Muons and magnets

Abstract:

Muon spin rotation (μSR) measurements have been performed on the organic superconductor κ-(BEDT-TTF)2Cu(SCN)2in order to investigate its exotic vortex behavior. Previously unobserved features of the μSR line shapes have been measured at low fields and temperatures. In the mixed state the existence of a lattice composed of linear vortices is demonstrated at low fields. The breakup of this well-ordered lattice has furthermore been observed as a function of both field and temperature. © 1997 The American Physical Society.

Abstract:

Muon spin rotation (μSR) experiments can be especially helpful in the study of various organic molecular materials. The technique has been used to study the magnetization in nitronyl nitroxide organic ferromagnets, the spin-density-wave ground state in various charge transfer salts, the dynamics of carriers in organic conductors and the vortex structures in organic superconductors. I outline the motivation for studying these new materials and review and discuss the results of recent work.

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 Pcapprox. 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 characteristic 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 approx.10 kbar. The origins of this are discussed.

Abstract:

Magnetisation measurements performed on the charge-transfer salt α-(BEDT-TTF)2TlHg(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 behavior 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 α-(BEDT-TTF)2KHg(SCN)4. This material possesses a spin-density-wave (SDW) groundstate exhibiting AMRO with pronounced minima attributable to quasi-one-dimensional (QID) 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.