Tailoring Graphene with Metals on Top

We study the effects of metallic doping on the electronic properties of graphene using density functional theory in the local density approximation in the presence of a local charging energy (LDA+U). The electronic properties are sensitive to whether graphene is doped with alkali or transition metals. We estimate the the charge transfer from a single layer of Potassium on top of graphene in terms of the local charging energy of the graphene sheet. The coating of graphene with a non-magnetic layer of Palladium, on the other hand, can lead to a magnetic instability in coated graphene due to the hybridization between the transition-metal and the carbon orbitals.Comment: 5 pages, 4 figure

An alternative approach for the dynamics of polarons in one dimension

We developed a new method based on functional integration to treat the dynamics of polarons in one-dimensional systems. We treat the acoustical and the optical case in an unified manner, showing their differences and similarities. The mobility and diffusion coefficients are calculated in the Markovian approximation in the strong coupling limit.Comment: 57 page

Exotic Superconducting Phases of Ultracold Atom Mixtures on Triangular Lattices

We study the phase diagram of two-dimensional Bose-Fermi mixtures of ultracold atoms on a triangular optical lattice, in the limit when the velocity of bosonic condensate fluctuations is much larger than the Fermi velocity. We contrast this work with our previous results for a square lattice system in Phys. Rev. Lett. {\bf 97}, 030601 (2006). Using functional renormalization group techniques we show that the phase diagrams for a triangular lattice contain exotic superconducting phases. For spin-1/2 fermions on an isotropic lattice we find a competition of $s$-, $p$-, extended $d$-, and $f$-wave symmetry, as well as antiferromagnetic order. For an anisotropic lattice, we further find an extended p-wave phase. A Bose-Fermi mixture with spinless fermions on an isotropic lattice shows a competition between $p$- and $f$-wave symmetry. These phases can be traced back to the geometric shapes of the Fermi surfaces in various regimes, as well as the intrinsic frustration of a triangular lattice.Comment: 6 pages, 4 figures, extended version, slight modification

Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)2_2X

High-resolution thermal expansion measurements have been performed for exploring the mysterious "structureless transition" in (TMTTF)$_{2}$X (X = PF$_{6}$ and AsF$_{6}$), where charge ordering at $T_{CO}$ coincides with the onset of ferroelectric order. Particularly distinct lattice effects are found at $T_{CO}$ in the uniaxial expansivity along the interstack $\textbf{\textit{c*}}$-direction. We propose a scheme involving a charge modulation along the TMTTF stacks and its coupling to displacements of the counteranions X$^{-}$. These anion shifts, which lift the inversion symmetry enabling ferroelectric order to develop, determine the 3D charge pattern without ambiguity. Evidence is found for another anomaly for both materials at $T_{int}$ $\simeq$ 0.6 $\cdot$ $T_{CO}$ indicative of a phase transition related to the charge ordering

Negative Hopping Magnetoresistance and Dimensional Crossover in Lightly Doped Cuprate Superconductors

We show that, due to the weak ferromagnetism of La$_{2-x}$Sr$_x$CuO$_4$, an external magnetic field leads to a dimensional crossover 2D $\to$ 3D for the in-plane transport. The crossover results in an increase of the hole's localization length and hence in a dramatic negative magnetoresistance in the variable range hopping regime. This mechanism quantitatively explains puzzling experimental data on the negative magnetoresistance in the N\'eel phase of La$_{2-x}$Sr$_x$CuO$_4$.Comment: 6 pages, 3 figures; published versio