Muons and magnets

Abstract:

We present the results of a muon-spin rotation/relaxation (mu SR) study in zero and longitudinal magnetic fields and temperatures down to 40 mK of para-cyanophenyl nitronyl nitroxide (pCNPNN). This layered material is expected to show ferromagnetic interactions on the basis of susceptibility measurements. In a zero-field mu SR experiment, the spins of muons which are implanted in diamagnetic states will precess in any local internal fields. Bence a study of precession frequencies and/or relaxation rates can be used to follow the magnetic order parameter as a function of temperature. We find strongly temperature dependent muon relaxation below similar to 0.17K similar to that found recently in 1-NAPNN, indicating a magnetic transition at this temperature.

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 0.5 T-30 T have been explored. A series of AMRO peaks originating from a approx.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 approx.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 approx.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:

Muon spin relaxation has been used to probe the highly anisotropic motion of polaronic charge carriers in the emeraldine base form of polyaniline. The measured diffusion rate along the polymer chain for negative polarons shows a metalliclike temperature dependence, which is saturated at low temperatures but becomes limited by phenylene ring librational scattering above 150 K. The interchain transport at room temperature is more than 4 orders of magnitude slower than the intrachain process and becomes further suppressed on cooling. © 1997 The American Physical Society.

Abstract:

A numerical model is used to derive the quantum oscillations in the magnetisation and magnetoresistance of quasi-two-dimensional α-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:

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.