Effect of electron-lattice coupling on charge and magnetic order in rare-earth nickelates

Abstract

We investigate the impact of electron-lattice coupling on the stability of various magnetic orders in rare-earth nickelates. We use the Hartree-Fock approximation, at zero temperature, to study an effective, two-band model with correlations characterized by a Hubbard U and a Hund's J. This is coupled to breathing-mode distortions of the octahedral oxygen cages, described semi-classically, with a Holstein term. We analyze the effect of the various parameters on the resulting phase diagram, in particular on the charge disproportionation and on the magnetic order. We confirm that the coupling to the lattice cooperates with Hund's coupling and thus encourages charge disproportionation. We also find that it favors the fully disproportionated, 4-site periodic magnetic order of type ⇑ 0 ⇓ 0. Other convergent magnetic phases, such as the collinear ↑↑↓↓ and non-collinear ↑→↓← states, do not couple to the lattice because of their lack of charge disproportionation. Novel phases, e.g. with charge disproportionation but no magnetic order, are also found to be stabilized in specific conditions.

Effects of reduced dimensionality, crystal field, electron-lattice coupling, and strain on the ground state of a rare-earth nickelate monolayer

“…We also include the coupling to breathing-mode distortions, which are treated semiclassically and coupled to electrons via a Holstein-like term. 33 Using this model, we show that the reduction from 3D to 2D promotes orbital occupancy of the d x 2 −y 2 orbitals, as they lower their energy through in-plane hopping relative to the d 3z 2 −r 2 which cannot hop as freely. This effect alone, even in the absence of favorable crystal-field splitting, is enough to give a cuprate-like phase, ensuring the preferential occupancy of d x 2 −y 2 and AFM order for reasonable values of Hubbard repulsion U and Hund's coupling J.…”

“…which comprises the band energy of e g electrons T , their mutual interactions Ĥe−e , lattice energy H l , and the electron-lattice coupling Ĥe−l . Like in the bulk case 33,36,37 , the two active orbitals per site are taken to have the symmetries of the Ni e g doublet; we discuss this more at the end of this section, where we comment on the applicability of this simple model to nickelates. We focus on a two-site unit cell that allows for (simple) magnetic and orbital order: the unit cell is coordinated through the rocksalt-type wavevector Q c = π(1, 1) to represent the symmetry of the charge order and of the breathing-mode distortion experimentally observed in ultrathin films and superlattices 38 .…”

“…This effect alone, even in the absence of favorable crystal-field splitting, is enough to give a cuprate-like phase, ensuring the preferential occupancy of d x 2 −y 2 and AFM order for reasonable values of Hubbard repulsion U and Hund's coupling J. This cuprate-like phase could not be stabilized in 3D, 33 but appears naturally in nickelate monolayers.…”

“…Another important ingredient in these materials is the presence of strong electron-lattice coupling. Its role for the rare-earth nickelates is well established: [25][26][27][28][29][30][31][32][33] it is responsible for the breathing-mode alternation of compressed and expanded oxygen octahedra in the ground state of (most) bulk nickelates. Whether such distortions are present in the ground-state of the proposed 2D monolayer heterostructure is an open question, and an impor-tant one given that this might interfere with cuprate-like physics.…”

Raman spectroscopic evidence for multiferroicity in rare earth nickelate single crystals

“…During recent years materials with the perovskite structure have become subject of intensive research in the field of photovoltaic cells [1], battery engineering [2] and cancer therapy [3]. In particular the nickelates with the general formula RNiO 3 (with R a trivalent rare earth) are good candidates for these applications by virtue of the interdependence in these compounds of the structural [4][5][6][7][8][9], electronic [10][11][12][13][14] and magnetic [15][16][17][18][19][20][21][22] order parameters. The most characteristic property of RNiO 3 is the transition from a high temperature metallic phase to a low temperature insulating phase.…”