Professor Stephen Blundell

Abstract:

We have carried out implanted positive muon studies on the molecular metal system d(n)(DMe-DCNQI)(2)Cu in order to understand better its novel magnetic properties. Examples of these salts at different levels of deuteration were studied. The fully deuterated (d(8)) salt shows a metal-insulator (MI) transition around 80 K and a magnetic transition around 7 K. The muon spin relaxation rate is enhanced below the MI transition, reflecting the localisation of spins along the Cu columns, however, the increase in muon spin relaxation rate occurs well above the metal-insulator (MI) transition and suggests a slowdown of the spin fluctuations around 120 K. At temperatures below 7 K a zero field precession signal was observed as a result of the 3D magnetic ordering of the Cu spins. For a muon site associated with the ring of the DCNQI molecule, the local field distribution was found to be consistent with the previously proposed magnetic structure. A sharp nuclear quadrupolar level crossing resonance (QLCR) was observed at 50 G which was assigned to resonance with the imine nitrogen on the DCNQI molecule.

Abstract:

The magnetoresistance and magnetization of single crystals of the organic charge-transfer salt α-(BEDT-TTF)2KHg(SCN)4 (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) have been studied in fields of up to 30 T and at temperatures as low as 20 mK. Five separate series of quantum oscillations have been observed in the low-temperature, low-field phase of this material and have been studied as a function of tilt angle of the field. It is proposed that two of these frequencies are the result of Stark quantum interference while the others are Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) oscillations due to closed Fermi surface pockets or conventional magnetic breakdown. The unconventional temperature dependence observed for some of these oscillations and the applicability of current models of the Fermi surface of α-(BEDT-TTF)2KHg(SCN)4 are discussed.

Abstract:

The magnetoresistance of α-(BEDT-TTF)2MHg(SCN)4 (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and M = NH4 or K) has been studied under pressures of up to 14.8 kbar and for temperatures down to 0.7 K. The ≃671 T (α) and ≃4270 T (β) Shubinkov-de Haas oscillations observed in the ambient pressure magentoresistence of α-(BEDT-TTF)2KHg(SCN)4 persist to the highest pressure while the other quantum oscillatory frequencies are removed under pressure. A strong second-harmonic component of the α-frequency oscillations is observed on the raw data at 1 bar and is initially suppressed by pressure but returns above ≃9 kbar. Furthermore, in the salt α-(BEDT-TTF)2NH4Hg(SCN)4 a similar pronounced second harmonic of the quantum oscillations has been observed at the highest pressures. The origins of these features of the data are discussed in the context of current models of the Fermi surfaces of these materials.

Abstract:

In undoped pure single crystals of the mixed valence compound SmB6 anomalous ESR absorption is observed in the frequency range v=40-120 GHz at temperatures of 1.8-4.2 K. The ESR for the case of the coherent ground state consists of two components corresponding to g-factors g1=1.907±0.003 and g2=1.890±0.003. The amplitude of both ESR lines strongly depends on temperature in the temperature range studied: the amplitude of the first line with g=gt increases and the amplitude of the second line decreases with temperature. A model based on consideration of intrinsic defects in the SmB6 crystalline lattice, with a density ∼1015-1016 cm-3, is suggested as an explanation for the anomalous ESR-behavior. In the frequency range v>70 GHz at T=4.2 K, in addition to the main ESR lines, a new magnetic resonance with a hysteretic field dependence is discovered. © 1996 American Institute of Physics.

Abstract:

Angle-dependent magnetoresistance oscillations (AMROs) have been studied in the isostructural charge-transfer salts α-(BEDT-TTF)2KHg(SCN)4 and α-(BEDT-TTF)2NH4Hg(SCN)4 (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) in steady fields of up to 30 T. The shapes of the approximately elliptical quasi-two-dimensional (Q2D) Fermi surfaces that these organic metals possess have been determined at 30 T and are found to be in broad agreement with recent band-structure calculations. The Fermi surface of the salt α-(BEDT-TTF)2KHg(SCN)4 undergoes a reconstruction at low fields and temperatures, resulting in a change in the dimensionality of the AMROs from Q2D character to quasi-one-dimensional character. This change is associated with the kink transition that is observed in magnetic field sweeps and is attributed to the formation of a spin-density wave ground state. The phase boundary of the change in the AMRO dimensionality has been followed to both the low-temperature high-field (about 23 T) and low-field high-temperature (about 8 K) extremes. The data are compared with recently proposed models of the AMROs and Fermi surfaces for these materials.