Muons and magnets

Abstract:

Electrons in cyclotron orbits around closed pockets in the Fermi surfaces of organic metals possess an oscillating real-space velocity. Higher harmonics of this real-space velocity lead to predicted cyclotron resonances additional to those normally expected. We calculate these resonances semiclassically using the Boltzmann transport equation. These higher harmonics are expected to occur remarkably often, and we show that they are found even in a very simple tight-binding model. A similar effect occurs in quasi-one-dimensional Fermi surfaces which are highly corrugated and in this case the oscillating part of the real-space velocity as it traverses the Fermi surface can couple directly to the microwave frequency. © 1997 The American Physical Society.

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.