454 research outputs found
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 associated with each hopping term . This analysis
also reveals a simple scaling law between the hopping terms and the
electron-phonon coupling which goes beyond the specific DFT
technique employed.Comment: 10 pages, 10 fig
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 -phases is
reexamined theoretically, based on 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 -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, alkali doping, could lead to
a novel 2D triangular superconducting state of this and similar surfaces.Comment: 4 pages, 4 figure
Direct evaluation of the isotope effect within the framework of density functional theory for superconductors
Within recent developments of density functional theory, its numerical implementation and of the superconducting density functional theory is nowadays possible to predict the superconducting critical temperature, Tc, with sufficient accuracy to anticipate the experimental verification. In this paper we present an analytical derivation of the isotope coefficient within the superconducting density functional theory. We calculate the partial derivative of Tc with respect to atomic masses. We verified the final expression by means of numerical calculations of isotope coefficient in monatomic superconductors (Pb) as well as polyatomic superconductors (CaC6). The results confirm the validity of the analytical derivation with respect to the finite difference methods, with considerable improvement in terms of computational time and calculation accuracy. Once the critical temperature is calculated (at the reference mass(es)), various isotope exponents can be simply obtained in the same run. In addition, we provide the expression of interesting quantities like partial derivatives of the deformation potential, phonon frequencies and eigenvectors with respect to atomic masses, which can be useful for other derivations and applications
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 Si (=Mn, Fe, Co,
Ni) thin films and Heusler alloy MnSi (=Fe, Co, Ni) films on Si(001).
Our calculations show that Si-termination of the 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 Si thin films in terms of the -Si bond-strength and the
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 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 MnSi compounds. In particular,
the CoMnSi/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
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