Prof Arzhang Ardavan

Abstract:

We report a spin resonance study of the family of quasi-two-dimensional organic (super)conductors beta"-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]S, where M is a 3d transition metal ion and S is a host solvent molecule. The spin systems for M = Cr3+ (S = 3/2) and M = Fe3+ (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 interactions. 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 = 2.5 T and T = 4 K, determining the threshold of full field penetration within the anion layers.

Abstract:

In order to clarify the origin of the "Rapid Oscillation" (RO) in (TMTSF)(2)PF6, 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.

Abstract:

We report high-field magnetotransport studies on quasi-two dimensional beta"-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]Y-. where Y is a solvent in the anionic layer. By changing the size of the solvent the low temperatures electronic behaviour varies from superconducting (for larger solvents, Y=C6H5NO2 and C6H5CN) to metallic (for smaller solvents, Y=C5H5N and CH2O2). These changes in the ground state are connected with modifications of the Fermi surface, which varies from having one or two pockets for the superconducting charge-transfer salts to at least four pockets in the case of metallic ones. When superconducting, the materials have very large in-plane critical fields (up to 32 T) and enhanced effective masses compared with the metallic compounds. The role of the charge-order fluctuations in stabilizing the superconducting ground state and the effects of intrinsic local disorder is discussed.