4,583 research outputs found
Electronic Properties of Strained Si/Ge Core-Shell Nanowires
We investigated the electronic properties of strained Si/Ge core-shell
nanowires along the [110] direction using first principles calculations based
on density-functional theory. The diameter of the studied core-shell wire is up
to 5 nm. We found the band gap of the core-shell wire is smaller than that of
both pure Si and Ge wires with the same diameter. This reduced band gap is
ascribed to the intrinsic strain between Ge and Si layers, which partially
counters the quantum confinement effect. The external strain is further applied
to the nanowires for tuning the band structure and band gap. By applying
sufficient tensile strain, we found the band gap of Si-core/Ge-shell nanowire
with diameter larger than ~3 nm experiences a transition from direct to
indirect gap.Comment: 4 figure
Diagrammatic quantum field formalism for localized electrons
We introduce a diagrammatic quantum field formalism for the evaluation of
normalized expectation values of operators, and suitable for systems with
localized electrons. It is used to develop a convergent series expansion for
the energy in powers of overlap integrals of single-particle orbitals. This
method gives intuitive and practical rules for writing down the expansion to
arbitrary order of overlap, and can be applied to any spin configuration and to
any dimension. Its applicability for systems with well localized electrons has
been illustrated with examples, including the two-dimensional Wigner crystal
and spin-singlets in the low-density electron gas.Comment: 13 pages, 0 figure
Lattice Green's function for crystals containing a planar interface
Flexible boundary condition methods couple an isolated defect to a
harmonically responding medium through the bulk lattice Green's function; in
the case of an interface, interfacial lattice Green's functions. We present a
method to compute the lattice Green's function for a planar interface with
arbitrary atomic interactions suited for the study of line defect/interface
interactions. The interface is coupled to two different semi-infinite bulk
regions, and the Green's function for interface-interface, bulk-interface and
bulk-bulk interactions are computed individually. The elastic bicrystal Green's
function and the bulk lattice Green's function give the interaction between
bulk regions. We make use of partial Fourier transforms to treat in-plane
periodicity. Direct inversion of the force constant matrix in the partial
Fourier space provides the interface terms. The general method makes no
assumptions about the atomic interactions or crystal orientations. We simulate
a screw dislocation interacting with a twin boundary in Ti using
flexible boundary conditions and compare with traditional fixed boundary
conditions results. Flexible boundary conditions give the correct core
structure with significantly less atoms required to relax by energy
minimization. This highlights the applicability of flexible boundary conditions
methods to modeling defect/interface interactions by \textit{ab initio}
methods
Non-Fermi liquid signatures in the Hubbard Model due to van Hove singularities
When a van-Hove singularity is located in the vicinity of the Fermi level,
the electronic scattering rate acquires a non-analytic contribution. This
invalidates basic assumptions of Fermi liquid theory and within perturbative
treatments leads to a non-Fermi liquid self-energy and transport
properties.Such anomalies are shown to also occur in the strongly correlated
metallic state. We consider the Hubbard model on a two-dimensional square
lattice with nearest and next-nearest neighbor hopping within the single-site
dynamical mean-field theory. At temperatures on the order of the low-energy
scale an unusual maximum emerges in the imaginary part of the self-energy
which is renormalized towards the Fermi level for finite doping. At zero
temperature this double-well structure is suppressed, but an anomalous energy
dependence of the self-energy remains. For the frustrated Hubbard model on the
square lattice with next-nearest neighbor hopping, the presence of the van Hove
singularity changes the asymptotic low temperature behavior of the resistivity
from a Fermi liquid to non-Fermi liquid dependency as function of doping. The
results of this work are discussed regarding their relevance for
high-temperature cuprate superconductors.Comment: revised version, accepted in Phys.Rev.
NB protein does not affect Influenza B virus replication in vitro and is not required for replication in or transmission between ferrets.
Elusive electron-phonon coupling in quantitative analyses of the spectral function
We examine multiple techniques for extracting information from angle-resolved
photoemission spectroscopy (ARPES) data, and test them against simulated
spectral functions for electron-phonon coupling. We find that, in the
low-coupling regime, it is possible to extract self-energy and bare-band
parameters through a self-consistent Kramers-Kronig bare-band fitting routine.
We also show that the effective coupling parameters deduced from the
renormalization of quasiparticle mass, velocity, and spectral weight are
momentum dependent and, in general, distinct from the true microscopic
coupling; the latter is thus not readily accessible in the quasiparticle
dispersion revealed by ARPES.Comment: A high-resolution version can be found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/KKBF.pd
Nonlocal spectral properties of disordered alloys
A general method is proposed for calculating a fully k-dependent, continuous,
and causal spectral function A(k,E) within the recently introduced nonlocal
version of the coherent-potential approximation (NLCPA). The method involves
the combination of both periodic and anti-periodic solutions to the associated
cluster problem and also leads to correct bulk quantities for small cluster
sizes. We illustrate the method by investigating the Fermi surface of a
two-dimensional alloy. Dramatically, we find a smeared electronic topological
transition not predicted by the conventional CPA.Comment: 17 pages, 5 figures, Submitted to: J. Phys.: Condens. Matter
Editorial receipt 25 May 200
A topological charge selection rule for phase singularities
We present an study of the dynamics and decay pattern of phase singularities
due to the action of a system with a discrete rotational symmetry of finite
order. A topological charge conservation rule is identified. The role played by
the underlying symmetry is emphasized. An effective model describing the short
range dynamics of the vortex clusters has been designed. A method to engineer
any desired configuration of clusters of phase singularities is proposed. Its
flexibility to create and control clusters of vortices is discussed.Comment: 4 pages, 3 figure
Kinks: Fingerprints of strong electronic correlations
The textbook knowledge of solid state physics is that the electronic specific
heat shows a linear temperature dependence with the leading corrections being a
cubic term due to phonons and a cubic-logarithmic term due to the interaction
of electrons with bosons. We have shown that this longstanding conception needs
to be supplemented since the generic behavior of the low-temperature electronic
specific heat includes a kink if the electrons are sufficiently strongly
correlatedComment: 4 pages, 1 figure, ICM 2009 conference proceedings (to appear in
Journal of Physics: Conference Series
Bosonic versus fermionic pairs of topological spin defects in monolayered high-T_c superconductors
The energy associated with bosonic and fermionic pairs of topological spin
defects in doped antiferromagnetic quantum spin-1/2 square lattice is estimated
within a resonating valence bond scenario, as described by a t-t'-J-like model
Hamiltonian, plus a t-perpendicular, responsible of a three-dimensional
screening of the electrostatic repulsion within the bosonic pairs. For
parameters appropriate for monolayered high-T_c superconductors, both fermionic
and bosonic pairs show x^2-y^2 symmetry. We find a critical value of doping
such that the energy of the bosonic pairs goes below twice the energy of two
fermionic pairs at their Fermi level. This finding could be related to the
onset of high-T_c superconductivity.Comment: 10 pages, 6 figures. To be published in Phys. Rev.
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