M. Ribault

Pressure-temperature phase diagram of α′-TTF [Pd(Dmit)2]2

Abstract Temperature dependence of resistivity (up to 24 Kbar) and of E.S.R. (at 1 bar) measurements suggest that centered monoclinic TTF[Pd(Dmit) 2 ] 2 could be an anisotropic and weak antiferromagnetic fluctuating metal. At low pressure, gradual charge and spin localizations at different temperatures are the signature of a Mott-Hubbard insulator. Above 20 Kbar, “high” superconducting onset temperatures (6.5 K) are observed.

Metallic State Stabilized under Pressure in (EDT-TTF)2(Pd(dmit)2)2

This letter reports upon transport properties of the (EDT-TTF)2 [Pd(dmit)2]2 compound. A nonlinear effect is observed below 50 K at ambient pressure. A resistivity bump, observed at ca. 40 K at ambient pressure, is shifted towards lower temperature under pressure and a metallic ground state is stabilized up to 10 kbar. In order to understand lack of superconductivity above 500 mK, low-dimensional electronic correlations are stressed on.

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

Abstract We present the first study of thermal conductivity of a quasi two dimensional organic superconductor. κ-(BEDT-TTF) 2 Cu(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.

Low-temperature in-plane thermal conductivity of untwinned YBa2Cu3O6.9 crystals

The thermal conductivity of high-quality untwinned single crystals of YBa2Cu3O6.9 was studied as a function of temperature (0.1 K < T < 1 K) and with heat current injected along two different orientations (a and b crystalline axes). We detect the presence of a linear term with comparable magnitude for the two orientations. This is in contrast with the high-temperature data, where an anisotropic behaviour is reported. The possible electronic origin of the linear term and its implication for the gap symmetry are discussed.

FERMI-SURFACE INSTABILITIES IN THE ORGANIC CONDUCTOR (TMTSF)2NO3: HIGH-PRESSURE STUDIES

The study of the magnetoresistance in the organic quasi-one-dimensional conductor (TMTSF)2NO3 at a pressure (8 kbar) exceeding the critical pressure required for the stabilization of a conducting state at low temperature has revealed the existence of quantum oscillations with a frequency of 190 T in good agreement with an extended Huckel-type calculation of the band structure. Unlike other members of the (TMTSF)2X series the ambient pressure SDW ground state in (TMTSF)2NO3 can be understood in terms of an instability involving the nesting of 2D semi-metallic Fermi surfaces.

Inhibited superconductivity induced by localization effects in (EDT-TTF)2[Pd(dmit)2]2

Abstract We report on transport properties of (EDT-TTF) 2 [Pd(dmit) 2 ] 2 . The occurence of a resistivity anomaly and of a metallic ground state, which is stabilized under pressure, is discussed within tight-binding band structure calculations.

Interplay between charge density waves and reentrant superconductivity in the pressure — temperature phase diagram of TTF(Ni(dmit)2)2

Abstract The pressure-temperature phase diagram of the molecular superconductor TTF(Ni(dmit) 2 ) 2 was determined by a.c. resistivity measurements up to 14 kbar. Increasing pressures induce electronic phase transitions between a high temperature metal and successive, semimetallic, insulating and reentrant superconducting ground states. This phase diagram is discussed in connection with ambient pressure charge density wave (CDW) instabilities. Their wave vector can be well accounted for by an original conduction band structure. This structure involves both the partially filled HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) bands of the acceptor slabs. The superconductivity coexists with a high temperature CDW instability and is in weak competition with low temperature CDW fluctuations. This explains the pressure increase of the superconducting temperature T c .

Symmetry of the superconducting order parameter in Bechgaard salts

Abstract Two decades after the discovery of superconductivity in a Bechgaard salt, the pairing mechanism is not yet identified. The first step towards such an identification is to determine the symmetry of the superconducting order parameter. Results on heat transport in the superconducting state of (TMTSF) 2 ClO 4 indicate a finite gap all over the Fermi surface. The magnitude of this gap is confirmed by the first experiments done on break junctions. A nodeless gap imposes a number of constraints on the symmetry of the order parameter,

In-plane symmetry of the superconducting gap in YBCO probed by the angular dependence of thermal magnetoresistance

Abstract We present a study of the thermal conductivity of a detwinned YBa 2 Cu 3 O 6.9 single crystal as a function of the angle between the crystal axes and a rotating magnetic field. Measurements were done at several different temperatures below T c and at a fixed field of 30 kOe. When the field rotates in a plane parallel to the CuO planes, thermal conductivity shows a four-fold angular variation with maxima at 45 degrees off the principal symmetry axes. This is caracteristic of an anisotropic superconducting gap associated with a d x 2 − y 2 order parameter. An upper limit on the weight of the possible s-wave component can be deduced from the angular positions of the extrema.

Fermi surface and physical properties of (TMTSF)2NO3

Abstract We studied the Fermi surface of the organic conductor (TMTSF) 2 NO 3 through the measurements of magnetic quantum oscillations. Well defined Shuvnikov-de Haas(SdH) oscillations with frequency of ca. 60T can be observed below 8K and above 6T at the ambient pressure. Under a pressure of 8kbar, different oscillations with frequency of ca. 180T is observed above 10T at 450mK. Electron structure at low temperature seems strongly modified upon application of pressure which may explain the absence of superconductivity and the field-induced spin-density waves. The angular magnetoresistance could not be fit neither by Yamajis model nor by Lebeds.