Electronic structure of a graphene-like artificial crystal of $NdNiO_3$

TL;DR

This study demonstrates a robust method combining advanced spectroscopy and first-principles calculations to analyze the electronic structure of ultrathin, graphene-like layers of $NdNiO_3$ in complex oxide heterostructures, revealing antiferro-orbital order and a valence-band gap.

Contribution

It introduces a novel approach to probe the electronic structure of buried ultrathin layers using bulk-sensitive spectroscopies and theoretical calculations.

Findings

Agreement between SX-ARPES data and first-principles calculations.

Observation of antiferro-orbital order in the artificial lattice.

Detection of a 265 meV valence-band gap.

Abstract

Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes, with typical information depths of a few Angstroms. Here, we use a combination of bulk-sensitive soft x-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard x-ray photoelectron spectroscopy (HAXPES) and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of $NdNiO_3$ confined between two 4-unit cell-thick layers of insulating $LaAlO_3$. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band (VB) bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers.