Zero bias anomalies in point-contact characteristics of αt-(BEDT-TTF)2I3

The zero-bias anomaly in point-contact characteristics of the organic superconductor α-(BEDT-TTF)2I3 is investigated as a function of temperature and magnetic field. It is found that the zero-bias anomaly is insensitive to magnetic fields up to 5 T. In contrast, a structure at 5 meV, conventionally designated as the superconducting gap - but which is 4 times larger than the expected BCS gap - is strongly affected by magnetic fields above 1 T

Resonant Raman scattering from superconducting single crystals of (BEDT-TTF)2I3

A study of low energetic resonant Raman scattering of (BEDT-TTF)2I3 superconducors was performed. The vanishing of phonon bands accompanied by a decrease of the electronic background was observed below Tc. We propose a theoretical explanation for this novel effect in terms of the Balseiro-Falicov model of phonon-superconducting amplitude mode interaction

Determination of the electron phonon coupling and the superconducting gap in β-(BEDT-TTF)2X crystals (X=I3, IAuI)

We performed point-contact and tunneling measurements on the organic superconductors beta-(BEDT-TTF)2I3 and beta-(BEDT-TTF)2IAuI, in the normal and superconducting states. The point-contact measurement in the normal state provides the Eliashberg function alpha2(omega)F (omega). This function has maxima at 4 and 15 mV, as well as a sharp peak at 1 mV which seems to indicate a very soft phonon strongly coupled to the electrons. The measurements in the superconducting state provide the superconducting gap, which for the tunneling measurements has a value 2Delta/kB Tc=4, just slightly above the BCS value

Band structure calculation and tunneling measurements in (BEDT-TTF)2X (X=I3, IAuI)

At about the same time when little proposed room-temperature superconductivity in organic polymers, it was suggested that the high-Tc of the more conventional A-15's is associated with their one-dimensional electronic band structure. When TTF-TCNQ was discovered in 1973, it was suggested that the electron-phonon coupling in this 1-D organic molecular crystal is responsible for the metal-to-insulator (Peierls) transition at 52 K2, and reducing λ will cause a crossover to a superconducting state. Since then, the electronic structure, the conduction mechanism, and the superconducting mechanism were subject to controversy. Therefore, it is of some importance to establish whether the electronic band structure, and conduction mechanism, are similar to those in more conventional metals, and whether the superconductivity mechanism is the normal BCS phonon-mediated interaction

Theory of Coexistence of Superconductivity and Ferroelectricity : A Dynamical Symmetry Model

We propose and investigate a model for the coexistence of Superconductivity (SC) and Ferroelectricity (FE) based on the dynamical symmetries $su(2)$ for the pseudo-spin SC sector, $h(4)$ for the displaced oscillator FE sector, and $su(2) \otimes h(4)$ for the composite system. We assume a minimal symmetry-allowed coupling, and simplify the hamiltonian using a double mean field approximation (DMFA). A variational coherent state (VCS) trial wave-function is used for the ground state: the energy, and the relevant order parameters for SC and FE are obtained. For positive sign of the SC-FE coupling coefficient, a non-zero value of either order parameter can suppress the other (FE polarization suppresses SC and vice versa). This gives some support to "Matthias' Conjecture" [1964], that SC and FE tend to be mutually exclusive. For such a Ferroelectric Superconductor we predict: a) the SC gap $\Delta$ (and $T_c$) will increase with increasing applied pressure when pressure quenches FE as in many ferroelectrics, and b) the FE polarization will increase with increaesing magnetic field up to $H_c$. The last result is equivalent to the prediction of a new type of Magneto-Electric Effect in a coexistent SC-FE material. Some discussion will be given of the relation of these results to the cuprate superconductors.Comment: 46 page

Point contact spectroscopy measurements on αt-(BEDT-TTF)2I3

We report point-contact spectroscopy 1-V data on superconducting αt-(BEDT-TTF)2I3. The I-V curves show structure in 3 regions: for energies of order 0.25 meV, of about 4,5 meV and for energies of about 10 meV. No structure is observed near the BCS gap (1.3-1.4 meV). We attribute the structure at 4.5 meV to interaction with the phonon possessing this frequency which was observed independently by Raman spectroscopy, and the structure near 0.25 meV to an excitation energy considerably smaller than the BCS gap

Point contact spectroscopy measurements of αt-(BEDT-TTF)2I3 : an alternative interpretation

We report on point contact measurements on the organic superconductor αt-(BEDT-TTF)2I3 (Tc~8K). The dV/dI vs V characteristics of homocontacts of this material show strong nonlinearities which, interpreted as a gap-structure, lead to extremly high values of Δ/κTc. Among other possibilities, we discuss the anomaly in terms of a strongly enhanced McMillan-Rowell structure Δ(ε) at ε = Δ BCS + α ph. The enhancement is caused by deviations from Eliashberg equations due to strong coupling (λ = 1-2) together with an Einstein phonon spectrum