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

Abstract

The metal-insulator (MI) transition in BaVS3 has been studied at ambient pressure and under hydrostatic pressure up to p = 26 kbar in the frequency range of 20 − 3000 cm −1 . The charge gap determined from the optical reflectivity is enhanced, ∆ ch (p)/kBTMI (p) ∼ 12. This ratio is independent of pressure indicating that the character of the transition does not vary along the p − T phase boundary. Above the critical pressure, pcr ∼ 20 kbar, metallic spectra were recorded in the whole temperature range, as expected from the shape of the phase diagram. Our results exclude the opening of a pseudogap above TMI at any pressure. Below TMI an unusually strong temperature dependence of the charge gap was observed, resulting in a ∆ ch (T ) deviating strongly from the mean field-like variation of the structural order parameter.A class of symmetry breaking phase transitions, characterized by an anomalously large gap parameter, has recently attracted much attention and has been investigated intensely in the wider framework of strongly correlated electron systems. Though in some manganites and nickelates the huge enhancement of the ∆/k B T c ratio, sometimes as large as ∼ 30, is accompanied by the opening of a pseudogap above T c [1, 2], in most cases a mean-field-like temperature dependence of ∆ is observed. In contrast, in first order metal-insulator transitions a more drastic, often discontinuous opening of the charge gap is seen. On the other hand, the transition in the inorganic spin-Peierls system CuGeO 3 looks almost like a first order one, as the opening of the singlet-triplet gap is much sharper than the BCS functional form and follows ∆ ∝ (T c − T ) β with β ≈ 0.1 instead of 0.5 [3]. In BaVS 3 , the detailed temperature dependence of neither the spin gap nor the charge gap has been measured so far. The present study of the infrared (IR) optical properties demonstrates that electron correlations play crucial role in BaVS 3 : they lead to a large enhancement of the ∆/k B T c ratio and simultaneously give rise to an abrupt temperature dependence of ∆.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. 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]. The suppression of the insulating phase is accompanied by a monotonic decrease of the spin gap [5]. Moreover, the phenomenon occurring at T MI ≈ 70 K at ambient pressure has been desc...

Infrared and Terahertz Spectroscopy of Strongly Correlated Electron Systems under Extreme Conditions

“…From 10.1 to 11.9 GPa, there is a much larger increase, which demonstrates a transition to metallic state at approximately 10 GPa. A similar study was also performed on the Cr-doped system V 1Àx Cr x O 2 , 66) and on other vanadium oxides with different compositions, i.e., V 2 O 3 with T c ¼ 220 K 67) and V 3 O 5 with T c ¼ 420 K. 68) Similar works have been reported on La 1Àx -Ca x MnO 3Ày , 69) and BaVS 3 , 70) where the pressure evolution of energy gap have been studied with IR-SR.…”

Infrared Conductivity of Elemental Bismuth under Pressure: Evidence for an Avoided Lifshitz-Type Semimetal-Semiconductor Transition

“…Ambient pressure spectra were measured in our usual optical cryostat [7]. Using a novel kerosene filled piston-cylinder cell [17] equipped with a large 2 mm diamond window we performed infrared reflection experiments on a sample mounted in contact with the window. A 2 angular miscut of the diamond allowed us to separate the contribution from the front and back interfaces.…”

Vanishing of the metal-insulator Peierls transition in pressurized BaVS3

“…This transition corresponds to a Peierls transition below T P = T MI that stabilizes a superstructure where the chain periodicity c is doubled. 5,6 The satellite reflections, evidencing the symmetry breaking at the MI transition, are located at the commensurate (C), (1, 0, 1 2 ) O reduced reciprocal wave vector in the orthorhombic setting. The superstructure consists of a tetramerization of the V chains, with an out-of-phase order between first neighboring chains.…”

“…In this context, the transition metal sulfide BaVS 3 , with both itinerant and localized electronic states together with a one-dimensional (1D) electronic anisotropy, is a model compound that displays, among its multiple instabilities, a metal-insulator (MI) transition that vanishes under pressure at a quantum critical point (QCP) at P cr ∼ 2 GPa. [3][4][5] Furthermore, in the vicinity of P cr , the restored metallic state behaves as a non-Fermi liquid (NFL). 4,5 The NFL behavior is ascribed to strong interactions of fermions with collective excitations of the incipient ordered phase.…”

“…[3][4][5] Furthermore, in the vicinity of P cr , the restored metallic state behaves as a non-Fermi liquid (NFL). 4,5 The NFL behavior is ascribed to strong interactions of fermions with collective excitations of the incipient ordered phase. In this paper, we propose that the MI transition associated with the Peierls ground state of BaVS 3 is destroyed by pressure, which induces a very efficient CDW depairing mechanism via a finite lifetime of the electron-hole pairs of the charge density wave (CDW) condensate, to lead finally to the QCP.…”