A study of the magnetoresistance of the charge-transfer salt at hydrostatic pressures of up to 20 kbar: evidence for a charge-density-wave ground state and the observation of pressure-induced superconductivity

Abstract

The magnetoresistance of single crystals of the quasi-two-dimensional (Q2D) organic conductor has been studied at temperatures between 700 mK and 300 K in magnetic fields of up to 15 T and hydrostatic pressures of up to 20 kbar. Measurements of the resistivity using a direct-current van der Pauw technique at ambient pressure show that the material undergoes a metal-to-insulator transition at ; below this temperature the resistivity increases by more than five orders of magnitude as the samples are cooled to 4.2 K. If the current exceeds a critical value, the sample resistivity undergoes irreversible changes, and exhibits non-ohmic behaviour over a wide temperature range. Below 30 K, either an abrupt increase of the resistivity by two orders of magnitude or bistable behaviour is observed, depending on the size and/or direction of the measurement current and the sample history. These experimental data strongly suggest that the metal - insulator transition and complex resistivity behaviour are due to the formation of a charge-density wave (CDW) with a well-developed domain structure. The magnetotransport data recorded under hydrostatic pressure indicate that pressure has the effect of gradually reducing the CDW ordering temperature. At higher pressures, there is a pressure-induced transition from the CDW state to a metallic, superconducting state which occurs in two distinct stages. Firstly, a relatively small number of Q2D carriers are induced, evidence for which is seen in the form of the magnetoresistance and the presence of Shubnikov - de Haas oscillations; in spite of the low carrier density, the material then superconducts below a temperature of . Subsequently, at higher pressures, the CDW state collapses, resulting in Q1D behaviour of the magnetoresistance, and eventual suppression of the superconductivity.

Robust superconducting state in the low-quasiparticle-density organic metalsβ″−(BEDT−TTF)4[(Bangura1, Coldea2, Singleton3 et al. 2005Phys. Rev. B

“…For T ജ 10 K , zz is initially ͑B Շ 3 T͒ proportional to B 2 ; at higher fields, the resistivity varies approximately linearly with B. Such behavior is entirely typical of metallic chargetransfer salts 21,27 and other layered systems; 28,29 the magnetoresistance is presumably mainly associated with the metallic regions of the samples.…”

C13NMR study of the charge-ordered state near the superconducting transition in the organic superconductorβ′′−(BEDT-T

“…Charge instability is also observed in quasi-two-dimensional organic conductors (BEDT-TTF) 2 X [bis(ethylenedithio)-tetrathiafulvalene] salts. In some BEDT-TTF salts, superconductivity appears directly after the suppression of the CO transition by applying chemical or mechanical pressure [10][11][12]. However, the CO transition temperature is much higher than the SC transition temperature T c [13,14], and the two phenomena thus appear * yihara@phys.sci.hokudai.ac.jp unrelated.…”

Superconductivity, incoherence and Anderson localization in the crystalline organic conductor (BEDT-TTF)3Cl2$middot$ 2H2O at high pressures

“…P c is the critical pressure at which the onset of the CDW state tends to zero temperature and the pressure that separates the region of saturating magnetoresistance from the region of diverging magnetoresistance in fields of up to 15 T as shown by the dotted line. In Reference [10] P c is found to be close to 12 kbar. state is frequently adjacent to a density-wave state caused by nesting of quasi-onedimensional (Q1D) pieces of the Fermi surface, and this leads to the suggestion that superconductivity is mediated by density wave fluctuations. The topology of the Fermi surface will obviously have an effect on its nesting properties.…”

“…In a pressure and temperature phase diagram the superconducting To whom correspondence should be addressed (p.goddard@physics.ox.ac.uk) Figure 1. The phase diagram of (BEDT-TTF) 3 Cl 2 ·2H 2 O proposed by Lubczynski et al [10]. P c is the critical pressure at which the onset of the CDW state tends to zero temperature and the pressure that separates the region of saturating magnetoresistance from the region of diverging magnetoresistance in fields of up to 15 T as shown by the dotted line.…”

“…Superposed over the top of this magnetoresistance they observe low-amplitude, low-frequency Shubnikovde Haas (SdH) oscillations, indicating that the Q2D carriers are still present [10]. Lubczynski et al go on to state that if the pressure is still further increased above 14 kbar the superconducting state is destroyed and the material becomes metallic.…”