Direct Evidence of the Competing Nature between Electronic and Lattice Breathing Order in Rare-Earth Nickelates

Abstract

Correlated electrons give rise to both exotic electronic and magnetic properties in rare-earth nickelates.Here we present evidence of the interfacial coupling between two nickelate systems EuNiO 3 (ENO) and LaNiO 3 (LNO) with different electronic and magnetic properties but with compatible structural registry giving rise to an electrostructural transition, unobserved in each constituent. Nominally, LNO remains in a paramagnetic-metallic 3 phase while orthorhombic ENO undergoes antiferromagnetic and insulating transitions. However, the ENO/LNO heterostructure displays a uniform rotational symmetry set by an entwined interface. This leads to an anomalous reduction of bond disproportionation in the ENO layer through the metal to insulator transition and concomitantly charge disproportionation (CD) opens the gap accompanied by antiferromagnetic ordering. Our results resolve a long-standing question in the physics of rare-earth nickelates, herein demonstrating that charge and bond disproportionation are competing mechanisms for the charge localization process in the rare-earth nickelate system.

Template Engineering of Metal-to-Insulator Transitions in Epitaxial Bilayer Nickelate Thin Films

“…To evidence the octahedral-distortion-mediated modification of the electronic structures in epitaxial nickelate bilayer samples further, synchrotron resonant X-ray diffraction analyses were implemented for LaNiO 3 (3 unit cells)/NdNiO 3 ( t = 7 and 14 unit cells) (001) thin films (for more details of the X-ray diffraction experiments, see the method part in the Supporting Information). , As shown in Figure c,d, X-ray photon energy was tuned to the Ni K -edge. While carrying out reciprocal L scans around the pseudocubic (0.5 0.5 0.5) film reflection, we monitored the resonant enhancement arising from charge ordering by disproportionation of the Ni charge valency. − For the LaNiO 3 (3 unit cells)/NdNiO 3 ( t = 7 unit cells) bilayer thin film (Figure c), no resonant peak related to the charge disproportionation was observed at both 14 and 150 K across the MIT ( T MIT ∼ 95 K for cooling in Figure b).…”

“… , As shown in Figure c,d, X-ray photon energy was tuned to the Ni K -edge. While carrying out reciprocal L scans around the pseudocubic (0.5 0.5 0.5) film reflection, we monitored the resonant enhancement arising from charge ordering by disproportionation of the Ni charge valency. − For the LaNiO 3 (3 unit cells)/NdNiO 3 ( t = 7 unit cells) bilayer thin film (Figure c), no resonant peak related to the charge disproportionation was observed at both 14 and 150 K across the MIT ( T MIT ∼ 95 K for cooling in Figure b). The temperature-independent energy dependence of the diffraction peak was due to the off-diagonal elements of the Ni K -edge anomalous scattering factor caused by the octahedral rotation .…”

PERSPECTIVE: Emergent phases in rare earth nickelate heterostructures

“…These emergent behaviors are linked to the fact that LNO layers in short periodic SLs are forced to follow the octahedral tilt pattern of highly dis-torted ENO layers but have zero or greatly reduced BD (see Fig. 2(d)-(e)) [77]. LaNiO 3 layers also follow the octahedral rotational pattern of NdNiO 3 in NdNiO 3 /LaNiO 3 superlattices [85].…”

“…The competition between interfacial unit cell and bulk-like unit cells were further investigated by growing nENO/nLNO SLs (Fig. 2(a)) [77,78]. The SL with n=8 uc remains metallic, similar to bulk LNO, whereas all short periodic SLs exhibit simultaneous MIT and antiferromagnetic transition (Fig.…”

“…2(c)). Moreover, the bond disproportionation (BD) and charge disproportionation (CD) compete with each other within these SLs [77,78], in contrast to their cooperative nature in bulk RENiO 3 [29,84]. These emergent behaviors are linked to the fact that LNO layers in short periodic SLs are forced to follow the octahedral tilt pattern of highly dis-torted ENO layers but have zero or greatly reduced BD (see Fig.…”