Electronic Structure and Phase Transition in V₂O₃: Importance of 3d Spin-Orbit Interaction and Lattice Distortion

Abstract

The 3d electronic structure and phase transition in pure and Cr doped V2O3 are theoretically investigated in relation to the 3d spin-orbit interaction and lattice distortion. A model consisting of the nearest-neighbor V ion pair with full degeneracy of the 3d orbitals is studied within the many-body point of view. It is shown that each V ion with S = 1 spin state has a large orbital magnetic moment ∼ 0.7µB and no orbital ordering occurs in the antiferromagnetic insulating (AFI) phase. The anomalous resonant Bragg reflection found in the AFI phase is attributed to the magnetic ordering. In the AFI and paramagnetic insulating (PI) phases, Jahn-Teller like lattice instability leads to tilting of the V ion pairs from the corundum c-axis and this causes large difference in the orbital occupation between the paramagnetic metal and the insulating phases, which is consistent with linear dichroic V 2p XAS measurements. To understand the AFI to PI transition, a model spin Hamiltonian is also proposed. The transition is found to be simultaneous order-disorder transition of the magnetic moments and tilting directions of the V ion pairs. Softening of elastic constant C44 and abrupt change in short range spin correlations observed at the transition are also explained.KEYWORDS: V 2 O 3 , electronic structure, phase transition, lattice distortion, orbital ordering, spin-orbit interaction, orbital moment, resonant x-ray scattering, V 2p XAS §1. IntroductionThe metal-insulator (Mott) transition in V 2 O 3 have been extensively studied ever since Mott suggested the possibility of the localization of electrons driven by Coulomb repulsion. 1)Above the transition temperature T N ∼ 155K, the material is paramagnetic metal (PM) and has the corundum structure with the trigonal symmetry (space group R3c). Below T N , it is an antiferromagnetic insulator (AFI) and the lattice distortion to the monoclinic structure (I2/a) takes place.2) Cr doped alloys (V 1−x Cr x ) 2 O 3 exhibit also interesting features. For 0.005 < x < 0.018, at a temperature above the AFI → PM transition, the material further undergoes a PM to paramagnetic insulator (PI) phase transition with no change in the crystal structure. For much higher Cr concentration x > 0.018, the PM phase disappears and a AFI → PI transition takes place.Although V 2 O 3 and its alloys are often referred to a typical example of the Mott-Hubbard system, there are experimental facts showing the complex aspects of these materials, which can not be explained by the simple single-band Hubbard model. In the AFI phase, they exhibit unusual magnetic order, which breaks corundum trigonal symmetry.8) Neutron scattering experiments have been performed by Bao et al. and abrupt change in spatial spin correlation functions has been reported both at the AFI → PM and the AFI → PI transitions. 9, 10)This switching of spin correlation indicates that the phase transitions can not be considered as usual order-disorder magnetic transition. Softening of the C 44 elastic constant has been observed in ultrasonic wa...

Orbital Switching and the First-Order Insulator-Metal Transition in ParamagneticV2O3

“…Furthermore, LDA+U results argue in favor of models without orbital degeneracy. This line of thought has been extended in detail by Tanaka [7], including spin-orbit coupling effects in a cluster-based approach. Mila et al [8] have proposed a model with S = 1 and orbital degeneracy for the antiferromagnetic insulating phase.…”

“…We choose ∆ trg ≃ 0.32 eV , completely consistent with the LDA. To study the MIT, we notice that external pressure decreases the quantity ∆ trg = E a1g − E e π g and leads to an increase in n a1g ; in particular, it leads to a reduction in ∆ trg across 0.28, at which point, a sudden increase in n a1g has been reported from cluster calculations [7]. To study this effect, we monitor the spectral function for different values of n a1g , with fixed total number of electrons.…”

On the Nature of the Magnetic Ground-State Wave Function of V2O3

“…Why do LDA (+U) results differ so much from optical data and from all theoretical descriptions 15,16,17 ? The answer to this question is already contained, implicitly, in the discussion of section V in Ref.…”

Electronic reconstruction of hexagonal FeS: a view from density functional dynamical mean-field theory

“…As considered in [42] we first assume that external pressure modifies the crystal field splittings of the 3d manifold, and we look for an electronic instability of the hexagonal phase as a function of applied pressure. This assumption is known to be plausible and it has already been successfully used by other groups to study the effect of pressure and/or lattice strain on the orbitalselective electronic reconstruction of correlated electron systems [43,44]. However, it is recognized that under external perturbations like pressure and strain, the hopping elements and the crystal field splittings are renormalized in non-trivial ways.…”

“…To our knowledge, our approach is not totally new: Mott and co-workers proposed such ideas in the 1970s [51]. Tanaka made a similar hypothesis in a cluster model approach for V 2 O 3 [43] and Savrasov et al [52] have employed similar ideology to study the volume collapse across the α δ − transition in Plutonium. In a correlated electron system, small changes in the local energy splittings lead to large dynamical SWT typically over a scale of few electron-Volts [45].…”