Bechgaard Salts Research Articles

Superconductivity and antiferromagnetism in quasi-one-dimensional organic conductors (Review Article)

We review the current understanding of superconductivity in the quasi-one-dimensional organic conductors of the Bechgaard and Fabre salt families. We discuss the interplay between superconductivity, antiferromagnetism, and charge-density-wave fluctuations. The connection to recent experimental observations supporting unconventional pairing and the possibility of a triplet spin order parameter for the superconducting phase is also presented.

Photoemission properties of quasi-one-dimensional conductors

We calculate the one-particle spectral function and the corresponding derived quantitiesfor a chain lattice with a one-dimensional conducting electron band, and with athree-dimensional long-range Coulomb electron–electron interaction treated within theG0W0 approximation. It is shown that due to the anisotropic acoustic dispersion of the plasmonmode, the quasi-particle peak as a standard Fermi liquid feature does not show upin the spectral function. Instead, the latter comprises only a broad maximumwith width of the order of plasmon energy. Such spectral properties are in thequalitative agreement with ARPES spectra of Bechgaard salts obtained in recentmeasurements. The present approach is appropriate for the treatment of wide energy scalesdefined by the width of the conducting band and the plasmon energy, and iscomplementary to earlier rigorous results obtained within the Luttinger liquidapproach in the asymptotic limit of low energies close to the chemical potential.

Correction: Retraction: Evidence for spin–charge separation in quasi-one-dimensional organic conductors

Nature 418, 614–617 (2002) In this Letter, we described the anomalous behaviour of the thermal conductivity k in quasi-one-dimensional Bechgaard salts. As we have since observed a conventional temperature dependence of k in another quasi-one-dimensional organic conductor, TTF-TCNQ, we remeasured thevalue of k in our Bechgaard salts and identified a heat leakage in the original experimental set-up, which becomes relevant for samples with very small values of k at low temperatures.

Spin-Wave Contribution to the Nuclear Spin-Lattice Relaxation in Triplet Superconductors

We discuss collective spin-wave excitations in triplet superconductors with an easy axis anisotropy for the order parameter. Using a microscopic model for interacting electrons, we estimate the frequency of such excitations in Bechgaard salts and ruthenate superconductors to be 1 and 20 GHz, respectively. We introduce an effective bosonic model to describe spin-wave excitations and calculate their contribution to the nuclear spin-lattice relaxation rate. We find that, in the experimentally relevant regime of temperatures, this mechanism leads to the power law scaling of 1/T1 with temperature. For two- and three-dimensional systems, the scaling exponents are 3 and 5, respectively. We discuss experimental manifestations of the spin-wave mechanism of the nuclear spin-lattice relaxation.

Tunnelling in organic superconductors

We present a simple scheme for computing the full current-voltage characteristics for tunnelling experiments within the framework of the non-equilibrium Keldysh Green function formalism. This formalism is flexible enough to address different pairing symmetries combined with magnetic fields at arbitrary bias voltages. We show how to apply these results to probe for the symmetry of the superconducting order parameter in the Bechgaard salts using tunnelling experiments.

Quantum statistics of charged particles and fingerprints of wigner crystallization in D dimensions

AbstractAfter a brief summary of the physical arguments underlying Wigner's original concept in 1934 of a quantal electron crystal, theoretical interpretation of a number of experimental findings are presented. These include (i) low‐density carriers in semiconductors in applied magnetic fields in both three, and recently two, dimensions; and (ii) low‐temperature phase diagram of underdoped high Tc cuprates; fullerides with relatively low Tc are also referred to in a related context. Interpretation of areas (i) and (ii) focuses on the relevance of both Fermi–Dirac and anyonic (fractional) statistics, the latter in relation to the proposed melting curve of the two‐dimensional (2D) magnetically induced Wigner solid into the Laughlin liquid phase, which is the seat of the fractional quantum Hall effect. A brief discussion follows of crystalline phases additional to the Wigner solid, namely Skyrmion and Hall crystals. Bose–Einstein statistics is then referred to, but now in relation to finite‐size confined quantal assemblies, with fingerprints of Wigner molecules the focus. Finally, quasi‐1D lattices are considered, both in Bechgaard salts and in the very recent single‐electron counting experiment of Bylander et al. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Triplet Superconducting Pairing and Density-Wave Instabilities in Organic Conductors

Using a renormalization group approach, we determine the phase diagram of an extended quasi-one-dimensional electron gas model that includes interchain hopping, nesting deviations, and both intrachain and interchain repulsive interactions. We find a close proximity of spin-density- and charge-density-wave phases and singlet d-wave and triplet f-wave superconducting phases. There is a striking correspondence between our results and recent puzzling experimental findings in the Bechgaard salts, including the coexistence of spin-density-wave and charge-density-wave phases and the possibility of a triplet pairing in the superconducting phase.

