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

Abstract

The first-order metal-insulator transition (MIT) in paramagnetic V2O3 is studied within the ab-initio scheme LDA+DMFT, which merges the local density approximation (LDA) with dynamical mean field theory (DMFT). With a fixed value of the Coulomb U = 6.0 eV , we show how the abrupt pressure driven MIT is understood in a new picture: pressure-induced decrease of the trigonal distortion within the strong correlation scenario (which is not obtained within LDA). We find good quantitative agreement with (i) switch of the orbital occupation of (a1g, e π g1 , e π g2 ) and the spin state S = 1 across the MIT, (ii) thermodynamics and dc resistivity, and (iii) the one-electron spectral function, within this new scenario.PACS numbers: 71.28+d,71.30+h,72.10-dThe spectacular metal-insulator transition (MIT) in the paramagnetic (P) state in V 2 O 3 has long been considered as a classic, and by now almost a textbook version of correlation-driven MIT in a one-band correlated system described by the one-band Hubbard model [1]. This conventional wisdom has been recently challenged by new experiments, clearly showing the coupled spinorbital character of the system, and necessitating a revision in terms of a multiband picture.The conventionally accepted picture rested upon the following argument. In the high-T phase, V 2 O 3 exists in the corundum structure, and with a 3d 2 state (V 3+ ), the two e σ g orbitals are empty, while the triply degenerate t 2g orbitals are filled by two electrons. This triple degeneracy is lifted by a small trigonal distortion, leading to a singly occupied a 1g orbital oriented along the c-axis, and doubly degenerate planar e π g orbitals, occupied by the second electron. Castellani et al. [2] proposed that strong covalent effects lead to a bonding singlet a 1g state involving two V ions along the c-axis. The remaining electron in the two-fold degenerate e π g orbitals gives rise to a S = 1/2 model with orbital degeneracy. This inspired development of theoretical techniques, culminating in the dynamical mean field theory (DMFT) [3], leading to considerable improvement in our understanding of the MIT. Within the basic picture [2], Rozenberg et al.[4] used DMFT for one-and two-orbital Hubbard models with Bethe density-of-states to study the MIT in V 2 O 3 .Recent polarized X-ray scattering results of Park et al.[5] require, however, an interpretation in terms of a spin S = 1 at each V site, with a mixed orbital e π g a 1g : e π g e π g = x : (1 − x) configuration. An exciting conclusion from these results is that the above ratio changes its value abruptly at the MIT, forcing one to abandon the one-band Hubbard model to describe V 2 O 3 . Notice that this implies an important role for the trigonal splitting, since the lower-lying orbital will be more "localized" when local Coulomb interactions are switched on. This raises questions concerning a possible link between the orbital "switching" and the drastic change in the electronic state, and to a possible common underlying origin.The high-spin ground state res...

Orbital-selective insulator-metal transition inV2O3under external pressure

“…2, we show the single particle DOS for the Mott insulator, obtained with U = 5.0 eV, U ′ = 3.0 eV and J H = 1.0 eV as correlation parameters for this system. These are slightly different from those used in our previous work, 29 but are roughly the same as those used by Held et al recently. 32 A clear Mott-Hubbard gap, E G = 0.2 eV , is seen, and, as expected from the orbital assignment, the e We now study the paramagnetic metallic state obtained as an instability of the correlated Mott insulator derived above.…”

“…29,30 we already showed the orbital selective character, as well as the evolution of the DOS at E F as a function of the occupation of the a 1g orbital. A clear first-order I-M transition around n a1g = 0.38 was found, involving, as described above, a discontinuous change in (selfconsistently determined) occupations of each orbital.…”

Mott-Kondo insulator behavior in the iron oxychalcogenides

“…The metallic phase is then the orbital-selective metal found in various contexts. [36][37][38] Once this selective localization occurs within DMFT, the low energy physical response is governed by strong scattering between the effectively Mott-localized and the renormalized, itinerant components of the matrix spectral function. The problem is thus effectively mapped onto a generalized spinful Falicov-Kimball [32] type of model, as has been noticed in earlier works.…”

“…The problem is thus effectively mapped onto a generalized spinful Falicov-Kimball [32] type of model, as has been noticed in earlier works. [36][37][38] Within DMFT, the itinerant fermion spectral function then shows a low-energy pseudogapped form, while the localized spectral function shows a power-law fall-off as a function of energy, as long as the renormalized Fermi energy (E F ) is pinned to the renormalized orbital energy of the localized orbital(s). This is understood from the mapping of the corresponding impurity model to that of the "X-ray edge", [39] where the orthogonality catastrophe destroys Fermi liquid behavior.…”

Thermodynamic Analysis of Metal-Insulator Transitions. Effect of Changes in the Density of States Function

“…Also, there are instances, such as the NiS 2−y Se y system, that exhibit a metal-insulator transition with rising temperature close to absolute zero [2]; the latter runs contrary to normal observations of an insulator-metal transition with rising T . These unusual situations have been rationalized lately by invoking the dynamic mean field theoretical approach, as championed by a number of research groups [3]. Their principal aim was to generate partial density-of-states curves which match experimental photoemission results, and to note changes concomitant to the occurrence of the metal-insulator transition.…”