Quantum and thermal phase transitions in the Bechgaard salts and their analogs

We present a group theoretical analysis of the superconducting state of the Bechgaard salts, e.g., (TMTSF)_2PF_6 or (TMTSF)_2ClO_6. We show that there are eight symmetry distinct superconducting states. Of these only the (fully gapped, even frequency, p-wave, triplet) 'polar state' is consistent with the full range of the experiments on the Bechgaard salts. The gap of the polar state is d(k) (psi_uk,0,0), where psi_uk may be any odd parity function that is translationally inv...

The crystal structures of the quasi-one-dimensional organic salts (TMTTF)$_2$PF$_6$ and (TMTSF)$_2$PF$_6$ were studied by pressure-dependent x-ray diffraction up to 10 GPa at room temperature. The unit-cell parameters exhibit a clear anomaly due to a structural phase transition at 8.5 and 5.5 GPa for (TMTTF)$_2$PF$_6$ and (TMTSF)$_2$PF$_6$, respectively.

Superconductivity in the Bechgaard salts series of quasi-one-dimensional organic conductors occurs on the verge of spin-density-wave ordering when hydrostatic pressure is applied. The sequence of instabilities is intimately connected to normal state anomalies in various quantities like the temperature dependence of electrical transport and nuclear spin-lattice relaxation rate. We discuss how such a connection takes its origin in the interference between the different pairing ...

We review the properties of quasi-one-dimensional organic superconductors: the Bechgaard salts and their sulfur analogs in their normal phase precursor tolong-range order. We go through the main observations made in the normal state of these systems at low magnetic field and tackle the issue of their description under the angles of the Fermi and Luttinger liquid pictures.

The symmetry of the superconductivity in Bechgaard salts is still unknown, though the triplet pairing has been established by Hc2 and NMR for (TMTSF)2PF6. The large upper critical field at T = 0K (Hc2 ~ 5 Tesla) both for H || a and H || b also indicates strongly the triplet pairing. Here we start with a low energy effective Hamiltonian and study the temperature dependence of the corresponding Hc2(T)'s. The present analysis suggests that one chiral f-wave superconductor sh...

Charge-ordering phenomena have been highly topical over the last few years. A phase transition towards a charge ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one dimensional organic charge-transfer salts (TMTTF)$_2$X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion (X = PF$_6$, AsF$_6$ and SbF$_6$). However, the relationship between the elect...

We discuss the possible existence of a superconducting phase at high magnetic field in organic quasi-one-dimensional conductors. We consider in particular (i) the formation of a Larkin-Ovchinnikov-Fulde-Ferrell state, (ii) the role of a temperature-induced dimensional crossover occuring when the transverse coherence length $\xi_z(T)$ becomes of the order of the lattice spacing, and (iii) the effect of a magnetic-field-induced dimensional crossover resulting from the localizat...

Motivated by recent experiments with Bechgaard salts, we investigate the competition between antiferromagnetism and triplet superconductivity in quasi one-dimensional electron systems. We unify the two orders in an SO(4) symmetric framework, and demonstrate the existence of such symmetry in one-dimensional Luttinger liquids. SO(4) symmetry, which strongly constrains the phase diagram, can explain coexistence regions between antiferromagnetic, superconducting, and normal phase...

We investigated the transport properties of the quasi one-dimensional organic metal (TMTSF)2ReO4 above the anion-ordering metal-insulator transition (T_{AO} \approx 180K). The pronounced conductivity anisotropy, a small and smoothly temperature dependent Hall effect, and a small, positive and temperature dependent magnetoresistance are analyzed within the existing Fermi-liquid and non-Fermi liquid models. We propose that the transport properties of quasi one-dimensional Bechg...

It is the saturation of the transition temperature Tc in the range of 24 K for known materials in the late sixties which triggered the search for additional materials offering new coupling mechanisms leading in turn to higher Tc's. As a result of this stimulation, superconductivity in organic matter was discovered in tetramethyl-tetraselenafulvalene-hexafluorophosphate, (TMTSF)2PF6, in 1979, in the laboratory founded at Orsay by Professor Friedel and his colleagues in 1962. A...