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

Abstract

Static electrical and magnetic properties of single crystal BaVS3 were measured over the structural (TS = 240K), metal-insulator (TMI = 69K), and suspected orbital ordering (TX = 30K) transitions. The resistivity is almost isotropic both in the metallic and insulating states. An anomaly in the magnetic anisotropy at TX signals a phase transition to an ordered low-T state. The results are interpreted in terms of orbital ordering and spin pairing within the lowest crystal field quasi-doublet. The disordered insulator at TX < T < TMI is described as a classical liquid of non-magnetic pairs.Spatial ordering of the occupancy of degenerate electronic orbitals plays important role in the diverse magnetic phenomena of transition metal compounds [1]. 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. As an alternative, the possibility of an orbitally ordered ground state was discussed [4], while other proposals emphasized the quasione-dimensional character of the material [5][6][7]. The crystal structure is certainly suggestive of a linear chain compound since along the c axis, the intrachain V-V distance is only 2.81Å, while in the a-b plane the interchain separation is 6.73Å [8,9]. It is thus somewhat surprising that our present studies show that electrically BaVS 3 is nearly isotropic. This means that BaVS 3 provides one of the few realizations of a Mott transition within the non-magnetic phase of a three-dimensional system. Since this case (or rather its D → ∞ counterpart) is much studied theoretically, but scarcely investigated experimentally, a good understanding of BaVS 3 should be valuable for strong correlation physics in general.BaVS 3 has a metal-insulator transition at T MI = 69K, accompanied by a sharp spike in the magnetic susceptibility [5,10]. The high temperature phase is a strongly correlated metal with mean free path in the order of the lattice constant. There is no sign of a sharp Fermiedge in the UPS/XPS spectra [6] and instead of a Pauli-susceptibility it exhibits Curie-Weiss like behavior. Though the magnetic susceptibility is similar to that of an antiferromagnet [10,11], no long-range magnetic order develops at the transition [9,12]. The transition is clearly seen in the thermal expansion anomaly [5], and in the specific heat [7]. The d-electron entropy right above T MI is estimated as ∼ 0.8R ln 2, and it seems that a considerable fraction of the electronic degrees of freedom is frozen even at room temperature [7]. It appears that the 69K transition is not symmetry breaking [13]: it is a pure Mott transition which does not involve either magnetic order or any static displacement of the atoms.Hints of long range order were found well below T MI , at T X = 30K, in rece...

Pressure-induced suppression of the spin-gapped insulator phase inBaVS3: An infrared optical study

“…At ambient pressure BaVS 3 exhibits a phase transition from a high-temperature paramagnetic "bad metal" phase to a low-temperature singlet insulator state at T MI ≈ 70 K [4]. This is a second order phase transition, as it has been pointed out recently by the comparison of the anomalies observed in different thermodynamic properties [5].…”

Dynamical mean-field theory using Wannier functions: A flexible route to electronic structure calculations of strongly correlated materials

“…In this phase, BaVS 3 is a quite bad metal, with unusual properties such as a CurieWeiss susceptibility from which the presence of local moments can be inferred. At ϳ70 K a second continuous phase transition takes place, 95,96 namely a metal-insulator transition ͑MIT͒ below which BaVS 3 becomes a paramagnetic insulator. A doubling of the primitive unit cell [97][98][99] is accompanying the MIT.…”

“…When approaching T MIT these nearly localized electrons should surely contribute to the Curie-Weiss form of the magnetic susceptibility. 93,96 In fact the "bad-metal" regime 95 above the MIT, including significant changes in the Hall coefficient, 91 might largely originate from scattering processes involving the E g electrons. But even if the A 1g bands become gapped at the MIT, from an effective single-particle LDA viewpoint the remaining E g bands may still ensure the metallicity of the system.…”

Structural aspects of the metal-insulator transition in BaVS3

“…BaVS 3 is a quasi-1D metal which exhibits anomalous electronic and magnetic properties [4][5][6][7][8] . Furthermore, BaVS 3 has a unique electronic structure.…”