W. Lubczynski

Magnetotransport studies of the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 under pressure: the relationship between carrier effective mass and critical temperature

Magnetotransport measurements have been carried out on the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 at temperatures down to 500 mK and in hydrostatic pressures up to 16.3 kbar. The observation of Shubnikov-de Haas and magnetic breakdown oscillations has allowed the pressure dependences of the area of the closed pocket of the Fermi surface and the carrier effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature Tc. The effective mass measured by the temperature dependence of the Shubnikov-de Haas oscillations is found to fall rapidly with increasing pressure up to a critical pressure Pc approximately=5 kbar. Above Pc a much weaker pressure dependence is observed; Tc also falls rapidly with pressure from 10.4 K at ambient pressure to zero at around Pc. This strongly suggests that the enhanced effective mass and the superconducting behaviour are directly connected in this organic superconductor. A simplified model of the band structure of kappa -(BEDT-TTF)2Cu(NCS)2 has been used to derive the bare band masses of the electrons from optical data. Comparisons of these parameters with cyclotron resonance data and the effective masses measured in the present experiments indicate that the greater part of the enhancement of the effective mass necessary for superconductivity in this material is due to quasiparticle interactions, with the electron-phonon interactions playing a secondary role. The dependence of Tc on the effective mass may be fitted satisfactorily to a suitably parametrized weak-coupling BCS expression, although this cannot be taken as a definitive proof of the nature of superconductivity in organic conductors.

The effects of open sections of the Fermi surface on the physical properties of 2D organic molecular metals

Abstract We report two cases in which the quasi-one-dimensional (Q1D) sections of Fermi surface (FS) in ET salts contribute to the low temperature behaviour of the magnetoresistance (MR). In κ-(ET)2Cu(NCS)2, breakdown orbits are observed between the Q2D and Q1D sections of FS; the MR oscillations contain a probable contribution from the Stark quantum interference effect. The results for the FS parameters are compared with theoretical calculations and the effects of many-body interactions are deduced. In α-(ET)2KHg(NCS)4, nesting of the Q1D open orbits gives a SDW ground state which considerably modifies the FS and leads to a field-induced transition known as the ‘kink’ between 22 and 23 T. Measurements of MR as a function of orientation of the crystal in the magnetic field reveal that the changes in FS at the ‘kink’ are rather subtle.

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

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.

Relationship between effective mass and superconducting critical temperature in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

Abstract We report high pressure magnetotransport on the organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 . The observation of Shubnikovde Haas and magnetic breakdown oscillations has allowed the pressure dependences of the Fermi surface topology and quasiparticle effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature T c . The data strongly suggest that the enhancement of the effective mass and the superconducting behaviour are directly connected. The results are fitted by calculations of the linearised Eliashberg equations.

The low-temperature phase of : II. Pressure dependence of the Shubnikov - de Haas oscillations

The magnetoresistance of (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and or K) has been studied under pressures of up to 14.8 kbar and for temperatures down to 0.7 K. The and Shubnikov - de Haas oscillations observed in the ambient pressure magnetoresistance of persist to the highest pressure while the other quantum oscillatory frequencies are removed under pressure. A strong second-harmonic component of the -frequency oscillations is observed on the raw data at 1 bar and is initially suppressed by pressure but returns above . Furthermore, in the salt a similar pronounced second harmonic of the quantum oscillations has been observed at the highest pressures. The origins of these features of the data are discussed in the context of current models of the Fermi surfaces of these materials.

Pressure and angle-dependent shubnikov-de haas studies of the spin-density-wave state of α-(BEDT-TTF)2KHg(SCN)4

Abstract Shubnikov-de Haas (SdH) experiments involving pressures of up to 15 kbar, temperatures as low as 500 mK and magnetic fields as high as 17 T have been carried out in order to probe the proposed phase transition of α-(BEDT-TTF)2KHg(SCN)4 from a spin-density-wave (SDW) state at a pressure of Pc∼ 5 kbar. SdH measurements have also been carried out with the sample at a variety of field orientations. Frequencies of λ≈181 T, α≈671 T, μ≈775 T, ν≈856 T and β≈4270 T are observed in the Fourier spectra of the ambient pressure oscillations. The λ, μ, ν and β oscillations and also the large second harmonic component of the α orbit SdH waveform are not observed in the high field (>23T) metallic state, implying that they are a characteristics of the ambient pressure SDW state. Pressure suppresses the λ, μ and ν frequencies but the α and β orbits are seen at all pressures. The large second harmonic of the waveform of the α orbit SdH is initially suppressed by pressure but re-emerges above ∼10 kbar. The origins of this are discussed.

Fermi surface studies of the pressure induced organic superconductor (ET)3Cl2.2H2O

Abstract The effects of temperature, pressure and magnetic field on the electrical transport of single crystal (ET)3Cl2.2H2O are reported. Increasing pressure gradually reduces the ordering temperature of a charge density wave ground state from ∼160 K at 1 bar to 6 K at 10.2 kbar. A superconducting state with TC>4 K is stabilised between 10.2 kbar and 13.5 kbar. Above 12.5 kbar, the observation of Shubnikov-de Haas oscillations allows the pressure dependences of the area of a closed Fermi surface pocket and the associated carrier effective mass to be deduced.

Fermi surface studies of tbe pressure induced organic superconductor (BEDT-TTF)/sub 3/Cl/sub 2/.2H/sub 2/O

Summary form only given. We present Fermi surface studies of the organic superconductor, (BEDT-TTF)/sub 3/Cl/sub 2/.2H/sub 2/O, in the pressure range 10-20 kbar, temperature range 700 mK-4.2 K and in magnetic fields up to 17 T. Superconductivity is observed above 700 mK in the pressure range 10-14.5 kbar and is accompanied by a single series of Shubnikov-de Haas oscillations, corresponding to ca. 0.5-1% of the first Brillouin zone. The frequency increases monotonically from 45 T at 12.5 kbar to 100 T at 20 kbar. The mass of the carriers obtained from fitting the temperature dependence of the amplitude of the Fourier transform to the Lifshitz-Kosevich model, is roughly pressure independent. The value of 0.8 m/sub e/ obtained is low for such materials; in other BEDT-TTF salts we have observed enhancement from the electronic band mass which has been associated with strong electron correlations in these narrow band conductors. Results of the present study will be discussed in relation to the crystal and electronic band structures and the Hall resistance, thermopower, susceptibility and optical reflectivity.

High field magnetotransport of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 under pressure

Abstract We report high pressure magnetotransport on the organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 . Pressure dependences of the area of the closed pocket of the Fermi surface, the effective mass of the carriers and the energy gap between Fermi surface sections have been measured. The effective mass is found to fall rapidly with increasing pressure in the region below a critical pressure P c ≈ 5 kbar . Above P c a much weaker pressure dependence is observed. The superconducting T c also falls rapidly with pressure, disappearing around P c . This demonstrates clearly that the enhancement of the transport mass and the superconducting transition are directly correlated in this organic superconductor.