Muons and magnets

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.

Abstract:

The Fermi surface topology and electronic bandwidth of the first ever organic superconductor containing magnetic ions, the charge transfer salt beta''-(BEDT-TTF)(4)[(H2O)Fe(C2O4)(3)].[6H5CN, have been studied using angle-dependent magnetoresistance oscillation (AMRO) and infrared (IR) reflectivity measurements. This material has a superconducting transition temperature of 7 It. AMRO data have been recorded at 1.4 K in a field of 15 T for all possible orientations of the sample with respect to the applied field. Several features with very strong azimuthal dependence are observed. The IR reflectivity is also very anisotropic; the reflectivity for E parallel to the needle axis is eight times greater than for the perpendicular direction. The results are discussed in relation to the calculated band structure and Fermi surface.

Abstract:

Examples from the (R-1,R-2-DCNQI)(2)X family of molecular conductors have been studied using mu SR in order to provide information about the microscopic magnetic properties of the various phases. For the fully deuterated dimethyl Cu salt (d(8)-DMe-Cu) there is a metal-insulator (MI) transition around 80 K and a magnetic transition around 7 K. The muon spin relaxation rate becomes enhanced in the region of the MI transition and below, reflecting the quenching of valence fluctuations and the appearance of localised spins on the Cu sites. A zero field precession signal develops below 7 K as a result of the 3D magnetic ordering of the Cu spins; the field distribution derived from the precession frequency is consistent with the proposed magnetic structure. In addition to the zero field studies, nuclear quadrupolar level crossing resonance between the muon and the imine nitrogen of the DCNQI has been used to study the temperature dependence of the electronic stale of the molecular conductor.

Abstract:

Muon spin rotation (mu SR) studies of the vortex lattice in the superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2) have revealed a crossover from a quasi-2d to a vortex-line lattice structure for fields below a characteristic field B-cr. The mu SR-lineshapes measured from the vortex-line lattice have allowed a re-evaluation of the in-plane penetration depth.