Control of hidden ground-state order in NdNiO3 superlattices

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.

Spatially Controlled Octahedral Rotations and Metal–Insulator Transitions in Nickelate Superlattices

“…The decoupling of MIT and spin ordering has also been reported for other RNiO 3 -based heterostructures and SLs with different boundary conditions. 2 , 9 , 47 , 48 The suppressed OOR along in-plane axes leads to a decrease of T N from ∼168 K for the 25-uc NNO film to ∼82 K for the n = 8 SL, following the trend in the bulk RNiO 3 phase diagram. 22 As n decreases from 8 to 6 uc, T N hardly changes due to the slightly altered OOR in this thickness range.…”

Disentangling lattice and electronic contributions to the metal–insulator transition from bulk vs. layer confined RNiO 3

“…This allows the seemingly paradoxical solutions with the amplitude of Q and ∆N showing opposite trends between the bulk and heterostructure. If we assume that the experimentally obtained metal-insulator transition temperature is more strongly correlated to ∆N than Q, while the XAS spectra splitting is more strongly correlated with Q, we can thus explain the seemingly paradoxical results in previous work (9).…”

“…However in the DFT calculations ∆N is relaxed at each Q . Referring to equation 5 we have on the DFT level (and noting the stationarity with respect to ∆N ): cQ = ∂EDF T ∂Q = kQ − 1 2 g DF T ∆N DF T (Q) [9] In the linear response regime, which accurately describes the DFT results for all structures we considered, we find:…”

“…Recent experiments (9) report that in NdNiO3/NdAlO3 (NNO/NAO) superlattices in which one or two monolayers of NNO are separated by many layers of the wide-gap insulator NAO, the MIT occurs at a much higher temperature than in the bulk, while the X-ray signatures of the lattice distortion are much less pronounced in the superlattices than in bulk. These experiments suggest that heterostructuring affects electronic and lattice properties differently and thus that a comparative examination of the two material forms can help disentangle the relative importance of electronic and lattice contributions to the metal-insulator transition.…”

“…We disentangle these effects by independently changing each. Motivated by recent experimental (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) and theoretical (8,13,(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36) results, we focus here on the rare earth nickelate family of materials. The concepts, formalism and findings are applicable to wide classes of materials.…”