Anomalous Metallic State in the Vicinity of Metal to Valence-Bond Solid Insulator Transition inLiVS2

Abstract

We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2, known for the formation of valence bond solid (VBS) state out of S = 1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first ordered transition from a paramagnetic metal to a VBS insulator at Tc ∼ 305 K upon cooling. The presence of VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudo-gap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K. A question that arises is whether or not similar melting of the VBS state and appearance of exotic metallic phases can occur in inorganic frustrated systems. In the inorganic systems, however, application of an external pressure is expected not to melt but to stabilize VBS due to a predominant volume effect. In CuIr 2 S 4 , the lattice shrinks appreciably in the VBS perhaps due to the formation of strongly bonded singlet molecules and the VBS can be stabilized through -pV (p : pressure, V : volume) term in the corresponding free energy [11][12]. Effects of negative pressure on the VBS states of inorganic systems, on the other hand, have not been investigated so far.The inorganic LiVO 2 in which the magnetic V 3+ ions (3d 2 , S = 1) form a triangular lattice is known to be a paramagnetic insulator with strong antiferromagnetic interactions between the localized S = 1 moments at high temperatures. Upon cooling, at T c ∼ 500 K, LiVO 2 exhibits a first ordered phase transition to a VBS state with a characteristic spin gap of ∼ 1600 K, evidenced by the formation of vanadium trimers. With this system, one can apply "negative" pressure by replacing oxygens with larger anions such as S and Se [13,14,16]. More over, the negative pressure may increase the overlap between V 3d and p-orbital (O 2p, S 3p, and Se 4p), and increase the electronic band width. Thus, this vanadium-based triangular system provides a good opportunity to study effects of negative pressure on VBS states in inorganic materials.In this Letter, we demonstrate that LiVS 2 is indeed an itinerant analogue of LiVO 2 with suppressed VBS. We found that in LiVS 2 a phase transition from a paramagnetic metal to a trimer VBS insulator occurs at T c ∼ 305 K that is lower than that of LiVO 2 . In LiVSe 2 with highest negative pressure, the phase transition is suppressed down to 2 K. In the high temperature metallic phase of LiVS 2 , strong temperature dependence of the bulk susceptibility, χ, was observed, which is similar to the pseudo-gap behavior found in underdoped superconducting cuprates. We argue this is an evidence for a pseudo-gap formation by short-range spin singlet fluctuations in the paramagnetic metallic phase of LiVS 2 .Powder samples of LiVS 2 , LiVSe 2 and their solid solution LiVS 2−x Se x were prepared by a soft-chemical method followed by a solid-state reactio...

Orbital reformation with vanadium trimerization ind2triangular lattice LiVO2revealed by51V NMR

“…[4][5][6][7][8][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.…”

“…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.…”

Ultrafast filling of an electronic pseudogap in an incommensurate crystal

“…The global energy resolution was 10 meV and the angular resolution 0.2 • . Reference band structure calculations for LiVS 2 [18] were performed within the local density approximation, using the Wien2K package [19]. TRPES experiments were performed with the FemtoARPES setup, using a Ti:Sapphire laser that generates 35 fs pulses centered at 790 nm with repetition rate of 30 kHz.…”

Spin-singlet trimer state induced by competing orbital order in triangular-lattice BaV10O15

“…3 At least for S = 1, the ground state is proposed to be a Néel state or a gapped spin-singlet state with orbital ordering, of which degeneracy is lifted into a spin-singlet trimer state in the presence of the strong electron-phonon coupling. 6 For real systems such as AMX 2 (A = Li, Na; M = Ti, V, Cr, X = O, S, Se), the moderate electron correlation gives a metal-insulator transition with the spin-singlet state 7,8 and an antiferromagnetic insulator with the reduced magnetic moment. 9 The presence or absence of the orbital ordering on the boundary of the metal-insulator transition has not been elucidated.…”