Jahn-Teller polarons and their superconductivity in a molecular conductor

We present a theoretical study of a possibility of superconductivity in a three dimensional molecular conductor in which the interaction between electrons in doubly degenerate molecular orbitals and an {\em intra}molecular vibration mode is large enough to lead to the formation of $E\otimes \beta$ Jahn-Teller small polarons. We argue that the effective polaron-polaron interaction can be attractive for material parameters realizable in molecular conductors. This interaction is the source of superconductivity in our model. On analyzing superconducting instability in the weak and strong coupling regimes of this attractive interaction, we find that superconducting transition temperatures up to 100 K are achievable in molecular conductors within this mechanism. We also find, for two particles per molecular site, a novel Mott insulating state in which a polaron singlet occupies one of the doubly degenerate orbitals on each site. Relevance of this study in the search for new molecular superconductors is pointed out.Comment: Submitted to Phys. Rev.

Temperature dependence of infrared spectral properties of the dimerized quasi 1-d TCNQ salts

Temperature dependence of spectral properties of dimerized TCNQ salts is explained in terms of isolated dimer model. Electron-electron correlation in these compounds is shown to be of major importance. The analysis of Re(1/σ (ω)) function, which can be calculated from reflectance data, is claimed to allow determination of temperature dependence of electron-molecular vibrations coupling parameters without ambiguity.Les variations thermiques des propriétés spectrales des sels TCNQ construits de dimères sont expliquées par le modèle de dimère isolé. Le rôle important des corrélations électron-électron dans ces composés est souligné. L'analyse de la fonction Re (1/σ (ω)) calculée en tenant compte de la réflexion a permis de déterminer sans ambiguïté la dépendance thermique des paramètres de couplage électron-phonon

Dissipative properties of three-layered composite structures. 4. Numerical experiment

Object and purpose of research. The study is concerned with a three-layer plate formed by two rigid anisotropic layers and a soft medium isotropic layer of viscoelastic polymer. Each rigid layer presents anisotropic structure formed by a finite number of randomly oriented orthotropic viscoelastic layers of composites. The paper is intended to study the influence of reinforcement orientation of rigid layers, relative thickness of the soft layer of an isotropic viscoelastic polymer and the ambient temperature on values values of natural frequencies and mechanical loss factors of the coupled damped oscillations in symmetric and assymetric plates. Subject matter and methods. Numerical experiment using a computer program implementing the previously proposed method for solving coupled differential equations of damped oscillations in anisotropic three-layer plates [2]. Main results. It was shown that in unsupported globally monoclinic symmetrical three-layer rectangular plate a bendingtorsional interaction occurs, generating mutual transformations of the eigenforms of the coupled oscillation modes if at least in one of the directions of the plate one of the eigenforms is characterized by an even number of quarters of the wave, and the other eigenform is characterized by an odd number of quarters of the wave. In unsupported globally orthotropic asymmetric three-layer rectangular plate interaction of bending modes of oscillations occurs in two mutually orthogonal planes, if both eigenforms are characterized by either an even or an odd number of wave quarters in main directions of the plate. It was found that each mode of natural oscillations of both symmetric and asymmetric three-layer plates has its own effective relative thickness of the soft layer of an isotropic viscoelastic polymer corresponding to the maximum level of dissipative properties. A further increase in the relative thickness is often accompanied by a decrease in values of the mechanical loss factors. The significant influence of ambient temperature on natural frequencies values and mechanical loss factors of all considered oscillation modes of symmetric and asymmetric unsupported rectangular three-layer composite plates is demonstrated. Conclusion. It was found that coupled damped oscillations of a symmetric three-layer plate are described by two systems of differential equations, with structures close to that of the systems of corresponding differential equations describing the damped oscillations of a quasi-homogeneous monoclinic plate. At the same time, the coupled damped oscillations of an asymmetric three-layer plate are described by two systems of differential equations that coincide with the systems of corresponding differrential equations describing the damped oscillations of a globally orthotropic three-layer plate

Modelling κ\mathsf \kappa Phase Organic Conductors

$\kappa$-phase organic conductors with bidimensional layers of orthogonal molecular dimers are modelled by tetramers and hexamers of appropriate geometry. The complex conductivity is calculated within the Hubbard model including the electron-intramolecular vibration coupling. The polarized optical conductivity data of two $\kappa$-phase charge-transfer salts of $bis$-(ethylenedithio)-tetrathiafulvalene: $\kappa$-(BEDT-TTF)$_2$[ Hg(SCN)$_2$Br] and $\kappa$-(BEDT-TTF)$_2$[ Hg(SCN)Cl$_2$] , are discussed

Inelastic electron tunneling spectroscopy of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) compounds

Inelastic electron tunneling spectra of thin films of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and its salts: a low temperature superconductor κ-(BEDT-TTF) 2Cu(NCS) 2 and an insulator (BEDT-TTF) 2Mo 6O 19 are measured at 4.2 K in the range 230–1650 cm −1 and compared with infrared spectra of the same compounds at room temperature. The obtained vibrational bands are assigned following theoretical normal mode calculations. Electron-molecular vibration coupling theory of charge-transfer organic compounds is extended to the case of electron tunneling spectra

Effect of vacancies on the spin density waves onset in Fe/Cr superlattices

International audienceThe spin density wave's onset in Cr based superlattices is considered within proximity of Fe interlayer boundaries and the effect of randomly located vacancies in Cr monolayers is examined. The study is performed for Fe/Cr, Fe/Cr/V superlattices with odd and even number of Cr monolayers. It is shown that the number of Cr monolayer determines the spin density wave's nodes onset in the perfect Fe/Cr super lattices. Pinning of Cr magnetic moments on vacancies destroys this determination and leads to appearance or disappearance of nodes