Low-dimensional Organic Superconductor Research Articles

Seebeck coefficient in correlated low-dimensional organic metals

We study the influence of inelastic electron-electron scattering on the temperature variation of the Seebeck coefficient in the normal phase of quasi-one-dimensional organic superconductors. The theory is based on the numerical solution of the semi-classical Boltzmann equation for which the collision integral equation is solved with the aid of the electronic umklapp scattering vertex calculated by the renormalization group method. We show that the one-loop renormalization group flow of momentum and temperature dependent umklapp scattering, in the presence of nesting alterations of the Fermi surface, introduce electron-hole asymmetry in the energy dependence of the anisotropic scattering time. This is responsible for the enhancement of the Seebeck coefficient with respect to the band $T$-linear prediction and even its sign reversal around the quantum critical point of the phase diagram, namely where the interplay between antiferromagnetism and superconductivity along with the strength of spin fluctuations are the strongest. Comparison of the results with available data on low dimensional organic superconductors is presented and critically discussed.

Electrical transport near quantum criticality in low-dimensional organic superconductors

We propose a theory of longitudinal resistivity in the normal phase of quasi-one-dimensional organic superconductors near the quantum critical point where antiferromagnetism borders with superconductivity under pressure. The linearized semi-classical Boltzmann equation is solved numerically, fed in by the half-filling electronic umklapp scattering vertex as derived from one-loop renormalization group calculations for the quasi-one-dimensional electron gas model. The momentum and temperature dependence of umklapp scattering has an important impact on the behaviour of longitudinal resistivity in the the normal phase. Resistivity is found to be linear in temperature around the quantum critical point at which spin-density-wave order joins superconductivity along the antinesting axis, to gradually evolve towards the Fermi liquid behaviour in the limit of weak superconductivity. A comparison is made between theory and experiments performed on the (TMTSF)$_2$PF$_6$ member of the Bechgaard salt series under pressure.

Inhomogeneous superconductivity in organic conductors: the role of disorder and magnetic field

Several experimental studies have shown the presence of spatially inhomogeneous phase coexistence of superconducting and non-superconducting domains in low dimensional organic superconductors. The superconducting properties of these systems are found to be strongly dependent on the amount of disorder introduced in the sample regardless of its origin. The suppression of the superconducting transition temperature Tc shows a clear discrepancy with the result expected from the Abrikosov–Gor’kov law giving the behavior of Tc with impurities. On the basis of the time dependent Ginzburg–Landau theory, we derive a model to account for this striking feature of Tc in organic superconductors for different types of disorder by considering the segregated texture of the system. We show that the calculated Tc quantitatively agrees with experiments. We also focus on the effect of superconducting fluctuations on the upper critical fields Hc2 of layered superconductors showing slab structure where superconducting domains are sandwiched by non-superconducting regions. We found that Hc2 may be strongly enhanced by such fluctuations.

Unconventional Superconductivity in a Quasi-One-Dimensional System (TMTSF)2X

The nature of superconductivity in a quasi-one-dimensional organic system has been controversial since its discovery in the early 80's. Here, we discuss recent developments regarding the issue based mainly on studies of the upper critical field H c2 (T) and the nuclear magnetic resonance (NMR) Knight shift. A strong upward curvature found in the temperature dependent upper critical field and an absence of the shift in the resonant adsorption frequency upon cooling through the superconducting transition strongly support the notion that the nature of the low dimensional organic superconductor is spin triplet.

From nodal liquid to nodal charge gap in a frustrated Hubbard model

We investigate the physics of frustrated 3-leg Hubbard ladders in the band limit, when hopping across the ladder's rungs (t$_{\perp}$) is of the same order as hopping along them (t) much greater than the onsite Coulomb repulsion (U). We show that this model exhibits a striking electron-hole asymmetry close to half-filling: the hole-doped system at low temperatures develops a Resonating Valence Bond (RVB)-like d-wave gap (pseudogap close to ($\pi$,0)) coinciding with gapless nodal excitations (nodal liquid); in contrast, the electron-doped system is seen to develop a Mott gap at the nodes, whilst retaining a metallic character of its majority Fermi surface. At lower temperatures in the electron-doped case, d-wave superconducting correlations -- here, coexisting with gapped nodal excitations -- are already seen to arise. Upon further doping the hole-doped case, the RVB-like state yields to d-wave superconductivity. Such physics is reminiscent of that exhibited by the high temperature cuprate superconductors--notably electron-hole asymmetry as noted by Angle Resolved PhotoEmission Spectroscopy (ARPES) and the resistivity exponents observed. This toy model also reinforces the importance of a more thorough experimental investigation of the known 3-leg ladder cuprate systems, and may have some bearing on low dimensional organic superconductors.

