Magnetic correlations and quantum criticality in the insulating antiferromagnetic, insulating spin liquid, renormalized Fermi liquid, and metallic antiferromagnetic phases of the Mott systemV2O3

Abstract

Magnetic correlations in all four phases of pure and doped vanadium sesquioxide (V 2 O 3 ) have been examined by magnetic thermal-neutron scattering. Specifically, we have studied the antiferromagnetic and paramagnetic phases of metallic V 2Ϫy O 3 , the antiferromagnetic insulating and paramagnetic metallic phases of stoichiometric V 2 O 3 , and the antiferromagnetic and paramagnetic phases of insulating V 1.944 Cr 0.056 O 3 . While the antiferromagnetic insulator can be accounted for by a localized Heisenberg spin model, the long-range order in the antiferromagnetic metal is an incommensurate spin-density wave, resulting from a Fermi surface nesting instability. Spin dynamics in the strongly correlated metal are dominated by spin fluctuations with a ''single lobe'' spectrum in the Stoner electron-hole continuum. Furthermore, our results in metallic V 2 O 3 represent an unprecedentedly complete characterization of the spin fluctuations near a metallic quantum critical point, and provide quantitative support for the self-consistent renormalization theory for itinerant antiferromagnets in the small moment limit. Dynamic magnetic correlations for បϽk B T in the paramagnetic insulator carry substantial magnetic spectral weight. However, they are extremely short-ranged, extending only to the nearest neighbors. The phase transition to the antiferromagnetic insulator, from the paramagnetic metal and the paramagnetic insulator, introduces a sudden switching of magnetic correlations to a different spatial periodicity which indicates a sudden change in the underlying spin Hamiltonian. To describe this phase transition and also the unusual short-range order in the paramagnetic state, it seems necessary to take into account the orbital degrees of freedom associated with the degenerate d orbitals at the Fermi level in V 2 O 3 . ͓S0163-1829͑98͒06443-1͔

Spin-1 effective Hamiltonian with three degenerate orbitals: An application to the case of V2O3

“…This situation was realized by Mila et al in their paper 19 where they took up an old suggestion by J. Allen 20 that "the magnetic and optical properties of all the phases of V 2 O 3 show a loss of V 3+ -ion identity". These findings, together with the results by inelastic neutron scattering quoted above 6,10 indicate that the vertical bond is quite stable and coupled to total spin S tot = 2 with the non polar part of the wavefunction given by |ψ ±1 = (|0 a | ± 1 b + | ± 1 a |0 b )/ √ 2 (where a and b indicate the two V centers). This state is clearly doubly degenerate, due to the freedom in the choice of the two degenerate states | ± 1 .…”

“…The importance of the orbital degrees of freedom in the physics of V 2 O 3 was later confirmed by several experimental facts. A series of neutron scattering experiments 6,10 showed evidence that the wave vector of the short range antiferromagnetic correlations in both paramagnetic phases was rotated by 30 degrees with respect to that observed in the AFI phase. In all three phases the atoms along the vertical pairs are ferromagnetically coupled; however one passes from a complete antiferromagnetic coupling along the three in-plane bonds in the high temperature paramagnetic phases to one ferromagnetic and two antiferromagnetic in the low temperature phase, with consequent breaking of the corundum trigonal symmetry.…”

“…2 Actually it is the only known example among transition-metal oxides to show a PM to PI transition. 6 In the paramagnetic phases the crystal has the corundum structure, in which the V ions are arranged in V-V pairs along the c-hexagonal axis and form a honeycomb lattice in the basal ab plane. Each V 3+ −ion has 3d 2 configuration and is surrounded by an oxygen octahedron with a small trigonal distortion which lifts the three-fold degeneracy of t 2g orbitals into a nondegenerate a 1g and doubly degenerate e g orbitals separated by the distortion energy ∆ t , with the e g states lying lower.…”

Orbitally driven spin pairing in the three-dimensional nonmagnetic Mott insulatorBaVS3:Evidence from single-crystal studies

“…It seems that the insulator phase does not arise from a magnetic instability of the metallic phase. Let us recall the case of V 2 O 3 where the onset of orbital order [3] leads to an insulating state whose magnetic ordering pattern cannot be anticipated from the short range order found in either of the neighboring phases [2]. We propose a similar picture for BaVS 3 : the metal-insulator transition involves both the spin and orbital degrees of freedom, and considering only the effect on spins, it amounts to a change of the spin hamiltonian.…”

“…To cite a well-known example: the interplay of magnetic and orbital long range ordering, and strong coupling to the lattice, account for the metal-insulator transitions of the V 2 O 3 system [2,3]. In contrast, the metal-insulator transition of the S = 1/2, 3d 1 electron system BaVS 3 is not associated either with magnetic long range order, or with any detectable static spin pairing.…”