Professor Stephen Blundell

Abstract:

The response of the superconducting charge carriers to an applied magnetic field has been probed in a number of organic superconductors using muSR. Systematic studies have been made of the in-plane superconducting penetration depth lambda in superconductors based on the ET and BETS molecules. These studies have revealed a strong correlation between lambda and T-c. This type of correlation was first noted in high T-c cuprates by Uemura et al who found a linear relation between T-c and lambda(-2). In the case of organic superconductors, however, we find that the overall con-elation is better described as T-c proportional to lambda(-3). Understanding this characteristic trend should provide some important clues about the nature of the superconductivity in the organic superconductors; several possible models are considered here for describing this behaviour. It is concluded that models in which the superconducting carrier concentration is tied to the nominal band filling are inconsistent with the observed scaling behaviour.

Abstract:

We present the results of numerical calculations of the energy levels and eigenfunctions of finite sections of a tight-binding square lattice in the presence of a perpendicular magnetic field. The energy spectrum of such a system, plotted as a function of magnetic field, resembles the Hofstadter butterfly found for the infinite system. In high magnetic fields, each eigenstate carries a persistent current which has a chirality associated with whether the eigenstate exists in the bulk or the edge of the system. We present simulations of electric dipole transitions between the chirally distinguished states for an isotropic lattice. These transitions correspond to harmonics in the cyclotron resonance.

Abstract:

Radical ion salts of the neutral organic radicals, 2-imidazolyl nitronyl nitroxide (2-IMNN) and 2-benz-imidazolyl nitronyl nitroxide (2-BIMNN), and a diradical cation salt of pyridine-2,6-diylbis(nitronyl nitroxide) (2,6-PYBNN), were prepared. The radical ion salts of 2-BIMNN, Li+(2-BIMNN)(.-) and (2-BIMNN).-BF4-, exhibit strong antiferromagnetic interactions with exchange coupling constants J/k = -42 K and -510 K, respectively, while the 2-IMNN salts show Curie-Weiss behaviour with Weiss constants theta = -9.0 K for Li+(2-IMNN)(.-) and theta = -1.8 K for (2-IMNN)Cl-.-(-), indicating weak interactions. The diradical cation salt (2,6-PYBNN)Cl-..+(-) 14 shows intramolecular ferromagnetic interaction with J/k = +4.7 K and intermolecular antiferromagnetic interaction with theta = -0.7 K, which is remarkably reduced from J/k = -57 K observed in the diradical 2,6-PYBNN.

Abstract:

We report millimetre-wave and magnetic measurements on single crystals of the high-spin molecular magnet, Cr-10(OMe)(20)(O2CCMe3)(10). The susceptibility data obtained using a SQUID magnetometer indicate that this molecule has a total spin of S = 15. The temperature dependence of the magnetisation allows us to estimate the intermolecular exchange constant. The EPR spectra were obtained at different frequencies, angles and temperatures. The results are compared with numerical simulations allowing determination of the Hamiltonian parameters.

Abstract:

We present zero-field muon spin relaxation (ZF-mu(+)SR) measurements on two examples of a new series of hybrid organometallic-inorganic layered magnets, namely ferromagnetic [FeCp2*][MnCr(ox)(3)] and ferrimagnetic [CoCp2*][FeFe(ox)(3)] (where ox = oxalate and Cp-* = pentame- thyl-cyclopentadienyl). Both materials show multi-component muon spin precession signals characteristic of quasistatic magnetic fields at several distinct muon sites. The temperature dependence of the precession frequencies allow critical exponents to be extracted. Possible muon sites are discussed on the basis of dipole field calculations.