Novel Spin-Orbital Phases Induced by Orbital Dilution

Abstract

We demonstrate that magnetic 3d impurities with S = 3/2 spins and no orbital degree of freedom induce changes of spin-orbital order in a 4d 4 Mott insulator with S = 1 spins. Impurities act either as spin defects which decouple from the surrounding ions, or trigger orbital polarons along 3d-4d bonds. The 4d-4d superexchange in the host J host competes with 3d-4d superexchange J imp -it depends on which orbital is doubly occupied. The spinorbital order within the host is totally modified at doping x = 1/4. Our findings provide new perspective for future theoretical and experimental studies of doped transitionmetal oxides. The well-known example are hole-doped La 1−x Sr x MnO 3 manganites with colossal magnetoresistance [3]. At low doping, orbital polarons emerge in an antiferromagnetic (AF) system by double-exchange mechanism [4], while at higher doping, spin order changes globally to a FM metal coexisting with an e g orbital liquid phase [5]. But a different scenario is also possible-frustrated spin-orbital interactions may lead in some cases to the collapse of any long range order and to a spin-orbital liquid suggested for LiNiO 2 [6]. However, spin and orbital energy scales are here quite different, and the reasons behind the absence of magnetic long range order are indeed more subtle and not yet fully understood [7]. KeywordsA rather unique example of a spin-orbital system are perovskite vanadates, where interesting competition between two types of spin-orbital order was observed [8]. The theoretical spin-orbital model describes an interplay between S = 1 spins and τ = 1/2 orbital doublet {yz, zx} active along the c axis [9], while xy orbitals are filled by one electron each. In addition, G-type AF (G-AF) order in YVO 3 is fragile and switches to C-type AF (C-AF) one in Y 1−x Ca x VO 3 [10], with staggered lines of one-dimensional (1D) FM order (Fig. 1), accompanied by G-type alternating orbital (G-AO) order. Already at low doping, x 0.02 finite spectral weight is generated within the Mott-Hubbard gap due to charge defects away from the VO 6 octahedra [11].

Spin-orbital model of stoichiometric LaMnO3 with tetragonal distortions

“…The models may employ the parameters derived from ab initio calculations or from experiment and thus serve as satisfactory and convincing explanation of interactions in the space of spin-orbital degrees of freedom [5,6]. Several nontrivial disordered [7,8] or ordered [9] phases arise as a generic consequence of spin-orbital exchange interactions, including novel phases found by orbital [10] or charge [11] dilution. Though such models have been successful in working out the peculiar physical properties of doped manganites [12,13], the structural aspects have usually been neglected.…”

Exotic Spin-Orbital Physics in Hybrid Oxides

“…Both for a single impurity and at low x ≤ 1/8 doping, the d 3 -d 4 bonds influence strongly spin-orbital order [28]. For a higher periodic doping x = 1/4 when half of the superexchange bonds are d 3 -d 4 hybrid bonds, the overall spin-orbital order is dictated by them [29]. One finds that only every second undoped vertical line b is FM, as in the C-AF host phase, and host spins are inverted on any other vertical line and the doublon flips from orbital a to b.…”

Spin, orbital and topological order in models of strongly correlated electrons

“…It is an effective dilution of the orbital degrees of freedom, as shown in Fig. 3(b)-(c), being a subject of papers [8] and [55]. Ruthenium atoms (being the host's atoms) are in 4d 4 configuration so their atomic state has spin S = 1, according to Hund's rule, and orbital angular momentum L = 1, where orbital degree of freedom is the double occupation (doublon) of one of the t 2g orbitals.…”