Hopping-resolved electron-phonon coupling in bilayer graphene

In this paper we investigate the electron-phonon coupling in bilayer graphene, as a paradigmatic case for multilayer graphenes where interlayer hoppings are relevant. Using a frozen-phonon approach within the context of Density Functional Theory (DFT) and using different optical phonon displacements we are able to evaluate quantitatively the electron-phonon coupling $\alpha_i$ associated with each hopping term $\gamma_i$. This analysis also reveals a simple scaling law between the hopping terms $\gamma_i$ and the electron-phonon coupling $\alpha_i$ which goes beyond the specific DFT technique employed.Comment: 10 pages, 10 fig

Vertex renormalization in dc conductivity of doped chiral graphene

The remarkable transport properties of graphene follow not only from the the Dirac-like energy dispersion, but also from the chiral nature of its excitations, which makes unclear the limits of applicability of the standard semiclassical Boltzmann approach. In this paper we provide a quantum derivation of the transport scattering time in graphene in the case of electron-phonon interaction. By using the Kubo formalism, we compute explicitly the vertex corrections to the dc conductivity by retaining the full chiral matrix structure of graphene. We show that at least in the regime of large chemical potential the Boltzmann picture is justified, and it is also robust against a small sublattice inequivalence which partly spoils the role of chirality.Comment: (pages late

Interplay of superconductivity with structural phases in a generalized t-J model

The phase diagram of the t-J-V model is discussed using a 1/N expansion in terms of X operators. It is shown that a flux phase of d-wave symmetry is stabilized by the Coulomb interaction V at intermediate dopings and competes with d-wave superconductivity. Since the flux wave instability is stronger than the superconducting one optimal doping is essentially determined by the onset of the flux phase. Below optimal doping the flux phase coexists with superconductivity at low and exists as a pseudo gap phase at higher temperatures. It is also found that the flux phase boundary is much less sensitive to impurity scattering than the boundary for superconductivity in agreement with experiments in Zn doped La-214 and (Y,Ca)-123.Comment: 4 pages, 3 figures, Proceed. M2S-HTSC-VI Housto

Strong-coupling properties of unbalanced Eliashberg superconductors

In this paper we investigate the thermodynamical properties of ``unbalanced'' superconductors, namely, systems where the electron-boson coupling $\lambda$ is different in the self-energy and in the Cooper channels. This situation is encountered in a variety of situation, as for instance in d-wave superconductors. Quite interesting is the case where the pairing in the self-energy is smaller than the one in the gap equation. In this case we predict a finite critical value $\lambda_c$ where the superconducting critical temperature $T_c$ diverges but the zero temperature gap is still finite. The specific heat, magnetic critical field and the penetration depth are also evaluated.Comment: 9 Revtex pages, 7 eps figures include

Spin susceptibility in small Fermi energy systems: effects of nonmagnetic impurities

In small Fermi energy metals, disorder can deeply modify superconducting state properties leading to a strong suppression of the critical temperature $T_c$. In this paper, we show that also normal state properties can be seriously influenced by disorder when the Fermi energy $E_{\rm F}$ is sufficiently small. We calculate the normal state spin susceptibility $\chi$ for a narrow band electron-phonon coupled metal as a function of the non-magnetic impurity scattering rate $\gamma_{\rm imp}$. We find that as soon as $\gamma_{\rm imp}$ is comparable to $E_{\rm F}$, $\chi$ is strongly reduced with respect to its value in the clean limit. The effects of the electron-phonon interaction including the nonadiabatic corrections are discussed. Our results strongly suggest that the recent finding on irradiated MgB$_2$ samples can be naturally explained in terms of small $E_{\rm F}$ values associated with the $\sigma$-bands of the boron plane, sustaining therefore the hypothesis that MgB$_2$ is a nonadiabatic metal.Comment: 7 pages, 6 eps figures, to appear on Eur. Phys. J.

Interactions and superconductivity in heavily doped MoS2

We analyze the microscopic origin and the physical properties of the superconducting phase recently observed in MoS$_2$. We show how the combination of the valley structure of the conduction band, the density dependence of the screening of the long range Coulomb interactions, the short range electronic repulsion, and the relative weakness of the electron-phonon interactions, makes possible the existence of a phase where the superconducting order parameter has opposite signs in different valleys, resembling the superconductivity found in the pnictides and cuprates

Spectroscopic and thermodynamic properties in a four-band model for pnictides

In this paper we provide a comprehesive analysis of different properties of pnictides both in the normal and superconducting state, with a particular focus on the optimally-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ system. We show that, by using the band dispersions experimentally measured by ARPES, a four-band Eliashberg model in the intermediate-coupling regime can account for both the measured hierarchy of the gaps and for several spectroscopic and thermodynamic signatures of low-energy renormalization. These include the kinks in the band dispersion and the effective masses determined via specific-heat and superfluid-density measurements. We also show that, although an intermediate-coupling Eliashberg approach is needed to account for the magnitude of the gaps, the temperature behavior of the thermodynamic quantities does not show in this regime a significant deviation with respect to weak-coupling BCS calculations. This can explain the apparent success of two-band BCS fits of experimental data reported often in the literature.Comment: 12 pages, 6 figures, final versio