2,461 research outputs found
Conductance and polarization in quantum junctions
We revisit the expression for the conductance of a general nanostructure -- such as a quantum point contact -- as obtained from the linear response theory. We show that the conductance represents the strength of the Drude singularity in the conductivity . Using the equation of continuity for electric charge we obtain a formula for conductance in terms of polarization of the system. This identification can be used for direct calculation of the conductance for systems of interest even at the {\it ab-initio} level. In particular, we show that one can evaluate the conductance from calculations for a finite system without the need for special ``transport'' boundary conditions
Spectrum of Electrons in Graphene as an Alternant Macromolecule and Its Specific Features in Quantum Conductance
An exact description of electrons based on the tight-binding model of
graphene as an alternant, plane macromolecule is presented. The model molecule
can contain an arbitrary number of benzene rings and has armchair- and
zigzag-shaped edges. This suggests an instructive alternative to the most
commonly used approach, where the reference is made to the honeycomb lattice
periodic in its A and B sublattices. Several advantages of the macromolecule
model are demonstrated. The newly derived analytical relations detail our
understanding of electron nature in achiral graphene ribbons and carbon
tubes and classify these structures as quantum wires.Comment: 13 pages 8 figures, revised in line with referee's comment
Distillation of local purity from quantum states
Recently Horodecki et al. [Phys. Rev. Lett. 90, 100402 (2003)] introduced an
important quantum information processing paradigm, in which two parties sharing
many copies of the same bipartite quantum state distill local pure states, by
means of local unitary operations assisted by a one-way (two-way) completely
dephasing channel. Local pure states are a valuable resource from a
thermodynamical point of view, since they allow thermal energy to be converted
into work by local quantum heat engines. We give a simple
information-theoretical characterization of the one-way distillable local
purity, which turns out to be closely related to a previously known operational
measure of classical correlations, the one-way distillable common randomness.Comment: 8 page
The conditional tunneling time for reflection using the WKB wave-function
We derive an expression for the conditional time for the reflection of a wave
from an arbitrary potential barrier using the WKB wavefunction in the barrier
region. Our result indicates that the conditional times for transmission and
reflection are equal for a symmetric barrier within the validity of the WKB
approach.Comment: 4 pages RevTeX, 1 eps figure include
Transport in Molecular Junctions with Different Metallic Contacts
Ab initio calculations of phenyl dithiol connected to Au, Ag, Pd, and Pt
electrodes are performed using non-equilibrium Green's functions and density
functional theory. For each metal, the properties of the molecular junction are
considered both in equilibrium and under bias. In particular, we consider in
detail charge transfer, changes in the electrostatic potential, and their
subsequent effects on the IV curves through the junctions. Gold is typically
used in molecular junctions because it forms strong chemical bonds with sulfur.
We find however that Pt and Pd make better electrical contacts than Au. The
zero-bias conductance is found to be greatest for Pt, followed by Pd, Au, and
then Ag
New mechanism for impurity-induced step bunching
Codeposition of impurities during the growth of a vicinal surface leads to an
impurity concentration gradient on the terraces, which induces corresponding
gradients in the mobility and the chemical potential of the adatoms. Here it is
shown that the two types of gradients have opposing effects on the stability of
the surface: Step bunching can be caused by impurities which either lower the
adatom mobility, or increase the adatom chemical potential. In particular,
impurities acting as random barriers (without affecting the adatom binding)
cause step bunching, while for impurities acting as random traps the
combination of the two effects reduces to a modification of the attachment
boundary conditions at the steps. In this case attachment to descending steps,
and thus step bunching, is favored if the impurities bind adatoms more weakly
than the substrate.Comment: 7 pages, 3 figures. Substantial revisions and correction
Kondo effect of an adsorbed cobalt phthalocyanine (CoPc) molecule: the role of quantum interference
A recent experimental study showed that, distorting a CoPc molecule adsorbed
on a Au(111) surface, a Kondo effect is induced with a temperature higher than
200 K. We examine a model in which an atom with strong Coulomb repulsion (Co)
is surrounded by four atoms on a square (molecule lobes), and two atoms above
and below it representing the apex of the STM tip and an atom on the gold
surface (all with a single, half-filled, atomic orbital). The Hamiltonian is
solved exactly for the isolated cluster, and, after connecting the leads (STM
tip and gold), the conductance is calculated by standard techniques. Quantum
interference prevents the existence of the Kondo effect when the orbitals on
the square do not interact (undistorted molecule); the Kondo resonance shows up
after switching on that interaction. The weight of the Kondo resonance is
controlled by the interplay of couplings to the STM tip and the gold surface,
and between the molecule lobes.Comment: 5 pages, 3 figura
Ballistic thermal conductance limited by phonon roughness scattering: A comparison of power-law and Gaussian roughness
In this work, we have investigated the influence of power-law roughness on the ballistic thermal conductance KTH for a nanosized beam adiabatically connected between two heat reservoirs. The sideways wall beam roughness is assumed to be power-law type, which is described by the roughness amplitude w, the in-plane roughness correlation length ξ and the roughness exponent 0≤H≤1. Distinct differences occur in between power-law and Gaussian wall roughness. For power-law roughness with low roughness exponents H (<0.5), the influence of phonon scattering can be rather destructive leading to significant deviations from the universal conductance value for flat beam walls. On the other hand for large roughness exponents (H>0.5) the conductance drop is significantly smaller than that of Gaussian roughness assuming similar roughness ratios w/ξ.
Effect of amplification on conductance distribution of a disordered waveguide
Introduction of optical gain to a disordered system results in enhanced
fluctuations [] of dimensionless
conductance , similar to the effect of Anderson localization in
passive medium. Using numerical simulations we demonstrate that despite of such
qualitative similarity, the whole distribution of conductance of amplifying
random media is drastically different from that of passive system with the same
value of .Comment: 4 pages, 4 figure
Quantum theory of dissipation of a harmonic oscillator coupled to a nonequilibrium bath; Wigner-Weisskopf decay and physical spectra
We extend the quantum theory of dissipation in the context of
system-reservoir model, where the reservoir in question is kept in a
nonequilibrium condition. Based on a systematic separation of time scales
involved in the dynamics, appropriate generalizations of the
fluctuation-dissipation and Einstein's relations have been pointed out. We show
that the Wigner-Weisskopf decay of the system mode results in a rate constant
which depending on the relaxation of nonequilibrium bath is dynamically
modified. We also calculate the time-dependent spectra of a cavity mode with a
suitable gain when the cavity is kept in contact with a nonequilibrium bath.Comment: Plain Latex, 28 pages, 2 PS figure
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