The thesis of Weida Wu entitled "Magnetic Field Induced Commensurability and Correlation Effects in Low Dimensional Organic Conductors", has been placed on deposit on August 16, 2004.

Any member of the University wishing to read the thesis may do so. Any objections should be submitted to me in writing. The principal advisor for this work was Prof. Paul Chaikin. The abstract is below

ABSTRACT

In this thesis we studied the ground state properties of Quasi-1-Dimensional organic superconductors, (TMTSF)2X (X=PF6, ClO4 ...) at commensurate/incommensurate magnetic fields. We used thermal, transport and Nuclear Magnetic Resonance (NMR) measurements on single crystal samples under high pressure (~10 kbar), high magnetic field (~8T) and low temperature (~100mK).

The Magic Angle Effect is a long standing mystery in Quasi-1-Dimensional Organic superconductors where large resistance dips were found when magnetic field is aligned at the commensurate angles (magic angles), which correspond to inter-chain directions. From thermoelectric transport measurements, we discovered giant Nersnt resonances at magic angles in (TMTSF)2PF6, where the Nernst signal rises to a peak and sharply drops to zero as magnetic field approaches a magic angle, then changes its sign and proceeds anti-symmetrically as magnetic field moves away from the magic angle. The sign change of the Nernst signal at the magic angles strongly suggests that the transport is effectively coherent 2-dimensional when the magnetic field is close to a magic angle. The sign of the Nernst signal is determined by the field component normal to the coherent planes. The amplitude of the peak Nernst signal reaches a maximum at ~1K as temperature is lowered, then falls off exponentially and diminishes below ~200mK. The Nernst signal is highly non-linear in magnetic field. Its temperature dependence at difference field strength seems to collapse to a single curve when normalized. Calculations based on Tight binding band structure and Boltzmann transport fails to explain either the angular dependence or the magnitude of the giant Nernst effect in (TMTSF)2PF6. Therefore, strong correlation effects must be considered in order to understand both the resistance and the Nernst magic angle effect. Present phenomenological models include field induced inter-plane decoupling and/or the presence of superconducting vortices. Other related magic angle models will be discussed.

NMR is a powerful tool to study electronic spin susceptibility. The Spin-Lattice Relaxation measurements show that there is no difference between magic angles and non-magic angles in the temperature dependence of spin-lattice relaxation rates (1/T1). 1/T1(T) approaches a Korringa-like relation at low temperature. Therefore, there is neither a spin gap nor a single particle gap involved in the magic angle effect. If there is a gap involved in the magic angle effect, it is at most a charge gap.

Daniel Marlow
Chair, Dept. of Physics