Control of hidden ground-state order in NdNiO$_3$ superlattices

TL;DR

This study reveals how atomically layered NdNiO$_3$ superlattices exhibit hidden electronic and magnetic phases, including a novel 2D phase where bulk order disappears, driven by interfacial effects and dimensional confinement.

Contribution

It demonstrates the control of hidden ground-state orders in NdNiO$_3$ superlattices through atomic layering, unveiling new phases and mechanisms in reduced dimensions.

Findings

Discovery of a new 2D phase with vanishing bulk magnetic and charge order.

Persistence of bulk-type ordering down to two unit cells with distinct insulating behavior.

Identification of interfacial electronic reconstruction as a mechanism for phase suppression.

Abstract

The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures, where one can constrain dimensionality and impose interfacial boundary conditions. Here, we unlock a host of unique, hidden electronic and magnetic phase transitions in NdNiO$_3$ while approaching the two-dimensional (2D) limit, resulting from the differing influences of dimensional confinement and interfacial coupling. Most notably, we discover a new phase in fully 2D, single layer NdNiO$_3$, in which all signatures of the bulk magnetic and charge ordering are found to vanish. In addition, for quasi two-dimensional layers down to a thickness of two unit cells, bulk-type ordering persists but separates from the onset of insulating behavior in a manner distinct from that found in the bulk or thin film nickelates. Using resonant x-ray spectroscopies, first-principles theory, and model calculations, we propose that the single layer phase suppression results from a new mechanism of interfacial electronic reconstruction based on ionicity differences across the interface, while the phase separation in multi-layer NdNiO$_3$ emerges due to enhanced 2D fluctuations. These findings provide insights into the intertwined mechanisms of charge and spin ordering in strongly correlated systems in reduced dimensions and illustrate the ability to use atomic layering to access hidden phases.