Pressure Induced Quantum Critical Point and Non-Fermi-Liquid Behavior inBaVS3

Abstract

The phase diagram of BaVS3 is studied under pressure using resistivity measurements. The temperature of the metal to nonmagnetic Mott insulator transition decreases under pressure, and vanishes at the quantum critical point pcr = 20kbar. We find two kinds of anomalous conducting states. The high-pressure metallic phase is a non-Fermi liquid described by ∆ρ ∝ T n where n =1.2-1.3 at 1K< T <60K. At p < pcr, the transition is preceded by a wide precursor region with critically increasing resistivity which we ascribe to the opening of a soft Coulomb gap.Understanding the Mott transition, and clarifying the nature of the phases on either side of the transition, is a matter of great importance. Though metal-insulator transitions are often accompanied by an ordering transition and/or influenced by disorder, one may speak about a "pure" Mott transition which is a local correlation effect in an ideal lattice fermion system, and takes place without breaking any global symmetry. Many aspects of this problem can be studied on the multifaceted behavior of BaVS 3 [1-3].The metal-insulator transition of the nearly isotropic 3D compound BaVS 3 offers a realization of the pure Mott transition in nature [3]. Under atmospheric pressure BaVS 3 has three transitions: the hexagonal-toorthorhombic transition at T S = 240K which has only a slight effect on the electrical properties; the metalinsulator transition at T MI = 69K, which does not seem to break any of the symmetries of the metallic phase; and the ordering transition at T X = 30K [4]. In spite of decades of effort, the character of the phases and the driving force of the transitions at T MI and T X , remain mysterious.Here we report the results of single crystal resistivity measurements under hydrostatic pressure in the range of 1bar ≤ p < 25kbar. These pressures encompass the entire insulating phase and part of a high-pressure low-T conducting phase. We report the first observation of the quantum critical point in BaVS 3 , and we characterize the strange metallic phase lying beyond the critical pressure p cr . On the metallic side of the phase boundary, we identify two regimes with anomalous properties: (i) a broad region at p < p cr in which the resistivity increases strongly with decreasing temperature, and (ii) a high-pressure non-Fermi-liquid state.Single crystals of BaVS 3 were grown by Tellurium flux method. The crystals, obtained from the flux by sublimation, have typical dimensions of 3 × 0.5 × 0.5 mm 3 . The resistivity was measured in four probe arrangement. The current was kept low enough to avoid the self-heating of the sample. For the high-pressure measurements the crystal was inserted into a self-clamping cell with kerosene as a pressure medium. The pressure was monitored in-situ by an InSb sensor. During cooling down the cell there was a slight pressure loss, but its influence on the temperature dependence of the resistivity was negligible. Above about 15 kbar the pressure was stable within 0.1 kbar in the whole temperature range.

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

“…At p = 26 kbar neither the opening of the gap nor the sharpening of the 100 cm −1 phonons can be detected indicating that the material remains metallic. This is in agreement with the shape of the p − T phase boundary and the value of the critical pressure p cr ≈ 20 kbar determined by resistivity measurements [8]. As the temperature is lowered, the onset of the insulating state is rather abrupt, and we observed only a weak change in the reflectivity spectra of the insulator between T MI and T = 7 K at any pressure.…”

“…The critical pressure above which the metallic phase extends over the whole temperature range is p cr ≈ 20 kbar [8]. The suppression of the insulating phase is accompanied by a monotonic decrease of the spin gap [5].…”

“…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]. The observation of the crystal symmetry lowering [6] in more recent X-ray experiments provided direct evidence for the second order character of the transition at T MI ≈ 70 K.The metallic nature of the compound is enhanced by the application of hydrostatic pressure and the transition temperature is suppressed at an average rate of ∆T MI /∆p ≈ 3.4 K/kbar [7,8]. The critical pressure above which the metallic phase extends over the whole temperature range is p cr ≈ 20 kbar [8].…”

Singular or non-Fermi liquids

“…Related properties have also been found in U 2 Pt 2 In [88,89] [250]. The SFL properties observed at the Mott insulator-to-metal transition in BaVS 3 [99] are also of related interest. A good example of an antiferromagnetic QCP in itinerant electrons is in the alloy series Cr 1−x V x for which the magnetic correlations have been measured [122].…”

1.5 GPa compact double-wall clamp cell for SANS and NSE studies at low temperatures and high magnetic fields

“…It has a pronounced influence on the structural, magnetic, electric, chemical and mechanical properties of condensed materials [11]. In addition, the experimental investigation of some of the most fascinating phenomena in condensed matter physics, such as quantum critical points [3,6,15], hidden order [2,15] or non-Fermi liquid behavior [6], often require the combination of low temperatures, high magnetic fields and high pressures. In this field of research, neutron scattering is an invaluable probe as it provides direct microscopic information on both the structural and magnetic correlations.…”