6,368 research outputs found
Parasitic pumping currents in an interacting quantum dot
We analyze the charge and spin pumping in an interacting dot within the
almost adiabatic limit. By using a non-equilibrium Green's function technique
within the time-dependent slave boson approximation, we analyze the pumped
current in terms of the dynamical constraints in the infinite-U regime. The
results show the presence of parasitic pumping currents due to the additional
phases of the constraints. The behavior of the pumped current through the
quantum dot is illustrated in the spin-insensitive and in the spin-sensitive
case relevant for spintronics applications
Exact dynamics of dissipative electronic systems and quantum transport: Hierarchical equations of motion approach
A quantum dissipation theory is formulated in terms of hierarchically coupled
equations of motion for an arbitrary electronic system coupled with grand
canonical Fermion bath ensembles. The theoretical construction starts with the
second--quantization influence functional in path integral formalism, in which
the Fermion creation and annihilation operators are represented by Grassmann
variables. Time--derivatives on influence functionals are then performed in a
hierarchical manner, on the basis of calculus--on--path--integral algorithm.
Both the multiple--frequency--dispersion and the non-Markovian reservoir
parametrization schemes are considered for the desired hierarchy construction.
The resulting formalism is in principle exact, applicable to interacting
systems, with arbitrary time-dependent external fields. It renders an exact
tool to evaluate various transient and stationary quantum transport properties
of many-electron systems. At the second--tier truncation level the present
theory recovers the real--time diagrammatic formalism developed by Sch\"{o}n
and coworkers. For a single-particle system, the hierarchical formalism
terminates at the second tier exactly, and the Landuer--B\"{u}ttiker's
transport current expression is readily recovered.Comment: The new versio
Optical properties of current carrying molecular wires
We consider several fundamental optical phenomena involving single molecules
in biased metal-molecule-metal junctions. The molecule is represented by its
highest occupied and lowest unoccupied molecular orbitals, and the analysis
involves the simultaneous consideration of three coupled fluxes: the electronic
current through the molecule, energy flow between the molecule and
electron-hole excitations in the leads and the incident and/or emitted photon
flux. Using a unified theoretical approach based on the non-equilibrium Green
function method we derive expressions for the absorption lineshape (not an
observable but a ueful reference for considering yields of other optical
processes) and for the current induced molecular emission in such junctions. We
also consider conditions under which resonance radiation can induce electronic
current in an unbiased junction. We find that current driven molecular emission
and resonant light induced electronic currents in single molecule junctions can
be of observable magnitude under appropriate realizable conditions. In
particular, light induced current should be observed in junctions involving
molecular bridges that are characterized by strong charge transfer optical
transitions. For observing current induced molecular emission we find that in
addition to the familiar need to control the damping of molecular excitations
into the metal substrate the phenomenon is also sensitive to the way in which
the potential bias si distributed on the junction.Comment: 56 pages, 8 figures; submitted to JC
Quantum transport in chains with noisy off-diagonal couplings
We present a model for conductivity and energy diffusion in a linear chain
described by a quadratic Hamiltonian with Gaussian noise. We show that when the
correlation matrix is diagonal, the noise-averaged Liouville-von Neumann
equation governing the time-evolution of the system reduces to the Lindblad
equation with Hermitian Lindblad operators. We show that the noise-averaged
density matrix for the system expectation values of the energy density and the
number density satisfy discrete versions of the heat and diffusion equations.
