Dimensionality-tuned electronic structure of nickelate superlattices explored by soft-x-ray angle resolved photoelectron spectroscopy

TL;DR

This study uses soft-x-ray ARPES to investigate how the electronic structure of nickelate superlattices changes with layer thickness and strain, revealing a transition from metallic to insulating behavior and signs of spin-density wave fluctuations.

Contribution

First direct k-space measurements of buried nickelate layers under strain, revealing the evolution of electronic coherence and Fermi surface features with layer thickness.

Findings

Quasiparticle coherence diminishes as LaNiO3 layers thin to 2 unit cells.

Residual Fermi surface persists in 2 uc superlattices, indicating incomplete insulating transition.

Evidence of spin-density wave fluctuations without full gap opening.

Abstract

The electronic and magnetic properties of epitaxial LaNiO3/LaAlO3 superlattices can be tuned by layer thickness and substrate-induced strain. Here, we report on direct measurements of the k-space-resolved electronic structure of buried nickelate layers in superlattices under compressive strain by soft x-ray photoemission. After disentangling strong extrinsic contributions to the angle-dependent signal caused by photoelectron diffraction, we are able to extract Fermi surface information from our data. We find that with decreasing LaNiO3 thickness down to two unit cells (2 uc) quasiparticle coherence becomes strongly reduced, in accord with the dimension-induced metal-to-insulator transition seen in transport measurements. Nonetheless, on top of a strongly incoherent background a residual Fermi surface can be identified in the 2 uc superlattice whose nesting properties are consistent with the spin-density wave (SDW) instability recently reported. The overall behavior of the Ni 3d spectra and the absence of a complete gap opening indicate that the SDW phase is dominated by strong order parameter fluctuations.