A model for electrical conductivity in (TMTSF)2ClO4, (TMTSF)2ReO4 and related materials

To describe quantitatively the diverse variations of the temperature-dependent electrical conductivities of tetramethyltetraselenafulvenium (TMTSF) salts, a model based on small Fermi (E_f) and small gap (E_g − E_f) energies and T⁻ⁿ dependence of scattering is presented. The difference between the conductivities of quenched and slowly cooled (TMTSF)₂ClO₄ is described by different values of E_f and E_g − E_f in the two cases. The model illustrates, in terms of variables E_f and E_g − E_f, the effect of pressure on the metal-insulator transition in (TMTSF)₂ReO₄, for example, and indicates a relation between E_f and E_g − E_f at the onset of superconductivity.     

Dynamics of isolated twin boundary in (TMTSF)

Intermittent and irregular motion of isolated twin boundary (kink) in organic crystal (TMTSF)₂PF₆ was studied at room temperature. Both the local velocity and the time of intermission are determined not only by external stress and temperature but also by the time (t w) elapsed after the backward passage and before the following forward one. When the kink moves after longer t w, its velocity becomes smaller and the time of intermission longer. Both tend to saturate for t w longer than 10² s. This result indicates that some disorder is induced in the lattice by the backward motion and it is relaxed during t w. We also found that the effect of the backward motion of one kink on its following motion is equivalent quantitatively to that of the forward motion of the pair-created counterpart. Received: 14 April 1998 / Received in final form and Accepted: 1st September 1998     

Band structure of (TMTSF)2X

The electronic structure of the organic conductors (TMTSF)₂X has been explored in terms of the tight binding band structures calculated for a sheet of TSF molecules. The Se 4d-orbitals appear to be critical in enhancing the interstack Se…Se interaction to the point that (TMTSF)₂X becomes pseudo two-dimensional. Based upon the present band structure study, it is discussed whether a normal metallic state or a spin density wave state provides the closed Fermi surface responsible for the Shubnikov-de Haas oscillations observed in (TMTSF)₂PF₆.     

Superconducting state of the organic conductor (TMTSF)2ClO4

(TMTSF)2ClO4 is a quasi-one-dimensional organic conductor and superconductor with Tc=1.4 K, and one of at least two Bechgaard salts observed to have upper critical fields far exceeding the paramagnetic limit. Nevertheless, the 77Se NMR Knight shift at low fields reveals a decrease in spin susceptibility chi(s) consistent with singlet spin pairing. The field dependence of the spin-lattice relaxation rate at 100 mK exhibits a sharp crossover (or phase transition) at a field Hs approximately 15 kOe, to a regime where chi(s) is close to the normal state value, even though Hc2> Hs.     

Low temperature thermodynamical properties of the organic chain conductor (TMTSF)AsF

We report on specific heat measurements of the quasi-one-dimensional organic salt (TMTSF)₂AsF₆ in its spin density wave state between 75 mK and 7 K. Similarly to (TMTSF)₂PF₆, we find discontinuities in the lattice contribution at 1.9 K an d 3.5 K ascribed to sub-spin density wave phases. Time-dependent effects due to dynamics of low-energy excitations in metastable states occur only below 0.2 K which yields an activation energy for the equilibrium energy relaxation process of 0.34 K, 4-5 times smaller than found for (TMTSF)₂PF₆. Finally the reduction of the low-energy excitations contribution to the specific heat in comparison to PF₆ reveals an intermediate cubic-like regime between 0.25 and 0.5 K that we tentatively describe as the phason contribution of the incommensurate spin density wave modulation. Received: 17 March 1998 / Revised: 27 July 1998 / Accepted: 22 September 1998     

Competing phases in the high field phase diagram of (TMTSF)(2)ClO(4)

A model is presented for the high field phase diagram of (TMTSF)(2)ClO(4), taking into account the anion ordering, which splits the Fermi surface into two bands. For strong enough field, the largest metal spin density wave critical temperature corresponds to the N=0 phase, which originates from two intraband nesting processes. At lower temperature, the competition between these processes puts at disadvantage the N=0 phase vs the N=1 phase, which is due to interband nesting. A first order transition then takes place from the N=0 to N=1 phase. We ascribe to this effect the experimentally observed phase diagrams.