Origin of the orbital and spin ordering in rare-earth titanates

Abstract

Rare-earth titanates RTiO3 are Mott insulators displaying a rich physical behavior, featuring most notably orbital and spin orders in their ground state. The origin of their ferromagnetic to antiferromagnetic transition as a function of the size of the rare-earth however remains debated. Here we show on the basis of symmetry analysis and first-principles calculations that although rare-earth titanates are nominally Jahn-Teller active, the Jahn-Teller distortion is negligible and irrelevant for the description of the ground state properties. At the same time, we demonstrate that the combination of two antipolar motions produces an effective Jahn-Teller-like motion which is the key of the varying spin-orbital orders appearing in titanates. Thus, titanates are prototypical examples illustrating how a subtle interplay between several lattice distortions commonly appearing in perovskites can produce orbital orderings and insulating phases irrespective of proper Jahn-Teller motions.ABO 3 oxide perovskites, with partly filled d states on the B site, exhibit a rich physical behavior originating from the intimate coupling between structural, electronic (charge and orbital) and magnetic degrees of freedom [1]. Typical examples are the rare-earth vanadateselectronic configuration on V 3+ ions) that exhibit two different spin and orbital orders yielding distinct symmetries for the ground state at low temperature [2,3]. With the electronic degeneracy of Ti 3+ -3d 1 -t 1 2g configuration -, rare-earth titanates R 3+ Ti 3+ O 3 are often expected to be another text book example of such a subtle interplay between orbital and spin orders.Rare-earth titanates are Mott insulators, which according to their small tolerance factor, adopt a common orthorhombic P bnm structure characterized by large oxygen cage rotations [4][5][6], i.e a − a − c + antiferrodistortive motions in Glazer's notations [7]. They also all undergo a magnetic phase transition to either a ferromagnetic (FM) ordering for small R= Lu-Gd+Y or a G-type antiferromagnetic (G-AFM) ordering for large R= SmLa [6,8,9].The nature of the very peculiar FM to G-AFM transition as a function of the rare-earth size is however puzzling and controversial [9]. On one hand, Ti 3+ is nominally a Jahn-Teller (JT) active ion and the JT distortion is commonly proposed as a key ingredient to explain the transition [10,11]. However, while such a distortion could be compatible with the ferromagnetic phase [12,13], it cannot provide a satisfying explanation for the purely antiferromagnetic phase [10]. On the other hand, some other works have proposed that the JT distortion is neither responsible for the insulating phase of these materials nor for the observed orbital orders [14,15].Instead, Mochizuki et al have suggested that specific orbital-orderings for the FM and AFM phases are triggered by the crystal field produced by the rareearth [13,16], with a potential competition with the JT distortion [11]. This latter model ultimately results in combinations of the three t 2g orbitals [9,13,16,...

Origins versus fingerprints of the Jahn-Teller effect in d -electron ABX3 perovskites

“…We have previously identified 33 We conclude that the electronically-induced Jahn-Teller distortion mode Q2and the geometrically induced steric Q2 + octahedral deformation mode are fully captured by a static mean-field method. This is in line with recent theoretical works that have demonstrated that static mean-field methods capable of inducing broken symmetry such as Density Functional Theory 13,26,30,33 in a polymorphous representation suffice to also explain (i) the trends in gapping and type of magnetic order across the ternary ABO3 series 33…”

A Living‐Dead Magnetic Layer at the Surface of Ferrimagnetic DyTiO3 Thin Films

“…Among transition‐metal perovskites, rare‐earth orthorhombic titanates RTiO 3 (R = La… Lu or Y) emerge as a particularly interesting family of Mott insulators . The most stable valence state of rare‐earths (except for Eu and Ce) is 3+, which imposes a 3+ valence state to the Ti that is in a spin ½ 3d 1 electronic state.…”

Tailoring Materials for Mottronics: Excess Oxygen Doping of a Prototypical Mott Insulator

“…In the undistorted cubic perovskite SrVO 3 , the three t 2g bands are degenerated with an effective band‐filling of 1/6 favoring the correlated metal phase. In the distorted titanates, however, the splitting of the t 2g levels gives rise to Mott insulating behavior . Stoichiometric LaTiO 3 is therefore a prototypical single band Mott insulator and, as indicated by its small Mott gap, already close to the transition into the correlated metal phase.…”