1,773 research outputs found
Zero-bias molecular electronics: Exchange-correlation corrections to Landauer's formula
Standard first principles calculations of transport through single molecules
miss exchange-correlation corrections to the Landauer formula. From Kubo
response theory, both the Landauer formula and these corrections in the limit
of zero bias are derived and calculations are presented.Comment: 4 pages, 3 figures, final version to appear in Phys. Rev. B, Rapid
Communication
Intensity distribution of scalar waves propagating in random media
Transmission of the scalar field through the random medium, represented by
the system of randomly distributed dielectric cylinders is calculated
numerically. System is mapped to the problem of electronic transport in
disordered two-dimensional systems. Universality of the statistical
distribution of transmission parameters is analyzed in the metallic and in the
localized regimes.In the metallic regime the universality of the transmission
statistics in all transparent channels is observed. In the band gaps, we
distinguish the disorder induced (Anderson) localization from the tunneling
through the system due to the gap in the density of states. We show also that
absorption causes rapid decrease of the mean conductance, but, contrary to the
localized regime, the conductance is self-averaged with a
Gaussian distribution
Phase Diagram for Anderson Disorder: beyond Single-Parameter Scaling
The Anderson model for independent electrons in a disordered potential is
transformed analytically and exactly to a basis of random extended states
leading to a variant of augmented space. In addition to the widely-accepted
phase diagrams in all physical dimensions, a plethora of additional, weaker
Anderson transitions are found, characterized by the long-distance behavior of
states. Critical disorders are found for Anderson transitions at which the
asymptotically dominant sector of augmented space changes for all states at the
same disorder. At fixed disorder, critical energies are also found at which the
localization properties of states are singular. Under the approximation of
single-parameter scaling, this phase diagram reduces to the widely-accepted one
in 1, 2 and 3 dimensions. In two dimensions, in addition to the Anderson
transition at infinitesimal disorder, there is a transition between two
localized states, characterized by a change in the nature of wave function
decay.Comment: 51 pages including 4 figures, revised 30 November 200
Fractional quantization of ballistic conductance in 1D hole systems
We analyze the fractional quantization of the ballistic conductance
associated with the light and heavy holes bands in Si, Ge and GaAs systems. It
is shown that the formation of the localized hole state in the region of the
quantum point contact connecting two quasi-1D hole leads modifies drastically
the conductance pattern. Exchange interaction between localized and propagating
holes results in the fractional quantization of the ballistic conductance
different from those in electronic systems. The value of the conductance at the
additional plateaux depends on the offset between the bands of the light and
heavy holes, \Delta, and the sign of the exchange interaction constant. For
\Delta=0 and ferromagnetic exchange interaction, we observe additional plateaux
around the values 7e^{2}/4h, 3e^{2}/h and 15e^{2}/4h, while antiferromagnetic
interaction plateaux are formed around e^{2}/4h, e^{2}/h and 9e^{2}/4h. For
large \Delta, the single plateau is formed at e^2/h.Comment: 4 pages, 3 figure
Quantum Aspects of Semantic Analysis and Symbolic Artificial Intelligence
Modern approaches to semanic analysis if reformulated as Hilbert-space
problems reveal formal structures known from quantum mechanics. Similar
situation is found in distributed representations of cognitive structures
developed for the purposes of neural networks. We take a closer look at
similarites and differences between the above two fields and quantum
information theory.Comment: version accepted in J. Phys. A (Letter to the Editor
Quantum Ballistic Evolution in Quantum Mechanics: Application to Quantum Computers
Quantum computers are important examples of processes whose evolution can be
described in terms of iterations of single step operators or their adjoints.
