Theory on the magnetic properties of (TMTSF)2 X at ambient pressure

We propose a theoretical scheme which explains the temperature and magnetic field dependences of susceptibility of Bechgaard salt, (TMTSF)₂ X, based on the phase Hamiltonian for the itinerant electrons derived from the bosonization of the fermion field. Calculation is made from the viewpoint of the weak spin density wave of itinerant electrons. The result shows almost similar tendency to the experimental finding.A part of this work was reported at International Conference on Synthetic Low Dimensional Conductors and Superconductors LES ARCS (December 1982) by S. Iwabuchi and H. Fukuyama: J. Phys. (Paris) Colloq. C3 [Suppl. 6]44 (1983)     

Subterahertz electrodynamics of (TMTSF)2X (X = ClO4, PF6) salts

The terahertz-subterahertz spectra of the complex permittivity and dynamic conductivity of polycrystalline (TMTSF)₂ClO₄ and (TMTSF)₂PF₆ samples are measured quantitatively. The spectra of (TMTSF)₂ClO₄ have absorption lines at frequencies of 7 and 30 cm^?1. The obtained temperature dependences of the line parameters in the range 5–300 K cast some doubt on the earlier concept of their phonon origin. An excitation is detected at temperatures below 20 K in the frequency range near 30 cm^?1, and its nature is related to the activation of a transverse acoustic phonon caused by the folding of the Brillouin zone due to the ordering of noncentrosymmetrical anions below 20 K. An increase in the carrier relaxation rate is found in this temperature range, which indicates a close relation between the electron and phonon subsystems in (TMTSF)₂ClO₄. Sings of additional low-energy excitations that should manifest themselves at frequencies below 1–2 cm^?1 are detected. (TMTSF)₂PF₆ containing centrosymmetrical anions has no absorption lines in the frequency range 3–20 cm^?1 and the temperature range 5–300 K.     

77Se NMR probe of magnetic excitations of the magic angle effect in (TMTSF)2PF6

We report 77Se spin-lattice relaxation rates for (TMTSF)2PF6, carried out in the regime where a set of spectacular transport anomalies known as the "magic angle effects" are observed. In situ resistance measurements (R(zz)) were used to verify the experimental conditions and give precise sample alignment information. We found that the 77Se T-11 exhibits no significant changes as the magnetic-field orientation is rotated through the magic angles, and conclude that there is no evidence for either a single-particle gap or a spin gap. The clearly observed field-induced spin-density wave transition temperature is also, unexpectedly, not enhanced at the magic angles.     

Effective electron Hamiltonian for a quasi one-dimensional system. The (TMTSF)2X and the (TMTTF)2X systems

In this paper we have introduced a variational approach to investigate the ground state of a model which includes both the Holstein electron-phonon interaction and the extended Hubbard electron-electron interaction. We have considered a variational state for the phonon subsystem which generalizes the previous used forms. This state allows to take into account the possibility of extended phonon mediated correlations. The effective electron Hamiltonian, which we have obtained, includes first and second neighbor electron-electron interaction terms. We have treated exactly, through a Lanczos method, this effective model in the one-dimensional case. We have applied our method to two Bechgaard salts and in these cases we have estimated the correlation parameters. We have shown that the introduction of electron-phonon interaction allows an estimate of the on site U and nearest-neighbor V Coulomb repulsion, which are in agreement with the experimental optical spectra of the above mentioned two compounds. Received: 30 October 1997 / Revised: 28 January 1998 / Accepted: 10 April 1998     

(TMTSF) 2F2PO2: An unusual member of the (TMTSF) 2X family of organic metals

Resistivity measurements were carried out as a function of temperature and pressure in (TMTSF) ₂F₂PO₂. A strong metal-insulator transition is observed at ～137K which is only incompletely suppressed by high pressures. The nature of the resistivity curves obtained at high pressure suggest the onset of a Fermi surface instability; however, the metallic-like transport at lower temperatures suggests that the energy gap covers only part of the Fermi surface. The surprising increase in ? (T) at temperatures below 20K (under pressure) may result from disorder induced localization arising from the strong electronic scattering from the randomly oriented dipole moments.     

Field-induced confinement in (TMTSF)2ClO4 under accurately aligned magnetic fields

We present transport measurements along the least conducting c direction of the organic superconductor (TMTSF)₂ClO₄  performed under an accurately aligned magnetic field in the low temperature regime. The experimental results reveal a two-dimensional confinement of the carriers in the (a, b) planes which is governed by the magnetic field component along the b^′ direction. This 2-D confinement is accompanied by a metal-insulator transition for the c axis resistivity. These data are supported by a quantum mechanical calculation of the transverse transport taking into account in self consistent treatment the effect of the field on the interplane Green function and on the intraplane scattering time.     

Hall effect in the normal phase of the organic superconductor (TMTSF)2PF6

We report accurate Hall effect measurements performed in the normal phase of the quasi-one-dimensional organic conductor (TMTSF)2PF6 at ambient pressure. The Hall coefficient is found to be strongly temperature dependent all the way from 300 K down to the spin density wave onset arising around 12 K. These new results emphasize the existence of a high temperature regime above 130 K where the Fermi liquid model is not satisfactory.     

Hall effect and conduction anisotropy in the organic conductor (TMTSF)2PF6

Both the Hall effect and the ab(')-plane conduction anisotropy are directly addressing the unconventional normal phase properties of the Bechgaard salt (TMTSF)2PF6. We found that the dramatic reduction of the carrier density deduced from recent optical data is not reflected in an enhanced Hall resistance. The pressure and temperature dependence of the b(')-direction resistivity reveal isotropic relaxation time and do not require explanations beyond the Fermi liquid theory. Our results allow a coherent-diffusive transition in the interchain carrier propagation, however the possible crossover to Luttinger liquid behavior is placed at an energy scale above room temperature.     

Infrared study on the formation of charge density waves in (TMTSF) 2X (X = Reo4− and PF6− at atmospheric pressure

The temperature dependence of the infrared absorption spectra (4000–200 cm^?) of (TMTSF) ₂Reo₄ and (TMTSF) ₂PF₆ are investigated with the aim at using the appearance of vibronic absorptions originated by electron-molecular vibration interactions to monitor the formation of charge density waves (CDW) on the donor stacks. The presence of vibronic bands at ≈1400, 436 and 264 cm^?1 in (TMTSF) ₂Re0₄ below 180 K shows that the anion ordering transition in this salt induces the formation of commensurate CDW. The absence of vibronic bands in the PF₆^? salt at 8K indicates that no CDW formation nor an underlying 2K_f lattice distortion takes place at the metal-insulator (MI) phase transition. These results show that, at atmospheric pressure both salts do not undergo a conventional Peierls type MI transition.     

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.     

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.