Professor Stephen Blundell

Abstract:

The muon spin rotation (mu SR) technique is primarily a powerful probe of the magnetic properties of materials; but it is also possible to measure charge-density related effects using muon quadrupolar level crossing resonance (QLCR). We present the first study of a charge density wave (CDW) using mu SR in TTF-TCNQ. The CDW develops below the metal-insulator transition at 54 K and we study the evolution of the CDW as a function of temperature using the QLCR resonances of the nitrogen atoms on the TCNQ molecules as the probe.

Abstract:

We present a model of a quasi-one-dimensional (Q1D) conductor with weak dispersion in the direction perpendicular to the chains in a magnetic field. Finite-energy electric dipole transitions between the eigenstates of the system constitute Fermi-surface traversal resonance, a Q1D analogue of cyclotron resonance. We extend the model to describe a (TMTSF)(2)X-like material, with dispersion in two directions perpendicular to the chains, and find that for certain orientations of the magnetic field large degeneracies occur between the magnetic-field induced states. These are angles at which maxima are observed experimentally in the d.c. conductivity, and thus we explain one class of angle-dependent magnetoresistance oscillations (AMRO) in terms of zero-energy electric dipole transitions between magnetic-field induced states.

Abstract:

We present the field dependent magnetization and magnetic susceptibility of an unusual metamagnet Co-2(OH)(2)(O2CC6H4CO2). It is characterized by a maximum in the susceptibility at the Neel temperature of 48 K and by spontaneous magnetization at a T-C of 43 K due to a small canting. It displays the characteristic kink of a metamagnetic transition in the field-dependent magnetization below T-N. Below T-C hysteresis is observed whose width increases to span +/- 5.2 T at 1.5 K. We propose a qualitative explanation of the rich phase behaviour.

Abstract:

Magnetic properties of purely organic radical ion salts and complexes based on nitronyl nitroxide (NN) derivatives were investigated. The salts and complexes are composed of N-alkylareniumyl NN as the radical cations and carboxylatoaryl, sulfonatoaryl and imidazolidyl NN as the radical anions. Tetracyanoquinodimethanide (TCNQ)(-) salts and complexes were also prepared. The results obtained for several complexes of a multifunctional molecule, 2-imidazoly NN (2-IMNN), are presented in detail, because 2-IMNN can be converted to various with p-carboxylatophenyl NN states by electron and proton transfers. The proton-transfer complex of 2-imidazoliniumyl NN (2-IMNN)(+*) with (p-CAPNN)(-*) shows magnetic behavior relevant to coexistence of intermolecular ferro- and antiferro-magnetic interactions. Several quinoliniumyl NN(QNNN)(+*)(p-CAPNN)(-*) complexes exhibit magnetic behavior similar to that observed in (2-IMNN)(+*)(p-CAPNN)(-*).

Abstract:

Muon-spin relaxation (mu SR) experiments on the conducting polymer poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) probe the dynamics of the highly mobile polarons created by the muon-implantation process in which muonium reacts with the polymer forming a radical state. The fluctuating spin density induced by the electronic spin defect rapidly diffusing up and down the chain leads to a characteristic relaxation, the temperature and field dependence of which permit the extraction of on-chain and inter-chain diffusion rates. These results provide information concerning: the charge-transport mechanism in DB-PPV.