A.A. House

Phase boundary in the dimensionality of angle-dependent magnetoresistance oscillations in the charge-transfer salt ?

Angle-dependent magnetoresistance oscillations (AMROs) have been studied in the isostructural charge-transfer salts and (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) in steady fields of up to 30 T. The shapes of the approximately elliptical quasi-two-dimensional (Q2D) Fermi surfaces that these organic metals possess have been determined at 30 T and are found to be in broad agreement with recent band-structure calculations. The Fermi surface of the salt undergoes a reconstruction at low fields and temperatures, resulting in a change in the dimensionality of the AMROs from Q2D character to quasi-one-dimensional character. This change is associated with the kink transition that is observed in magnetic field sweeps and is attributed to the formation of a spin-density wave ground state. The phase boundary of the change in the AMRO dimensionality has been followed to both the low-temperature high-field (about 23 T) and low-field high-temperature (about 8 K) extremes. The data are compared with recently proposed models of the AMROs and Fermi surfaces for these materials.

The low-temperature phase of : I. Angle and temperature dependence of the Shubnikov - de Haas and de Haas - van Alphen oscillations

The magnetoresistance and magnetization of single crystals of the organic charge-transfer salt (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) have been studied in fields of up to 30 T and at temperatures as low as 20 mK. Five separate series of quantum oscillations have been observed in the low-temperature, low-field phase of this material and have been studied as a function of tilt angle of the field. It is proposed that two of these frequencies are the result of Stark quantum interference while the others are Shubnikov - de Haas (SdH) and de Haas - van Alphen (dHvA) oscillations due to closed Fermi surface pockets or conventional magnetic breakdown. The unconventional temperature dependence observed for some of these oscillations and the applicability of current models of the Fermi surface of are discussed.

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.

Magnetoresistance studies on (BEDT-TTF)4(Mo6Cl8)Cl6.xCH2Cl2 under pressure

Abstract The charge on the (BEDT-TTF) molecules in the charge-transfer salt (BEDT-TTF) 4 (Mo 6 Cl 8 )Cl 6 .xCH 2 Cl 2 is not always distributed equally between the two crystallographically inequivalent molecular sites but depends on thermodynamic parameters. To investigate the resulting phase diagram of (BEDT-TTF) 4 (Mo 6 Cl 8 )Cl 6 .xCH 2 Cl 2 , magnetoresistance studies on single crystals were performed at temperatures as low as 800 mK, in fields of up to 16 T and at pressures of up to 1.6 GPa. At ambient pressure this salt shows a metal-insulator (MI) transition at ∼ 40 K that is thought to be due to a spin density wave state. Pressure reduces the MI transition temperature, suppressing it above 1 GPa. The magnetoresistance of this material at moderate pressures is strongly negative with resistance drops of up to 90 % at 16 T and 800 mK. With increasing pressure the magnetoresistance becomes more positive. Between 0.4 GPa to 0.7 GPa the magnetoresistance develops a hysteretic, temperature-independent peak in its field dependence. However, to achieve a positive magnetoresistance pressures in excess of 1.5 GPa are necessary.

Magnetic-field dependent Fermi surfaces in quasi-2D organic conductors

We review recent studies of charge-transfer salts of the ion bis(ethylenedithio)tetrathiafulvalene which exhibit spin-density wave ground states, concentrating on techniques involving high magnetic fields and experiments carried out by the oxford group and coworkers.

Fermi surface studies of low-dimensional organic conductors based on BEDT-TTF

Abstract This paper provides an introduction to charge-transfer salts of the ion bis(ethylenedithio)tetrathiafulvalene (ET) and their band-structure, and reviews some recent experiments on the salts involving high magnetic fields carried out by the Oxford group and coworkers.

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.

Angle dependent magnetoresistance oscillation study of the fermi surface of β″-(BEDT-TTF)2AuBr2

Abstract Angle dependent magnetoresistance oscillations (AMRO) have been studied for the charge-transfer salt β″-BEDT-TTF)2AuBr2. The temperature range 1,5 K–4.2 K and field region 0.5T–30 T have been explored. A series of AMRO peaks originating from a ∼40 T elliptical quasi-two-dimensional (Q2D) Fermi surface (FS) pocket have been observed at all fields. The measured orientation of this pocket is in good agreement with band structure calculations. Above a field of ∼10 T a second series of (Q2D) AMRO peaks emerge, indicating a field-induced reconstruction of the FS. It is proposed that this results from a shifting of the nesting vector of the spin density wave which is believed to be present in this material. The high field series of peaks correspond to a ∼222 T elliptical FS pocket with major axis aligned along the direction in which band structure calculations predict Q1D Fermi sheets. It is thus attributed to imperfect nesting of the Q1D FS sheets.

Temperature dependence of the angle-dependent magnetoresistance oscillations in α-(BEDT-TTF)2KHg(SCN)4

Abstract Angle dependent magnetoresistance oscillations (AMRO) have been studied for the charge-transfer salt α-(BEDT-TTF) 2 KHg(SCN) 4 . This material possesses a spin-density-wave (SDW) groundstate exhibiting AMRO with pronounced minima attributable to quasi-one-dimensional (Q1D) sections of Fermi surface (FS). Increasing the temperature or the magnetic field produces a change in the AMRO with the strong minima disappearing while pronounced peaks simultaneously evolve. These maxima are ascribable to a Q2D FS pocket. The change in form of the AMRO is thus associated with a reordering of the FS. This is brought about by removal of the SDW state causing reversion of the FS to the form close to that predicted by bandstructure calculations. The phase boundary between these two regimes has been tracked using AMRO from the high temperature limit to the high field limit for the first time and the shape of the Q2D sections of FS derived.

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 Shubnikov-de Haas and magnetic breakdown oscillations has allowed the pressure dependencies of the Fermi surface topology and the carrier 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.