Professor Stephen Blundell

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.

Abstract:

The magnetoresistance of single crystals of the quasi-two-dimensional (Q2D) organic conductor (BEDT-TTF)3Cl2 2H2O has been studied at temperatures between 700 mK and 300 K in magnetic fields of up to 15 T and hydrostatic pressures of up to 20 kbar . Measurements of the resistivity using a direct-current van der Pauw technique at ambient pressure show that the material undergoes a metal-to-insulator transition at ̃150 K; below this temperature the resistivity increases by more than five orders of magnitude as the samples are cooled to 4.2 K. If the current exceeds a critical value, the sample resistivity undergoes irreversible changes, and exhibits non-ohmic behaviour over a wide temperature range. Below 30 K, either an abrupt increase of the resistivity by two orders of magnitude or bistable behaviour is observed, depending on the size and/or direction of the measurement current and the sample history. These experimental data strongly suggest that the metal-insulator transition and complex resistivity behaviour are due to the formation of a charge-density wave (CDW) with a welldeveloped domain structure. The magnetotransport data recorded under hydrostatic pressure indicate that pressure has the effect of gradually reducing the CDW ordering temperature. At higher pressures, there is a pressure-induced transition from the CDW state to a metallic, superconducting state which occurs in two distinct stages. Firstly, a relatively small number of Q2D carriers are induced, evidence for which is seen in the form of the magnetoresistance and the presence of Shubnikov-de Haas oscillations; in spite of the low carrier density, the material then superconducts below a temperature of ̃2-3 K. Subsequently, at higher pressures, the CDW state collapses, resulting in QID behaviour of the magnetoresistance, and eventual suppression of the superconductivity.

Abstract:

Magnetization and magnetotransport measurements have been used to study the composition dependence of the electronic properties of the Ruddlesden-Popper phases Sr2NdMn2O7 and Sr1.9Nd1.1Mn2O7. Although their behaviour differs in detail, both compounds show a colossal magnetoresistance (CMR) effect (>10 000% in 14 T) in the temperature range 4.2 ≤ T/K ≤ 100. However, neither material shows a transition to a ferromagnetic state above 4.2 K, and both materials have higher resistivities (>103 Ω cm for 4.2 ≤ T/K ≤ 100) than the metallic oxides previously found to show CMR. In view of the low conductivity and the absence of ferromagnetism, the CMR of these phases is not readily explained by a doubleexchange mechanism.