Dimensional Organic Superconductor Research Articles

Heat conduction in κ-(BEDT-TTF) 2X superconductors

We present the first study of thermal conductivity of a quasi two dimensional organic superconductor. κ-(BEDT-TTF) 2Cu(NCS) 2 presents features which have already been detected in the high T c cuprates. Notably we observed an increase of the thermal conductivity at the superconducting transition which is suppressed by a moderate magnetic field. At low temperatures we clearly resolved a residual electronic contribution close to the universal limit value which is consistent with an anisotropic pairings with lines of nodes in the superconducting gap function.

First order vortex phase transition in the organic superconductor {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2}

The local magnetic induction is measured in the quasi-two dimensional organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 by using a micro Hall probe. The small step is observed on the temperature dependence of the local magnetic induction. This result implies the existence of the first-order phase transition of the vortex system in this organic superconductor as is already reported in high-T c oxide and other organic superconductors.

The relationship between the critical temperature Tc and the quasiparticle effective mass in quasi-two dimentional organic superconductors

We will discuss theoretically the relationship between the superconducting critical temperature T c and the effective mass of holes in the quasi-two dimensional organic superconductors by means of the gauge invariant carrieron model, which is based on the gauge invarient effective Lagrangian density.

Effective mass and combination frequencies of de Haas-van Alphen oscillations in κ-(BEDT-TTF) 2Cu(NCS) 2

De Haas-van Alphen (dHvA) measurements in the two dimensional organic superconductor κ-(BEDT-TTF) 2Cu(NCS) 2 have been performed down to .44K in a wide magnetic field range up to 30T. In the range between 20T and 30T, we find dHvA frequencies corresponding to the closed (α) and magnetic breakdown (β) orbits, and many combination frequencies of α and β as well. The observation of the combination frequencies β-α and 2β-2α is not understood by conventional theories because both the torque and magnetic interaction effects are negligibly small. The effective masses and the field dependence of these oscillations are reported.

High-field Shubnikov-de Haas effect in the organic superconductor α-(BEDT-TTF) 2NH 4Hg(SCN) 4

We report Shubnikov-de Haas (SdH) measurements on the quasi-two dimensional organic superconductor α-(BEDT-TTF) 2NH 4Hg(SCN) 4 in pulsed fields up to 50 T. Analysis of the field and temperature-dependence of the oscillations reveals several unusual features, such as a field-dependent effective mass parameter, non-linear Dingle plots and an anomalously large harmonic ratio. These results suggest departures from the Lifshitz-Kosevich formalism, arising from the highly two-dimensional nature of the material.

Angular magnetoresistance oscillations and magnetic breakdown in θ-(BEDT-TTF) 2I 3

We have conducted a systematic investigation of the angular dependent magnetoresistance oscillations (ADMRO) and Shubnikov-de Haas oscillations (SdH) in the quasi-two dimensional organic superconductor θ-(BEDT-TTF) 2I 3 . The Fermi-surface derived from the AMDRO is found to correspond in size to the SdH oscillation orbit β, which is due to magnetic breakdown. The identification of this orbit highlights the sensitivity of the ADMRO. The ADMRO anisotropy is interpreted in terms of magnetic breakdown probability.

Revisiting the superconducting phase diagram of (TMTSF) 2ClO 4

We present results of a search for evidence of reentrance at high magnetic field of the superconducting state in the quasi-one dimensional organic superconductor (TMTSF) 2ClO 4, for a specific orientation of the field at low temperature. This search was motivated by the theoretical work of Lebed' and of Dupuis & Montambaux, and in part by that of Teŝanovic and coworkers, who predict novel magnetic field-temperature phase diagrams in quasi-1-D and quasi-2-D superconductors in a strong magnetic field. In the quasi-1-D case, a field directed both normal to the chains and along the layers is proposed to induce exotic oscillations of the critical temperature at high field, while for the 2-D case, quantum effects associated with Landau quantization lead to similar predictions. We have investigated the superconducting transition in a quasi-1-D candidate material, (TMTSF) 2ClO 4, to 0.06 K with field oriented along the b′-axis (±0.02°). From mostly constant field temperature sweeps, we find positive curvature in H c2(T) below about T c/2. In addition, a resistivity decrease is observed to persist to 7 T, the highest field employed, with a slightly positive slope ∂H/∂T above 5T. The possibility of these features being representative of high field superconductivity is discussed.

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.

Magnetic penetration depth and flux melting in κ-(BEDT-TTF)2Cu[N(CN)2]Br

We have performed muon spin relaxation studies in the quasi-2 dimensional organic superconductor κ-(ET) 2 Cu[N(CN) 2 ]Br, applying an external field of 3 kG perpendicular to the conducting planes of single crystal specimens. The relaxation rate in field-cooled measurements shows a linear variation for T ≤ 0.4 T c . This behavior is consistent with an anisotropic pairing state with line nodes in the superconducting energy gap. Zero-field-cooling measurements reveal that magnetic flux is not well-pinned until T ≤ 5 K, well below T c = 12 K.

Hall effect in (DMET)2I3 under pressure

The Hall effect under pressure is presented for the quasi one dimensional organic superconductor, (DMET)2I3. Below 20–30 K, Hall coefficient decreases from the high temperature almost constant value. Such variation of Hall effect with temperature is discussed comparing with other organic superconductors, such as (DMET)2Au(CN)2, ß-(BEDT-TTF)2I3 and κ-(BEDT-TTF)2Cu(NCS)2

Structure of Di-Tetramethyltetraselen-afulvalenium Perchlorate (TMTSF)2 CϱO4 in its Low Temperature (T ∼ 10 K) Ordered Phase

Abstract At T ∼ 24K, the quasi one dimensional organic superconductor (TMTSF)2CϱO4 shows a structural transition leading to a doubling (a × 2b × c) of the unit cell. The neutron average structure (i.e. superlattice reflections not included) at T = 7K has already been published. In the present paper, we report the results of a rigid body refinement of the low temperature (T ∼ 10K) structure from both the earlier set of neutron main reflection intensities and an extra set of X-ray superlattice intensities. This improved structural description clearly shows that the anion orientations alternate in the b direction only and that there is no significant TMTSF displacement from the average position. We compare these results with recent structural data obtained on (TMTSF)2ReO4 which exhibits below T ∼ 180 K an insulating ground state instead of the metallic one for the perchlorate salt.

The three dimensional stabilization of superconductivity at 12 K in the one dimensional organic superconductor (TMTSF)2PF6 under 11 kbar

The three dimensional stabilization of superconductivity at 12 K in the one dimensional organic superconductor (TMTSF)2PF6 under 11 kbar