Orbital reformation with vanadium trimerization ind2triangular lattice LiVO2revealed by51V NMR

Abstract

LiVO 2 is a model system of the valence bond solid (VBS) in 3d 2 triangular lattice. The origin of the VBS formation has remained controversial. We investigate the microscopic mechanism by elucidating the d orbital character via on-site 51 V NMR measurements in a single crystal up to 550 K across a structural transition temperature T c . The Knight shift, K, and nuclear quadrupole frequency, δν, show that the 3d orbital with local trigonal symmetry are reconstructed into a d yz d zx orbital order below T c . Together with the NMR spectra with three-fold rotational symmetry, we confirm a vanadium trimerization with d-d σ bonds. The Knight shift extracts the large Van-Vleck orbital susceptibility, χ VV = 3.6 × 10 −4 , in a paramagnetic state above T c , which is comparable to the spin susceptibility. The results suggest that orbitally induced Peierls transition in the proximity of the frustrated itinerant state is the dominant driving force of the trimerization transition. PACS numbers: 71.30.+h, 75.40.Gb Valence bond ordering in two-and three-dimensional systems is a manifestation of geometrical frustration.1 Across Mott transition, a valence bond solid (VBS) phase resides nearby unconventional metal 2,3 and superconducting phases 3 in triangular lattice systems. The VBS transition can accompany by d orbital orders with versatile textures, depending on lattice geometry and band filling. 4-9 However, d orbital orders governed by frustrated intersite interactions are not apparent from lattice distortions and need to be detected by microscopic probes. Here we show a tool for detailed characterization of d 2 orbital ordering in a model material LiVO 2 with the triangular lattice.ABX 2 (A:alkali metal; B: transition metal; X: chalcogen ion) with an ordered lock-type structure is a prototype of triangular-lattice antiferromagnet. 9 For Ti 3+ and V 3+ , t 2g orbital degrees of freedom play crucial roles in determining magnetic ground states. Among ABX 2 , LiVO 2 has been extensively investigated as a candidate of VBS.10-14 The crystal structure is comprised of the triangular lattice of V 3+ (3d 2 ) ions in a rhombohedral R3c structure [ Fig. 1(a)]. A trigonal elongation of VO 6 along the c axis splits the t 2g sublevel into a lower a 1g singlet and an upper e15 Calculated orbital occupations are almost equivalent in the a 1g and e ′ g orbitals due to the small trigonal field of 25 meV compared to the bandwidth.15 Below a first-order structural transition temperature T c ∼ 450-500 K, a superlattice Fig. 1(a)] appears in x-ray and electron diffraction measurements.11-14 Recent pair distribution function analyses of synchrotron data propose lowersymmetry lattice distortions depending on the layers. 16 Although the way of displacements of vanadium ions is compatible to trimerization, 12,13,16 the orbital character has not been accessible by the diffraction measurements due to Li deficiency and crystal twinning. Thus, the origin of the spin-singlet state in LiVO 2 remains controversial and to be elucidated via direct obse...

Possible phase separation and weak localization in the absence of a charge-density wave in single-phase1T-VS2

“…From the electron spin resonance of the V 3+ ion 37,38 , we take A iso = −85 kOe/µ B to obtain χ d = 6 ×10 −5 emu/mol, which corresponds to a density of states g(E F ) ≈ 2.0 states eV −1 cell −1 . We finally obtain χ orb = χ 0 − χ d ≈ 2 × 10 −4 emu/mol, comparable with the values reported in the related compound LiVO 2 39 .…”

Orbital Molecules in the New Spinel GaV2O4

“…In both VO2 and LiVO2, the distortion is driven by orbital polarizations that result in significant d-d σ-bonding interactions. 17,18 This is similar to the orbitally induced Peierls transition used to describe the formation of orbital dimer states in the spinels CuIr2S4 and MgTi2O4. 19 However, we have recently found a very different type of orbital molecule behavior in the spinel AlV2O4.…”

Hidden spin-orbital hexagonal ordering induced by strong correlations in LiVS2

“…In the insulator, a spin-Peierlslike lattice distortion occurs [4], leading to a trimerization in the quasi-two-dimensional arrangement of VS 6 octahedra [21]. Orbital ordering has long been suggested to be a vital ingredient of the low-temperature phase in many layered vanadates [22] and experimental evidence appeared recently [23,24]. Profound questions exist in view of the T -dependent electronic states and in general concerning the interplay of the various degrees of freedom in vicinity to the MIT in the case of LiVS 2 .…”