Professor Stephen Blundell

Abstract:

Angle dependent magnetoresistance oscillations (AMRO) have been studied for the charge-transfer salt beta ''-(BEDT-TTF)(2)AuBr2. The temperature range 1.5 K-4.2 K and field region 0.5 T-30 T have been explored. A series of AMRO peaks originating from a similar to 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 similar to 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 similar to 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.

Abstract:

A numerical model is used to derive the quantum oscillations in the magnetisation and magnetoresistance of quasi-two-dimensional alpha-phase BEDT-TTF charge-transfer salts in high magnetic fields. Recent experimental results are simulated and the standard Lifshitz-Kosevich formalism is shown to be no longer appropriate.

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 alpha-(BEDT-TTF)(2)KHg(SCN)(4) from a spin-density-wave (SDW) state at a pressure of P-c similar to 5 kbar. SdH measurements have also been carried out with the sample at a variety of field orientations. Frequencies of lambda approximate to 181 T, alpha approximate to 671 T, mu approximate to 775 T, nu approximate to 856 T and beta approximate to 4270 T are observed in the Fourier spectra of the ambient pressure oscillations. The lambda, mu, nu and beta oscillations and also the large second harmonic component of the alpha orbit SdH waveform are not observed in the high field (>23T) metallic state, implying that they are a characteristic of the ambient pressure SDW state. Pressure suppresses the lambda, mu and nu frequencies but the a and P orbits are seen at all pressures. The large second harmonic of the waveform of the a orbit SdH is initially suppressed by pressure but reemerges above similar to 10 kbar. The origins of this are discussed.

Abstract:

Magnetisation measurements performed on the charge-transfer salt alpha-(BEDT-TTF)(2)TlHg(SCN)(4) in pulsed magnetic fields reveal the existence of eddy current ''resonances'' in the high field state, indicating the presence of deep minima in the transverse magnetoresistivity (or ideal conducting phases). Their behaviour can be explained qualitatively in terms of enhanced conductivity due to the quantum Hall effect, which becomes possible even when quasi-one-dimensional carriers are present.

Abstract:

Angle dependent magnetoresistance oscillations (AMRO) have been studied for the charge-transfer salt alpha-(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.