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.

Robustness of a local Fermi Liquid against Ferromagnetism and Phase Separation

We study the properties of Fermi Liquids with the microscopic constraint of a local self-energy. In this case the forward scattering sum-rule imposes strong limitations on the Fermi-Liquid parameters, which rule out any Pomeranchek instabilities. For both attractive and repulsive interactions, ferromagnetism and phase separation are suppressed. Superconductivity is possible in an s-wave channel only. We also study the approach to the metal-insulator transition, and find a Wilson ratio approaching 2. This ratio and other properties of Sr_{1-x}La_xTiO_3 are all consistent with the local Fermi Liquid scenario.Comment: 4 pages (twocolumn format), can compile with or without epsf.sty latex style file -- Postscript files: fig1.ps and fig2.p

Studies of bosons in optical lattices in a harmonic potential

We present a theoretical study of bose condensation and specific heat of non-interacting bosons in finite lattices in harmonic potentials in one, two, and three dimensions. We numerically diagonalize the Hamiltonian to obtain the energy levels of the systems. Using the energy levels thus obtained, we investigate the temperature dependence, dimensionality effects, lattice size dependence, and evolution to the bulk limit of the condensate fraction and the specific heat. Some preliminary results on the specific heat of fermions in optical lattices are also presented. The results obtained are contextualized within the current experimental and theoretical scenario.Comment: Revised version, References updated, a new section on Fermions added, 14 pages, 16 figure

Bose-Einstein condensation in multilayers

The critical BEC temperature $T_{c}$ of a non interacting boson gas in a layered structure like those of cuprate superconductors is shown to have a minimum $T_{c,m}$, at a characteristic separation between planes $a_{m}$. It is shown that for $a<a_{m}$, $T_{c}$ increases monotonically back up to the ideal Bose gas $T_{0}$ suggesting that a reduction in the separation between planes, as happens when one increases the pressure in a cuprate, leads to an increase in the critical temperature. For finite plane separation and penetrability the specific heat as a function of temperature shows two novel crests connected by a ridge in addition to the well-known BEC peak at $T_{c}$ associated with the 3D behavior of the gas. For completely impenetrable planes the model reduces to many disconnected infinite slabs for which just one hump survives becoming a peak only when the slab widths are infinite.Comment: Four pages, four figure

Small Polarons in Transition Metal Oxides

The formation of polarons is a pervasive phenomenon in transition metal oxide compounds, with a strong impact on the physical properties and functionalities of the hosting materials. In its original formulation the polaron problem considers a single charge carrier in a polar crystal interacting with its surrounding lattice. Depending on the spatial extension of the polaron quasiparticle, originating from the coupling between the excess charge and the phonon field, one speaks of small or large polarons. This chapter discusses the modeling of small polarons in real materials, with a particular focus on the archetypal polaron material TiO2. After an introductory part, surveying the fundamental theoretical and experimental aspects of the physics of polarons, the chapter examines how to model small polarons using first principles schemes in order to predict, understand and interpret a variety of polaron properties in bulk phases and surfaces. Following the spirit of this handbook, different types of computational procedures and prescriptions are presented with specific instructions on the setup required to model polaron effects.Comment: 36 pages, 12 figure

Helix packing motif common to the crystal structures of two undecapeptides containing dehydrophenylalanine residues: implications for the de novo design of helical bundle super secondary structural modules

De novo designed peptide based super secondary structures are expected to provide scaffolds for the incorporation of functional sites as in proteins. Self-association of peptide helices of similar screw sense, mediated by weak interactions, has been probed by the crystal structure determination of two closely related peptides: Ac-Gly1-Ala2-&#916;Phe3-Leu4-Val5-&#916;Phe6-Leu7-Val8-&#916;Phe9-Ala10-Gly11-NH2 (I) and Ac-Gly1-Ala2-&#916;Phe3-Leu4-Ala5-&#916;Phe6-Leu7-Ala8-&#916;Phe9-Ala10-Gly11-NH2 (II). The crystal structures determined to atomic resolution and refined to R factors 8.12 and 4.01%, respectively, reveal right-handed 310-helical conformations for both peptides. CD has also revealed the preferential formation of right-handed 310-helical conformations for both molecules. Our aim was to critically analyze the packing of the helices in the solid state with a view to elicit clues for the design of super secondary structural motifs such as two, three, and four helical bundles based on helix-helix interactions. An important finding is that a packing motif could be identified common to both the structures, in which a given peptide helix is surrounded by six other helices reminiscent of transmembrane seven helical bundles. The outer helices are oriented either parallel or antiparallel to the central helix. The helices interact laterally through a combination of N-H ... O, C-H ... O, and C-H ... &#960; hydrogen bonds. Layers of interacting leucine residues are seen in both peptide crystal structures. The packing of the peptide helices in the solid state appears to provide valuable leads for the design of super secondary structural modules such as two, three, or four helix bundles by connecting adjacent antiparallel helices through suitable linkers such as tetraglycine segment