169 research outputs found
A tight-binding model for MoS monolayers
We propose an accurate tight-binding parametrization for the band structure
of MoS monolayers near the main energy gap. We introduce a generic and
straightforward derivation for the band energies equations that could be
employed for other monolayer dichalcogenides. A parametrization that includes
spin-orbit coupling is also provided. The proposed set of model parameters
reproduce both the correct orbital compositions and location of valence and
conductance band in comparison with ab initio calculations. The model gives a
suitable starting point for realistic large-scale atomistic electronic
transport calculations.Comment: 35 pages, 8 figure
Chaos in one-dimensional lattices under intense laser fields
A model is investigated where a monochromatic, spatially homogeneous laser
field interacts with an electron in a one-dimensional periodic lattice. The
classical Hamiltonian is presented and the technique of stroboscopic maps is
used to study the dynamical behavior of the model. The electron motion is found
to be completely regular only for small field amplitudes, developing a larger
chaotic region as the amplitude increases. The quantum counterpart of the
classical Hamiltonian is derived. Exact numerical diagonalizations show the
existence of universal, random-matrix fluctuations in the electronic energy
bands dressed by the laser field. A detailed analysis of the classical phase
space is compatible with the statistical spectral analysis of the quantum
model. The application of this model to describe transport and optical
absorption in semiconductor superlattices submitted to intense infrared laser
radiation is proposed.Comment: 9 pages, RevTex 3.0, EPSF (6 figures), to appear in Europhys. J.
Coherent transport through graphene nanoribbons in the presence of edge disorder
We simulate electron transport through graphene nanoribbons of experimentally
realizable size (length L up to 2 micrometer, width W approximately 40 nm) in
the presence of scattering at rough edges. Our numerical approach is based on a
modular recursive Green's function technique that features sub-linear scaling
with L of the computational effort. We identify the influence of the broken A-B
sublattice (or chiral) symmetry and of K-K' scattering by Fourier spectroscopy
of individual scattering states. For long ribbons we find Anderson-localized
scattering states with a well-defined exponential decay over 10 orders of
magnitude in amplitude.Comment: 8 pages, 6 Figure
RKKY Interaction in Disordered Graphene
We investigate the effects of nonmagnetic disorder on the
Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction in graphene by studying
numerically the Anderson model with on-site and hopping disorder on a honeycomb
lattice at half filling. We evaluate the strength of the interaction as a
function of the distance R between two magnetic ions, as well as their lattice
positions and orientations. In the clean limit, we find that the strength of
the interaction decays as 1/R^3, with its sign and oscillation amplitude
showing strong anisotropy. With increasing on-site disorder, the mean amplitude
decreases exponentially at distances exceeding the elastic mean free path. At
smaller distances, however, the oscillation amplitude increases strongly and
its sign changes on the same sublattice for all directions but the armchair
direction. For random hopping disorder, no sign change is observed. No
significant changes to the geometrical average values of the RKKY interaction
are found at small distances, while exponential suppression is observed at
distances exceeding the localization length.Comment: 4+\epsilon\ pages, 5 figure
RKKY Interactions in Graphene: Dependence on Disorder and Gate Voltage
We report the dependence of Ruderman-Kittel-Kasuya-Yoshida\,(RKKY)
interaction on nonmagmetic disorder and gate voltage in grapheme. First the
semiclassical method is employed to reserve the expression for RKKY interaction
in clean graphene. Due to the pseudogap at Dirac point, the RKKY coupling in
undoped grapheme is found to be proportional to . Next, we investigate
how the RKKY interaction depends on nonmagnetic disorder strength and gate
voltage by studying numerically the Anderson tight-binding model on a honeycomb
lattice. We observe that the RKKY interaction along the armchair direction is
more robust to nonmagnetic disorder than in other directions. This effect can
be explained semiclassically: The presence of multiple shortest paths between
two lattice sites in the armchair directions is found to be responsible for the
reduceddisorder sensitivity. We also present the distribution of the RKKY
interaction for the zigzag and armchair directions. We identify three different
shapes of the distributions which are repeated periodically along the zigzag
direction, while only one kind, and more narrow distribution, is observed along
the armchair direction. Moreover, we find that the distribution of amplitudes
of the RKKY interaction crosses over from a non-Gaussian shape with very long
tails to a completely log-normal distribution when increasing the nonmagnetic
disorder strength. The width of the log-normal distribution is found to
linearly increase with the strength of disorder, in agreement with analytical
predictions. At finite gate voltage near the Dirac point, Friedel oscillation
appears in addition to the oscillation from the interference between two Dirac
points. This results in a beating pattern. We study how these beating patterns
are effected by the nonmagnetic disorder in doped graphene
Resilient Quantum Computation in Correlated Environments: A Quantum Phase Transition Perspective
We analyze the problem of a quantum computer in a correlated environment
protected from decoherence by QEC using a perturbative renormalization group
approach. The scaling equation obtained reflects the competition between the
dimension of the computer and the scaling dimension of the correlations. For an
irrelevant flow, the error probability is reduced to a stochastic form for long
time and/or large number of qubits; thus, the traditional derivation of the
threshold theorem holds for these error models. In this way, the ``threshold
theorem'' of quantum computing is rephrased as a dimensional criterion.Comment: 4.1 pages, minor correction and an improved discussion of Eqs. (4)
and (14
Nonperturbative Scaling Theory of Free Magnetic Moment Phases in Disordered Metals
The crossover between a free magnetic moment phase and a Kondo phase in low
dimensional disordered metals with dilute magnetic impurities is studied.
We perform a finite size scaling analysis of the distribution of the Kondo
temperature as obtained from a numerical renormalization group calculation of
the local magnetic susceptibility and from the solution of the self-consistent
Nagaoka-Suhl equation. We find a sizable fraction of free (unscreened) magnetic
moments when the exchange coupling falls below a disorder-dependent critical
value . Our numerical results show that between the free moment
phase due to Anderson localization and the Kondo screened phase there is a
phase where free moments occur due to the appearance of random local pseudogaps
at the Fermi energy whose width and power scale with the elastic scattering
rate .Comment: 4 pages, 6 figure
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