Prof Arzhang Ardavan

Abstract:

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C 60 and N@C 70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS 2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S≥1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C 60, indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C 70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T 2) ever observed for a molecular electron spin, being 0.25 ms at 170 K. © 2006 American Institute of Physics.

Abstract:

We report a spin resonance study of the family of quasi-two-dimensional organic (super)conductors β′-(BEDT-TTF) 4 [(H 3O)M(C 2O 4) 3]S, where M is a 3d transition metal ion and S is a host solvent molecule. The spin systems for M = Cr 3+ (S = 3/2) and M = Fe 3+ (S = 5/ 2) are investigated by means of both resonant and field modulation techniques in the frequency range between 50 and 313 GHz. The role of the different solvent molecules in determining the degree of spin-orbit coupling and the local symmetry at the metal ion site is established. The low temperature behaviour of intensities, positions and widths of the resonant lines shows significant modifications of the spin-orbit coupling, and of the inter-and intra-ionic spin-spin inter actions. Despite the onset of a weak antiferromagnetic internal field at low temperature, the ultimate narrowing of the lines suggests spin-lattice interactions may still be the dominant relaxation process. Diamagnetic screening in the mixed state of the superconducting samples for fields parallel to the quasi-two-dimensional layers induces additional lineshifts only below B = 2.5T and T = 4K, determining the threshold of full field penetration within the anion layers. © Springer Science+Business Media, LLC.

Abstract:

In order to clarify the origin of the "Rapid Oscillation" (RO) in (TMTSF) 2PF 6, we studied the magnetoresistance anisotropy in the Field-Induced Spin Density Wave (FISDW) phase. We have found that in the FISDW insulating state, the Fermi surface is not totally gapped; the remaining 2D metallic pockets are quantized in magnetic field and give rise to the RO. Decreasing temperature does not change the size and orientation of the closed pockets, rather, it causes depopulation of the delocalized states in favor of the localized ones, resulting in the disappearance of the RO. © Springer Science+Business Media, LLC 2006.