Resonant Nernst Effect in the Metallic and Field-Induced Spin Density Wave States of(TMTSF)2ClO4

We examine an unusual phenomenon where, in tilted magnetic fields near magic angles parallel to crystallographic planes, a "giant" resonant Nernst signal has been observed by Wu et al. [Phys. Rev. Lett. 91, 056601 (2003)] in the metallic state of an organic conducting Bechgaard salt. We show that this effect appears to be a general feature of these materials and is also present in the field-induced spin density wave phase with even larger amplitude. Our results place conditions on any model that treats the metallic state as a state with finite Cooper pairing.

From organic superconductors to DNA: Fragment orbital‐based model

AbstractA semi‐empirical valence bond/Hartree–Fock (VB/HF) method is developed to calculate one‐ and two‐electron interactions between molecular fragments in conducting supramolecular stacks. This fragment orbital‐based formalism allows for consistent extraction of an effective Hamiltonian defined as a “frontier orbital” model. This Hamiltonian quantitatively describes transfer and electrostatic interactions between conducting electrons, while reducing the active space so dramatically that the electronic eigenstates of very large systems may be investigated. The capabilities of the VB/HF method are illustrated on two different supramolecular stacks involving a π–π interacting fragment. In the first part of this study, the framework of the VB/HF method is used to evaluate the relative magnitude of the electronic interactions between conduction electrons in organic conductors and superconductors derived from Bechgaard salts. In the second part of this study, the VB/HF formalism is extended to derive an effective model for conduction holes along doped DNA double strands. Transferable intra‐ and intersite parameters were first evaluated from VB/HF calculations carried out on nucleoside pairs. From this interaction databank, the effective Hamiltonian of any type of nucleoside sequence can be defined. The thermalized charge distribution for a single hole delocalized along a DNA sequence containing 240 Watson–Crick pairs is then calculated and compared with the experimental yields of damage revealed by photocleavage experiments. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Effects of the electron-phonon interaction of the charge ordering in quasi-one-dimensional charge-transfer salts

The Bechgaard salts and their sulfur analogs are considered to be in quasi-one-dimensional correlated electron systems with a quarter-filled π band and exhibit various types of charge and/or spin ordering. A variety of mechanisms have been proposed in an attempt to interpret the variety of instabilities, featuring dimensionality, which can be turned by pressure as well as chemically, intrachain dimerization due to the anion columns, and competing on-site and even long-range Coulomb repulsions. Here we investigate quantum and thermal phase transitions in the Bechgaard salts and their sulfur analogs with particular emphasis on lattice degree of freedom. We consider the one-dimensional extended Peierls-Hubbard model with long-range Coulomb and electron-lattice interactions. Due to the intrinsic dimerization accompanied by the anion columns, we introduce alternation to transfer integrals, nearest-neighbor Coulomb interactions and spring constants. We fully demonstrate the interplay between charge ordering and lattice instability within the Hartree approximation. Besides the purely electronic charge and spin density waves which are well established both experimentally and theoretically, we reveal a lattice-tetramerized phase stabilized essentially in Peierls-Hubbard model. Our results inspected and compared with the results of experiments.

Study of rapid oscillations in (TMTSF) 2FSO 3 under pressure and under very high magnetic fields

(TMTSF) 2FSO 3 is very special in that its electric transport properties are described by electrons from two-dimensional Fermi surfaces (FS) whereas those in most other Bechgaard salts are intrinsically explained with electrons from weakly warped quasi-one- dimensional FS. Conventional Shubnikov-dc Haas oscillations, described with the Lifshitz-Kosevich formula, has been observed. In this study, we measured magnetoresistance along the least conducting direction ( c *) under pressures of up to 11 kbars and magnetic fields of up to 55 T, generated in a pulsed magnet, and found that the oscillations preserved two-dimensional nature to the highest measured field. There is no field-induced electronic transition. Pressure dependence and magnetic field dependence of parameters of oscillations are presented and discussed.

