant bands exhibit a quasi-2D character, the $d{z^2}$-dominant band displays a finite $k_z$ dispersion. Finite energy gaps are identified on all observed bands across multiple high-symmetry directions. Systematic temperature-dependent analysis characterizes the superconducting nature of the gap on the $d{z^2}$-derived band, revealing a large gap $\Delta\sim 18$ meV and a ratio $2\Delta/k_BT_c\sim 8$ exceeding the weak-coupling BCS limit. The suppression of spectral weight near the Fermi level persists above the superconducting transition temperature. Ubiquitous waterfall-like spectral features evidence the presence of electron interactions. These results underscore the role of the $d_{z^2}$ orbital and correlations, placing constraints on theoretical models for nickelate superconductivity.