Electronic states of the dimerized Hubbard and t– J models for low-dimensional organic superconductors

Density-matrix-renormalization-group (DMRG) technique and Lanczos diagonalization of small clusters are used to study the one-dimensional dimerized Hubbard model at quarter filling. We present a precise calculation of the charge gap as a function of the interaction and dimerization strengths and discuss the metal–insulator transition of quasi-one-dimensional organic compounds. We also discuss the validity of the reduction of the present model to the single-band Hubbard model at half filling.

Organic Superconductors as Stages for Exploration of Superconductivity in High Magnetic Fields

The low dimensionality of organic superconductors leads to pronounced anisotropy in the upper critical field. In the vicinity of the field direction parallel to the superconducting plane, the critical field shoots out due to the suppression of the orbital pair-breaking effect. Organic superconductors are suitable for the study of the high-field state related to the spin effect under an aligned field, since they are of a high crystalline quality. The reported experimental results covering the behavior at low temperatures are reviewed first, and the breakthrough of the BCS Pauli paramagnetic limit is discussed. The potential of the low-dimensional organic superconductor for study of the effect of electronic spectrum quantization is argued.

Concerning the first-order phase transition in the low-dimensional organic superconductor . Crystal and electronic band structures below the phase transition

Concerning the first-order phase transition in the low-dimensional organic superconductor . Crystal and electronic band structures below the phase transition

Physical Features of Low Dimensional Organic Super-conductors

Abstract Since the discovery of the organic superconductivity more than 10 years ago with a critical temperature (TS) of 1 K in quasi one dimensional (1D) conductors based on the tetramethyltetraselenafulvalene molecule, more than 40 organic superconductors are now known, achieving TS up to about 13 K. These salts are made of π quasi-planar donors or acceptors, orbitals of which overlap very anisotropically. As a consequence of the 1D or 2D anisotropy of the electron gas, the superconducting ground state competes with charge or spin density wave ground states and the electronic properties are strongly influenced by the occurrence of order-disorder transitions involving molecular orientational or conformational degrees of freedom. In this framework we review the present understanding of the physical properties of the main families of low dimensional organic superconductors and we compare their physical features with those shown by the family of 3D fullerenes exhibiting TS higher than 30 K.

Hall Effect under Pressure in Low Dimensional Organic Superconductors

Hall coefficient, RH(T) of low-dimensional organic superconductors, (DMET)2X and (BEDT-TTF)2X have been examined systematically under pressure of 0 - 10 kbar with the interest of understanding the electronic states in relation to superconductivity and spin density wave in these materials. Even in the metallic states in these materials, RH(T) are found to be remarkably temperature-dependent. We show that at least some of these temperature dependence is related to phase transition or its precursor, which has not been paid attention previously, and which could be discussed with superconductivity.

Novel Fermi-surface topological effects of magnetoresistance anisotropy in low-dimensional organic superconductors

Abstract The commensurability effect of magnetoresistance anisotropy found in the quasi-one-dimensional organic superconductor (TMTSF) 2 X is explained in terms of the resonant drift motion of electrons in the field direction. This is the common picture with the model for the angular dependent magnetoresistance oscillation effect observed in the quasi-two-dimensional organic superconductors (BEDTTTF) 2 X.

Commensurability effect of magnetoresistance anisotropy in the quasi-one-dimensional conductor tetramethyltetraselenafulvalenium perchlorate, (TMTSF)2ClO4.

We have observed a clear series of fine structures superposed on the angular dependence of magnetoresistance in the metallic and magnetic-field-induced spin-density-wave phases in the low-dimensional organic superconductor (TMTSF${)}_{2}$${\mathrm{ClO}}_{4}$. They appear at the field angles whose tangent satisfies a fractional condition reflecting the crystal geometry. This new oscillatory phenomenon is considered to result from a resonance effect, i.e., commensurate versus incommensurate, between two periodicities in the electron motion along an open orbit under tilted magnetic fields.