666 research outputs found
Controlled dephasing in single-dot Aharonov-Bohm interferometers
We study the Fano effect and the visibility of the Aharonov-Bohm oscillations
for a mesoscopic interferometer with an embedded quantum dot in the presence of
a nearby second dot. When the electron-electron interaction between the two
dots is considered the nearby dot acts as a charge detector. We compute the
currents through the interferometer and detector within the Keldysh formalism
and the self-energy of the non-equilibrium Green functions is found up to the
second order in the interaction strength. The current formula contains a
correction to the Landauer-B\"{uttiker} formula. Its contribution to transport
and dephasing is discussed. As the bias applied on the detector is increased,
the amplitude of both the Fano resonance and Aharonov-Bohm oscillations are
considerably reduced due to controlled dephasing. This result is explained by
analyzing the behavior of the imaginary part of the self-energy as a function
of energy and bias. We investigate as well the role of the ring-dot coupling.
Our theoretical results are consistent to the experimental observation of Buks
{\it et al.} [Nature {\bf 391}, 871 (1998)].Comment: 24 pages, 8 figure
Resonant and coherent transport through Aharonov-Bohm interferometers with coupled quantum dots
A detailed description of the tunneling processes within Aharonov-Bohm (AB)
rings containing two-dimensional quantum dots is presented. We show that the
electronic propagation through the interferometer is controlled by the spectral
properties of the embedded dots and by their coupling with the ring. The
transmittance of the interferometer is computed by the Landauer-B\"uttiker
formula. Numerical results are presented for an AB interferometer containing
two coupled dots. The charging diagrams for a double-dot interferometer and the
Aharonov Bohm oscillations are obtained, in agreement with the recent
experimental results of Holleitner {\it et al}. [Phys. Rev. Lett. {\bf 87},
256802 (2001)] We identify conditions in which the system shows Fano line
shapes. The direction of the asymetric tail depends on the capacitive coupling
and on the magnetic field. We discuss our results in connection with the
experiments of Kobayashi {\it et al} [Phys. Rev. Lett. {\bf 88}, 256806 (2002)]
in the case of a single dot.Comment: 30 pages, 12 figure
Mesoscopic Fano Effect in a spin splitter with a side-coupled quantum dot
Cataloged from PDF version of article.We investigate the interplay between the spin interference and the Fano effect in a three-lead mesoscopic ring with a side-coupled quantum dot (QD). A uniform Rashba spin-orbit coupling and a perpendicular magnetic field are tuned such that the ring operates as a spin splitter in the absence of the QD: one lead is used to inject unpolarized electrons and the remaining (output) leads collect almost polarized spin currents. By applying a gate potential to the quantum dot a pair of spin-split levels sweeps the bias window and leads to Fano interference. The steady-state spin and charge currents in the leads are calculated for a finite bias applied across the ring via the non-equilibrium Green's function formalism. When the QD levels participate to transport we find that the spin currents exhibit peaks and dips whereas the charge currents present Fano lineshapes. The location of the side-coupled quantum dot and the spin splitting of its levels also affect the interference and the output currents. The opposite response of output currents to the variation of the gate potential allows one to use this system as a single parameter current switch. We also analyze the dependence of the splitter efficiency on the spin splitting on the QD. (C) 2012 Elsevier B.V. All rights reserved
Spin-flip Effects in the Mesoscopic Spin-Interferometer
We investigate the properties of the electron spin-transmission through an
Aharonov-Bohm interferometer with an embedded multilevel quantum dot containing
magnetic impurities. A suitable formalism is developed. The amplitude and the
phase of the flip- and nonflip-transmittance are calculated numerically as
function of the magnetic field and the gate potential applied on the dot. The
effects induced by the exchange interaction to spin-dependent
magnetoconductance fluctuations and transmittance phase are shown.Comment: 10 pages, 9 figure
A partition-free approach to transient and steady-state charge currents
We construct a non-equilibrium steady state and calculate the corresponding
current for a mesoscopic Fermi system in the partition-free setting. To this
end we study a small sample coupled to a finite number of semi-infinite leads.