NMR Studies of Exotic Members of Bechgaard Salt with Non- centrosymmetric Anions under Pressure

The electronic properties of (TMTSF) 2FSO 3 have been investigated under pressure of 0.65 GPa by measuring 77Se-NMR lineshape, spin-lattice relaxation rate ( T 1 − 1 ) and spin-spin relaxation rate ( T 1 − 1 ) measurements on a single crystal sample. The NMR signal was found abnormally broadened below 90 K. Below 40 K, a narrow component appears and coexists with the broader one. The temperature dependence of T 1 − 1 suggests that the broader component comes from a metallic phase and the narrow one from an insulating phase. The activation energy determined for the insulating component was found three times lower than the value found at ambient pressure. From the T 1 − 1 measurements, the broadening of the metallic component was confirmed as inhomogeneous. An unusual double peak anomaly of T 1 − 1 suggests the existence of extremely slow fluctuations of the inhomogeneous local fields below 90 K at this pressure.

Ring-exchange mechanism for triplet superconductivity in a two-chain Hubbard model: Possible relevance to Bechgaard salts

The density-matrix renormalization group method is used to study the ground state of the two-chain zigzag-bond Hubbard model at quarter filling. We show that, with a proper choice of the signs of hopping integrals, the ring exchange mechanism yields ferromagnetic spin correlations between interchain neighboring sites, and produces the attractive interaction between electrons as well as the long-range pair correlations in the spin-triplet channel, thereby leading the system to triplet superconductivity. We argue that this novel mechanism may have possible relevance to observed superconductivity in Bechgaard salts.

Optical conductivity of the one-dimensional dimerized Hubbard model at quarter filling

We investigate the optical conductivity in the Mott insulating phase of the one-dimensional extended Hubbard model with alternating hopping terms (dimerization) at quarter band filling. Optical spectra are calculated for the various parameter regimes using the dynamical density-matrix renormalization group method. The study of limiting cases allows us to explain the various structures found numerically in the optical conductivity of this model. Our calculations show that the dimerization and the nearest-neighbor repulsion determine the main features of the spectrum. The on-site repulsion plays only a secondary role. We discuss the consequences of our results for the theory of the optical conductivity in the Bechgaard salts.

Keldysh study of point-contact tunneling between superconductors

We revisit the problem of point-contact tunnel junctions involving one-dimensional superconductors and present a simple scheme for computing the full current-voltage characteristics within the framework of the non-equilibrium Keldysh Green function formalism. We address the effects of different pairing symmetries combined with magnetic fields and finite temperatures at arbitrary bias voltages. We discuss extensively the importance of these results for present-day experiments. In particular, we propose ways of measuring the effects found when the two sides of the junction have dissimilar superconducting gaps and when the symmetry of the superconducting states is not the one of spin-singlet pairing. This last point is of relevance for the study of the superconducting state of certain organic materials like the Bechgaard salts and, to some extent, for ruthenium compounds.

Field-induced spin-density wave in(TMTSF)2NO3

Interlayer magnetoresistance of the Bechgaard salt (TMTSF)$_2$NO$_3$ is investigated up to 50 teslas under pressures of a few kilobars. This compound, the Fermi surface of which is quasi two-dimensional at low temperature, is a semi metal under pressure. Nevertheless, a field-induced spin density wave is evidenced at 8.5 kbar above $\sim$ 20 T. This state is characterized by a drastically different spectrum of the quantum oscillations compared to the low pressure spin density wave state.

Tunnelling in Organic Superconductors

We discuss the possibility of deciding on the symmetry of the superconducting phase in the organic superconductors (Bechgaard salts), using tunnelling experiments. We first briefly review the properties of organic superconductors, and the possibility to have unconventional (triplet) superconductivity in these systems. We then present a simple scheme for computing the full current-voltage characteristics for tunnelling experiments within the framework of the non-equilibrium Keldysh Green function formalism. This formalism is flexible enough to address different pairing symmetries combined with magnetic fields and finite temperatures at arbitrary bias voltages. We then discuss extensively how to apply these results to probe for the symmetry of the superconducting order parameter.

SO(4) Theory of Antiferromagnetism and Superconductivity in Bechgaard Salts

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, demonstrating the existence of such symmetry in one-dimensional Luttinger liquids. SO(4) symmetry strongly constrains the phase diagram, leading to coexistence regions of antiferromagnetic, superconducting, and normal phases, as observed in (TMTSF)(2)PF(6). We predict a sharp neutron scattering resonance in superconducting samples.

Applications of quantum field theory in condensed matter

We present a brief review of some applications of quantum field theory in condensed matter systems. These include isotropic and anisotropic antiferromagnetic chains, strongly correlated organic conductors, such as the Bechgaard salts, carbon nanotubes and high-Tc superconductors. The review is by no means exhaustive and points to a vast range of new interesting possible applications.

Quantum Physics in One Dimension

Quantum Physics in One Dimension