Based on this, Hamiltonian evolution of processes with associated step
operators is investigated here. The main limitation of this paper is to
processes which evolve quantum ballistically, i.e. motion restricted to a
collection of nonintersecting or distinct paths on an arbitrary basis. The main
goal of this paper is proof of a theorem which gives necessary and sufficient
conditions that T must satisfy so that there exists a Hamiltonian description
of quantum ballistic evolution for the process, namely, that T is a partial
isometry and is orthogonality preserving and stable on some basis. Simple
examples of quantum ballistic evolution for quantum Turing machines with one
and with more than one type of elementary step are discussed. It is seen that
for nondeterministic machines the basis set can be quite complex with much
entanglement present. It is also proved that, given a step operator T for an
arbitrary deterministic quantum Turing machine, it is decidable if T is stable
and orthogonality preserving, and if quantum ballistic evolution is possible.
The proof fails if T is a step operator for a nondeterministic machine. It is
an open question if such a decision procedure exists for nondeterministic
machines. This problem does not occur in classical mechanics.Comment: 37 pages Latexwith 2 postscript figures tar+gzip+uuencoded, to be
published in Phys. Rev.
Metal-insulator transition in a two-dimensional electron system: the orbital effect of in-plane magnetic field
The conductance of an open quench-disordered two-dimensional (2D) electron
system subject to an in-plane magnetic field is calculated within the framework
of conventional Fermi liquid theory applied to actually a three-dimensional
system of spinless electrons confined to a highly anisotropic (planar)
near-surface potential well. Using the calculation method suggested in this
paper, the magnetic field piercing a finite range of infinitely long system of
carriers is treated as introducing the additional highly non-local scatterer
which separates the circuit thus modelled into three parts -- the system as
such and two perfect leads. The transverse quantization spectrum of the inner
part of the electron waveguide thus constructed can be effectively tuned by
means of the magnetic field, even though the least transverse dimension of the
waveguide is small compared to the magnetic length. The initially finite
(metallic) value of the conductance, which is attributed to the existence of
extended modes of the transverse quantization, decreases rapidly as the
magnetic field grows. This decrease is due to the mode number reduction effect
produced by the magnetic field. The closing of the last current-carrying mode,
which is slightly sensitive to the disorder level, is suggested as the origin
of the magnetic-field-driven metal-to-insulator transition widely observed in
2D systems.Comment: 19 pages, 7 eps figures, the extension of cond-mat/040613
Wigner Function Description of the A.C.-Transport Through a Two-Dimensional Quantum Point Contact
We have calculated the admittance of a two-dimensional quantum point contact
(QPC) using a novel variant of the Wigner distribution function (WDF)
formalism. In the semiclassical approximation, a Boltzman-like equation is
derived for the partial WDF describing both propagating and nonpropagating
electron modes in an effective potential generated by the adiabatic QPC. We
show that this quantum kinetic approach leads to the well-known stepwise
behavior of the real part of the admittance (the conductance), and of the
imaginary part of the admittance (the emittance), in agreement with the latest
results, which is determined by the number of propagating electron modes. It is
shown, that the emittance is sensitive to the geometry of the QPC, and can be
controlled by the gate voltage. We established that the emittance has
contributions corresponding to both quantum inductance and quantum capacitance.
Stepwise oscillations in the quantum inductance are determined by the harmonic
mean of the velocities for the propagating modes, whereas the quantum
capacitance is a significant mesoscopic manifestation of the non-propagating
(reflecting) modes.Comment: 23 pages (latex), 3 figure
Electron orbital valves made of multiply connected armchair carbon nanotubes with mirror-reflection symmetry: tight-binding study
Using the tight-binding method and the Landauer-B\"{u}ttiker conductance
formalism, we demonstrate that a multiply connected armchair carbon nanotube
with a mirror-reflection symmetry can sustain an electron current of the
-bonding orbital while suppress that of the -antibonding orbital over
a certain energy range. Accordingly, the system behaves like an electron
orbital valve and may be used as a scanning tunneling microscope to probe
pairing symmetry in d-wave superconductors or even orbital ordering in solids
which is believed to occur in some transition-metal oxides.Comment: 4 figures, 12 page
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