Initially, the whole system of quasi-free fermions is in a grand canonical
equilibrium state. At t = 0 we turn on a potential bias on the leads and let
the system evolve. We study how the charge current behaves in time and how it
stabilizes itself around a steady state value, which is given by a
Landauer-type formula.Comment: 14 pages, submitte
Adiabatic non-equilibrium steady states in the partition free approach
Consider a small sample coupled to a finite number of leads, and assume that
the total (continuous) system is at thermal equilibrium in the remote past. We
construct a non-equilibrium steady state (NESS) by adiabatically turning on an
electrical bias between the leads. The main mathematical challenge is to show
that certain adiabatic wave operators exist, and to identify their strong limit
when the adiabatic parameter tends to zero. Our NESS is different from, though
closely related with the NESS provided by the Jak{\v s}i{\'c}-Pillet-Ruelle
approach. Thus we partly settle a question asked by Caroli {\it et al} in 1971
regarding the (non)equivalence between the partitioned and partition-free
approaches
Time-dependent transport via the generalized master equation through a finite quantum wire with an embedded subsystem
The authors apply the generalized master equation to analyze time-dependent
transport through a finite quantum wire with an embedded subsystem. The
parabolic quantum wire and the leads with several subbands are described by a
continuous model. We use an approach originally developed for a tight-binding
description selecting the relevant states for transport around the bias-window
defined around the values of the chemical potential in the left and right leads
in order to capture the effects of the nontrivial geometry of the system in the
transport. We observe a partial current reflection as a manifestation of a
quasi-bound state in an embedded well and the formation of a resonance state
between an off-set potential hill and the boundary of the system.Comment: RevTeX (pdf-LaTeX), 12 pages with 19 included jpg figure
Kadanoff-Baym approach to time-dependent quantum transport in AC and DC fields
We have developed a method based on the embedded Kadanoff-Baym equations to
study the time evolution of open and inhomogeneous systems. The equation of
motion for the Green's function on the Keldysh contour is solved using
different conserving many-body approximations for the self-energy. Our
formulation incorporates basic conservation laws, such as particle
conservation, and includes both initial correlations and initial embedding
effects, without restrictions on the time-dependence of the external driving
field. We present results for the time-dependent density, current and dipole
moment for a correlated tight binding chain connected to one-dimensional
non-interacting leads exposed to DC and AC biases of various forms. We find
that the self-consistent 2B and GW approximations are in extremely good
agreement with each other at all times, for the long-range interactions that we
consider. In the DC case we show that the oscillations in the transients can be
understood from interchain and lead-chain transitions in the system and find
that the dominant frequency corresponds to the HOMO-LUMO transition of the
central wire. For AC biases with odd inversion symmetry odd harmonics to high
harmonic order in the driving frequency are observed in the dipole moment,
whereas for asymmetric applied bias also even harmonics have considerable
intensity. In both cases we find that the HOMO-LUMO transition strongly mixes
with the harmonics leading to harmonic peaks with enhanced intensity at the
HOMO-LUMO transition energy.Comment: 16 pages, 9 figures. Submitted at "Progress in Nonequilibrium Green's
Functions IV" conferenc
Enhancement of linear and nonlinear optical properties of deoxyribonucleic acid-silica thin films doped with rhodamine
In this work, we present the linear and nonlinear optical properties of DNA as functional material, incorporated into a silica material matrix with rhodamine organic dye. We observed that even low concentration of DNA affects the aggregate behavior of the dyes in silica films. The samples with DNA showed higher transmittance and fluorescence efficiency. Moreover, the presence of DNA has been found to significantly enhance the nonlinear optical response of the systems. In this way, we prove that silica materials can provide suitable matrices for hybridization with functional molecules and can be utilized as active optical waveguide materials with enhanced nonlinear optical properties
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