Prof Arzhang Ardavan

Abstract:

The Hofstadter butterfly is the energy spectrum of an infinite square lattice, plotted as a function of the magnetic field. We illustrate a method of calculating similar spectra for finite lattices in a magnetic field, using methods that consider the appropriate molecular orbitals, and find that the spectra resemble the Hofstadter butterfly. We relate the bonding and antibonding orbitals used to describe small systems to the Landau levels of the infinite system. This approach provides an unusual, but instructive, method of introducing the physics of Landau levels from the basic quantum mechanics of small systems. © 2004 American Association of Physics Teachers.

Abstract:

We present a theoretical study of the emission from a superluminal polarization current whose distribution pattern rotates (with an angular frequency omega) and oscillates (with a frequency Omega) at the same time and that comprises both poloidal and toroidal components. This type of polarization current is found in recent practical machines designed to investigate superluminal emission. We find that the superluminal motion of the distribution pattern of the emitting current generates localized electromagnetic waves that do not decay spherically, i.e., that do not have an intensity diminishing as RP(-2) with the distance RP from their source. The nonspherical decay of the focused wave packets that are emitted by the polarization currents does not contravene conservation of energy: The constructive interference of the constituent waves of such propagating caustics takes place within different solid angles on spheres of different radii (RP) centered on the source. For a polarization current whose longitudinal distribution (over an azimuthal interval of length 2pi) consists of m cycles of a sinusoidal wave train, the nonspherically decaying part of the emitted radiation contains the frequencies Omega +/- momega; i.e., it contains only the frequencies involved in the creation and implementation of the source. This is in contrast to recent studies of the spherically decaying emission, which was shown to contain much higher frequencies. The polarization of the emitted radiation is found to be linear for most configurations of the source.

Abstract:

We report high-field magneto transport measurements on beta''- BEDT-TTF4[(H3O)M(C2O4)(3)]C5H5N, where M = Ga3+, Cr3+ or Fe3+. In spite of the differing M ions, these compounds have very similar Fermi surfaces. We observe four distinct Shubnikov-de Haas frequencies, corresponding to four Fermi-surface pockets; the frequencies exhibit the additive relationship expected for a compensated semimetal. The compounds show paramagnetic behaviour and no superconductivity down to 0.5 K, in contrast to other materials of the same family with different solvent molecules. (C) 2003 Elsevier B.V. All rights reserved.

Abstract:

We show that the coexistence of a quasi-two-dimensional Fermi-surface exhibiting Landau quantisation and a charge-density wave leads to a screening of electromagnetic fields at the sample Surface. Hall potential experiments demonstrate that this screening is responsible for some of the unusual phenomena observed in the high-field CDWx state of alpha-(BEDT-TTF)(2)KHg(SCN)(4).

Abstract:

We applied magneto-optical techniques to investigate the electron spin resonance (ESR) spectra of single crystal and powder samples of the organic superconductor beta"-(BEDT- TTF)(4)[(H3O)Fe (C2O4)(3)]C6H5CN. Data were obtained in magnetic fields up to 10 Tesla using both cylindrical and rectangular resonators at 55, 63 and 72 GHz and transmission measurements at 95 and 190 GHz with field modulation. Analysis of the high field-high frequency ESR lines for different field orientations and temperatures allows the determination of the g-value anisotropy and the zero-field splitting parameters for the Fe3+ ions (S = 5/2), showing that they occupy a site with distorted octahedral symmetry. The temperature and frequency dependences of the magneto-optical spectra are used to identify the nature of the interaction between localised Fe spins and between the local moments and the delocalised carriers.