2,766 research outputs found
Universal conductance reduction in a quantum wire
Even a single point defect in a quantum wire causes a conductance reduction.
In this paper we prove (without any approximations) that for any point impurity
this conductance reduction in all the sub-bands is exactly 2e^2/h. Moreover, it
is shown that in the case of a surface defect, not only is the conductance
minimum independent of the defect characteristics, but the transmission matrix
also converges to universal (defect-independent) values. We also discuss
particle confinement between two arbitrarily weak point defects.Comment: 4 pages, 4 figures (Revtex
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
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
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.
The Smallest Molecular Switch
Ab-initio total energy calculations reveal benzene-dithiolate (BDT) molecules
on a gold surface, contacted by a monoatomic gold STM tip to have two classes
of low energy conformations with differing symmetries. Lateral motion of the
tip or excitation of the molecule cause it to change from one conformation
class to the other and to switch between a strongly and a weakly conducting
state. Thus, surprisingly, despite their apparent simplicity these Au/BDT/Au
nanowires are shown to be electrically bi-stable switches, the smallest
two-terminal molecular switches to date. Experiments with a conventional or
novel self-assembled STM are proposed to test these predictions.Comment: 8 pages, 3 figure
Distributions of Conductance and Shot Noise and Associated Phase Transitions
For a chaotic cavity with two indentical leads each supporting N channels, we
compute analytically, for large N, the full distribution of the conductance and
the shot noise power and show that in both cases there is a central Gaussian
region flanked on both sides by non-Gaussian tails. The distribution is weakly
singular at the junction of Gaussian and non-Gaussian regimes, a direct
consequence of two phase transitions in an associated Coulomb gas problem.Comment: 5 pages, 3 figures include
Quantized Conductance of a Single Magnetic Atom
A single Co atom adsorbed on Cu(111) or on ferromagnetic Co islands is
contacted with non-magnetic W or ferromagnetic Ni tips in a scanning tunneling
microscope. When the Co atom bridges two non-magnetic electrodes conductances
of 2e^2/h are found. With two ferromagnetic electrodes a conductance of e^2/h
is observed which may indicate fully spin-polarized transport.Comment: 3 pages, 2 figure
DC Conductance of Molecular Wires
Inspired by the work of Kamenev and Kohn, we present a general discussion of
the two-terminal dc conductance of molecular devices within the framework of
Time Dependent Current-Density Functional Theory. We derive a formally exact
expression for the adiabatic conductance and we discuss the dynamical
corrections. For junctions made of long molecular chains that can be either
metallic or insulating, we derive the exact asymptotic behavior of the
adiabatic conductance as a function of the chain's length. Our results follow
from the analytic structure of the bands of a periodic molecular chain and a
compact expression for the Green's functions. In the case of an insulating
chain, not only do we obtain the exponentially decaying factors, but also the
corresponding amplitudes, which depend very sensitively on the electronic
properties of the contacts. We illustrate the theory by a numerical study of a
simple insulating structure connected to two metallic jellium leads.Comment: 15 pgs and 9 figure
- …