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

Theoretical investigation of FeTe magnetic ordering under hydrostatic pressure

We investigate the pressure phase diagram of FeTe, predicting structural and magnetic properties in the normal state at zero temperature within density functional theory (DFT). We carefully examined several possible different crystal structures over a pressure range up to $\approx 30$ GPa: simple tetragonal (PbO type), simple monoclinic, orthorhombic (MnP type), hexagonal (NiAs and wurzite type) and cubic (CsCl and NaCl type). We predict pressure to drive the system through different magnetic ordering (notably also some ferromagnetic phases) eventually suppressing magnetism at around 17GPa. We speculate the ferromagnetic order to be the reason for the absence of a superconducting phase in FeTe at variance with the case of FeSe.Comment: 11 figure

Triangular Mott-Hubbard Insulator Phases of Sn/Si(111) and Sn/Ge(111) Surfaces

The ground state of Sn/Si(111) and Sn/Ge(111) surface $\alpha$-phases is reexamined theoretically, based on $ab-initio$ calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density + Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed $\Gamma$-point photoemission intensity, as well asfor the partial metallization observed above above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, $e.g.$ alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.Comment: 4 pages, 4 figure

Electronic, dynamical and superconducting properties of CaBeSi

We report first-principles calculations on the normal and superconducting state of CaBe(x)Si(2-x) (x=1), in the framework of density functional theory for superconductors (SCDFT). CaBeSi is isostructural and isoelectronic to MgB2 and this makes possible a direct comparison of the electronic and vibrational properties and the electron-phonon interaction of the two materials. Despite the many similarities with MgB2 (e.g. sigma bands at the Fermi level and a larger Fermi surface nesting), according to our calculations CaBeSi has a very low critical temperature (Tc ~ 0.4 K, consistent with the experiment). CaBeSi exhibits a complex gap structure, with three gaps at Fermi level: besides the two sigma and pi gaps, present also in MgB2, the appearance of a third gap is related to the anisotropy of the Coulomb repulsion, acting in different way on the bonding and antibonding electronic pi states.Comment: 6 pages, 5 figure

First-principles study of thin magnetic transition-metal silicide films on Si(001)

In order to combine silicon technology with the functionality of magnetic systems, a number of ferromagnetic (FM) materials have been suggested for the fabrication of metal/semiconductor heterojunctions. In this work, we present a systematic study of several candidate materials in contact with the Si surface. We employ density-functional theory calculations to address the thermodynamic stability and magnetism of both pseudomorphic CsCl-like $M$Si ($M$=Mn, Fe, Co, Ni) thin films and Heusler alloy $M_2$MnSi ($M$=Fe, Co, Ni) films on Si(001). Our calculations show that Si-termination of the $M$Si films is energetically preferable during epitaxy since it minimizes the energetic cost of broken bonds at the surface. Moreover, we can explain the calculated trends in thermodynamic stability of the $M$Si thin films in terms of the $M$-Si bond-strength and the $M$ 3d orbital occupation. From our calculations, we predict that ultrathin MnSi films are FM with sizable spin magnetic moments at the Mn atoms, while FeSi and NiSi films are nonmagnetic. However, CoSi films display itinerant ferromagnetism. For the $M_2$MnSi films with Heusler-type structure, the MnSi termination is found to have the highest thermodynamic stability. In the FM ground state, the calculated strength of the effective coupling between the magnetic moments of Mn atoms within the same layer approximately scales with the measured Curie temperatures of the bulk $M_2$MnSi compounds. In particular, the Co$_2$MnSi/Si(001) thin film has a robust FM ground state as in the bulk, and is found to be stable against a phase separation into CoSi/Si(001) and MnSi/Si(001) films. Hence this material is of possible use in FM-Si heterojunctions and deserves further experimental investigations.Comment: 13 pages, 8 figure