Transport coefficients are given in terms of model Hamiltonian parameters. We
discuss conditions on the Hamiltonian under which the noise-averaged
expectation value of the total energy remains constant. For chains placed
between two heat reservoirs, the gradient of the energy density along the chain
is linear.Comment: 6 pages, to appear in J. Chem. Phy
Theoretical study of electronic transport through a small quantum dot with a magnetic impurity
We model a small quantum dot with a magnetic impurity by the Anderson
Hamiltonian with a supplementary exchange interaction term. The transport
calculations are performed by means of the Green functions within the equation
of motion scheme, in which two decoupling procedures are proposed, for high and
low temperatures, respectively. The paper focuses on the charge fluctuations
for such a system, aspect not addressed before, as well as on the Kondo
resonance. We show a specific role of the excited state, which can be observed
in transport and in spin-spin correlations. Our studies show on a new many-body
feature of the phase shift of transmitted electrons, which is manifested in a
specific dip. In the Kondo regime, our calculations complement existing
theoretical results. The system shows three Kondo peaks in the density of
states: one at the Fermi energy and two side peaks, at a distance corresponding
to the singlet-triplet level spacing. The existence of the central peak is
conditioned by a degenerate state (the triplet) below the Fermi energy.Comment: 12 pages, 4 figure
Hartree-Fock theory of a current-carrying electron gas
State-of-the-art simulation tools for nonequilibrium quantum transport systems typically take the current-carrier occupations to be described in terms of equilibrium distribution functions characterized by two different electrochemical potentials, while for the description of electronic exchange and correlation, the local density approximation (LDA) to density functional theory is generally used. However, this involves an inconsistency because the LDA is based on the homogeneous electron gas in equilibrium, while the system is not in equilibrium and may be far from it. In this paper, we analyze this inconsistency by studying the interplay between nonequilibrium occupancies obtained from a maximum entropy approach and the Hartree-Fock exchange energy, single-particle spectrum and exchange hole, for the case of a two-dimensional homogeneous electron gas. The current dependence of the local exchange potential is also discussed. It is found that the single-particle spectrum and exchange hole have a significant dependence on the current, which has not been taken into account in practical calculations since it is not captured by the commonly used functionals. The exchange energy and the local exchange potential, however, are shown to change very little with respect to their equilibrium counterparts. The weak dependence of these quantities on the current is explained in terms of the symmetries of the exchange hole
Electron transport through an interacting region: The case of a nonorthogonal basis set
The formula derived by Meir and Wingreen [Phys. Rev. Lett. {\bf 68}, 2512
(1992)] for the electron current through a confined, central region containing
interactions is generalized to the case of a nonorthogonal basis set. As in the
original work, the present derivation is based on the nonequilibrium Keldysh
formalism. By replacing the basis functions of the central region by the
corresponding elements of the dual basis, the lead- and central
region-subspaces become mutually orthogonal. The current formula is then
derived in the new basis, using a generalized version of second quantization
and Green's function theory to handle the nonorthogonality within each of the
regions. Finally, the appropriate nonorthogonal form of the perturbation series
for the Green's function is established for the case of electron-electron and
electron-phonon interactions in the central region.Comment: Added references. 8 pages, 1 figur
Mobilities and Scattering Times in Decoupled Graphene Monolayers
Folded single layer graphene forms a system of two decoupled monolayers being
only a few Angstroms apart. Using magnetotransport measurements we investigate
the electronic properties of the two layers conducting in parallel. We show a
method to obtain the mobilities for the individual layers despite them being
jointly contacted. The mobilities in the upper layer are significantly larger
than in the bottom one indicating weaker substrate influence. This is confirmed
by larger transport and quantum scattering times in the top layer. Analyzing
the temperature dependence of the Shubnikov-de Haas oscillations effective
masses and corresponding Fermi velocities are obtained yielding reduced values
down to 66 percent in comparison to monolayers.Comment: 4 pages, 5 figure
Tunable graphene system with two decoupled monolayers
The use of two truly two-dimensional gapless semiconductors, monolayer and bilayer graphene, as current-carrying components in field-effect transistors (FET) gives access to new types of nanoelectronic devices. Here, we report on the development of graphene-based FETs containing two decoupled graphene monolayers manufactured from a single one folded during the exfoliation process. The transport characteristics of these newly-developed devices differ markedly from those manufactured from a single-crystal bilayer. By analyzing Shubnikov-de Haas oscillations, we demonstrate the possibility to independently control the carrier densities in both layers using top and bottom gates, despite there being only a nanometer scale separation between them
Signatures of spin in the n=1/3 Fractional Quantum Hall Effect
The activation gap Delta of the fractional quantum Hall state at constant
filling n =1/3 is measured in wide range of perpendicular magnetic field B.
Despite the full spin polarization of the incompressible ground state, we
observe a sharp crossover between a low-field linear dependence of Delta on B
associated to spin texture excitations and a Coulomb-like behavior at large B.
From the global gap-reduction we get information about the mobility edges in
the fractional quantum Hall regime.Comment: 4 pages, 3 figure
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