Professor Stephen Blundell

Abstract:

We describe how muon-spin rotation (mu+SR) can be particularly effective in determining the onset temperature of 3D ordering in low-dimensional molecular magnets, even when the low dimensionality means that this transition is masked in data from bulk thermodynamic probes such as heat capacity measurements. We illustrate the use of mu+SR with various copper-based magnets, including copper pyrazine dinitrate, Cu(C4H4N2)(NO3)(2), which orders below 0. 107 K, and also for the organic radical-ion salt DEOCC-TCNQF(4) which appears to be one of the most ideal examples of 1D S = 1/2 Heisenberg anti ferromagnets yet discovered. (c) 2007 Elsevier Ltd. All rights reserved.

Abstract:

On cooling through the transition temperature T(c) of a conventional superconductor, an energy gap develops as the normal-state charge carriers form Cooper pairs; these pairs form a phase-coherent condensate that exhibits the well-known signatures of superconductivity: zero resistivity and the expulsion of magnetic flux (the Meissner effect). However, in many unconventional superconductors, the formation of the energy gap is not coincident with the formation of the phase-coherent superfluid. Instead, at temperatures above the critical temperature a range of unusual properties, collectively known as 'pseudogap phenomena', are observed. Here we argue that a key pseudogap phenomenon-fluctuating superconductivity occurring substantially above the transition temperature-could be induced by the proximity of a Mott-insulating state. The Mott-insulating state in the kappa-(BEDT-TTF)2X organic molecular metals can be tuned, without doping, through superconductivity into a normal metallic state as a function of the parameter t/U, where t is the tight-binding transfer integral characterizing the metallic bandwidth and U is the on-site Coulomb repulsion. By exploiting a particularly sensitive probe of superconducting fluctuations, the vortex-Nernst effect, we find that a fluctuating regime develops as t/U decreases and the role of Coulomb correlations increases.

Abstract:

On cooling through the transition temperature Tc of a conventional superconductor, an energy gap develops as the normal-state charge carriers form Cooper pairs; these pairs form a phase-coherent condensate that exhibits the well-known signatures of superconductivity: zero resistivity and the expulsion of magnetic flux (the Meissner effect). However, in many unconventional superconductors, the formation of the energy gap is not coincident with the formation of the phase-coherent superfluid. Instead, at temperatures above the critical temperature a range of unusual properties, collectively known as 'pseudogap phenomena', are observed. Here we argue that a key pseudogap phenomenon-fluctuating superconductivity occurring substantially above the transition temperature-could be induced by the proximity of a Mott-insulating state. The Mott-insulating state in the κ-(BEDT-TTF) 2X organic molecular metals can be tuned, without doping, through superconductivity into a normal metallic state as a function of the parameter t/U, where t is the tight-binding transfer integral characterizing the metallic bandwidth and U is the on-site Coulomb repulsion. By exploiting a particularly sensitive probe of superconducting fluctuations, the vortex-Nernst effect, we find that a fluctuating regime develops as t/U decreases and the role of Coulomb correlations increases. ©2007 Nature Publishing Group.

Abstract:

Magnetic materials in which the fundamental building block is molecular have attracted a great deal of interest because they capitalize on the flexibility inherent in carbon chemistry. Some compounds incorporating chemically stable free radicals are purely organic and show long-range magnetic order at very low temperatures, but the most useful molecular magnets incorporate transition metal or lanthanide ions, with the molecular groups providing a bridge to mediate exchange interactions between the ions. Some of these materials exhibit a spin crossover effect between low-spin and high-spin states. Other molecular magnets form model low-dimensional magnetic compounds that can be used to test models of quantum spin systems. Molecular nanomagnets are complex molecules containing a number of metal ions whose individual moments conspire to create a giant magnetic moment associated with the entire molecule. These systems can be used to exhibit quantum tunnelling of magnetization and are also candidate systems for quantum computing applications.