R. Bennaceur

Correlation effects in ET compounds

Abstract The ET salt family is described by a two parameter 2D Fermi surface model, in which bandwidth and departure from perfect nesting can be varied. We have studied the effect of spin fluctuations on the normal state properties in a RPA approach. The calculated NMR relaxation rate exhibits a peak in 1/T1T, which strongly decreases when the departure from perfect nesting of the Fermi surface and the bandwidth increase. These results are in perfect agreement with NMR experiments done in (BEDT-TTF)2 X (X= Cu[N(CN)2]Br, Cu(NCS)2), at least qualitatively. We investigate the superconductivity pairing due to spin fluctuations and a model of quasi-2D electrons close to SDW instability, described by a two-parameter Fermi surface model, in which bandwidth and departure from perfect nesting can be varied. The coupling constants of various channels are calculated. They strongly depend on the nesting properties of the Fermi surface and on the bandwidth.

NMR Anomalies in Quasi-2D Organic Superconductors: Effects of Spin Fluctuations in a Fermi Liquid Approach

The κ-(BEDT-TTF)2X organic superconductors are described by a two parameter 2D Fermi surface model, in which bandwidth and departure from perfect nesting can be varied. We have studied the spin fluctuations effect on the normal state properties in a Fermi liquid approach using the RPA approximation. The calculated NMR relaxation rate exhibits a peak in 1/(T1T), which strongly decreases when the departure from perfect nesting of the Fermi surface and the bandwidth increase. These results are in good agreement with NMR experiments done in κ-(ET)2X at least qualitatively. In conclusion, we have shown that, in the normal state and with a Fermi liquid approach, the spin fluctuations, which are present in the system due to an imperfect nesting property of the Fermi surface, can induce anomalies of the magnetic properties. Besides, we can restore the usual behaviour like the Korringa law by increasing the bandwidth or by considering a more imperfect nesting. Our calculation reproduces qualitatively the applied pressure relaxation rate experiment done in κ-(ET)2X salt.

Study of magnetic properties within an imperfect nested Fermi surface

Abstract We investigate the quasi-particle behaviour of a quasi-2D electrons system within an imperfect nesting peoperty of the Fermi surface. Our study treats the effects of spin fluctuations on a metallic phase. Within a Fermi liquid approach, we show that spin fluctuations can induce unusual quasi-particle properties and abnormal temperature dependences in the metallic phase of the relaxation rate.

New phase induced by a magnetic field in Bi2212 superconductors

Abstract We have performed a mean field treatment of a simple model of quasi-two-dimensional d-wave superconductor, assuming two possible order parameters : a superconducting one and a magnetic one. We have shown that a magnetic field can induce a mixed phase in which both orders coexist. We propose that such a simple model might provide a possible explanation for the anomalies observed in the thermal conductivity of Bi 2 Sr 2 CaCu 2 O 8+δ in a moderate applied field.

New symmetry breaking in high Tc superconductors

Abstract We study a nearly square 2D Fermi surface with strong nesting properties and a Van Hove singularity at the Fermi level, as a model for high Tc superconductors, such as Bi2Sr2CaCu2O8 (Bi2212). Such a model is consistent with a recent photoemission data. We assume an effective (phenomenological) attractive coupling between electrons as well as an exchange coupling. The {Ln(kT/Ef)}2 behaviour of the superconducting response favours a d-wave superconducting instability. When a magnetic field is applied perpendicularly to the electron plane, an orbital effect restore a 1D behaviour for the magnetic response, with an effective exchange constant rapidly increasing with the field. Therefore, we expect, above a critical field, a coexistence of superconductivity and spin density wave (SDW). This new symmetry breaking should open a gap on the whole Fermi surface. We propose to ascribe to this affect the anomalies observed in the thermal conductivity of Bi2212 and the apparent disappearance of the quasi-particles in an applied field.

Normal state properties of ET organic conductors

Abstract We study quasi-2D organic conductors particularly κ-(ET) 2 Cu[N(CN) 2 ]Br and κ-(ET) 2 Cu[N(CN) 2 ]Cl in which the Fermi surface exhibits nesting properties. The nesting induces specific properties. In particular, it leads to a spin density wave (SDW) ground state below a critical pressure. The phase diagram is obtained within a mean field theory. Nesting properties lead to important effect of antiferromagnetic fluctuations on the normal state. It may explain the abnormal temperature dependence of the observed NMR spin-lattice relaxation rate. To discuss these effects in our calculation, we use an analytical dispersion relation which fits the Fermi surface determined by band structure calculations.

Marginal Fermi liquid behaviour within a Fermi liquid theory

Abstract Abnormal properties of the normal state of high T C superconductors have lead to models which discard the normal Fermi liquid picture. We propose a phenomenological model to describe the electrons in a marginal Fermi liquid approach of a quasi-2D conductor exhibiting a Van Hove singularity at the Fermi Level. We assume a self-energy with a Fermi liquid behaviour near the Fermi energy and a marginal behaviour for energies higher than a crossover energy ω C . This leads to the destruction of the Van Hove singularity and to the opening of a pseudogap near the Fermi level. We show that the Pauli susceptibility is temperature dependent and we can relate the pseudogap width with the crossover energy ω C .