7,028 research outputs found
Excitonic Dynamical Franz-Keldysh Effect
The Dynamical Franz-Keldysh Effect is exposed by exploring near-bandgap
absorption in the presence of intense THz electric fields. It bridges the gap
between the DC Franz- Keldysh effect and multi-photon absorption and competes
with the THz AC Stark Effect in shifting the energy of the excitonic resonance.
A theoretical model which includes the strong THz field non-perturbatively via
a non-equilibrium Green Functions technique is able to describe the Dynamical
Franz-Keldysh Effect in the presence of excitonic absorption.Comment: 4 pages in revtex with 5 figures included using epsf. Submitted to
Physical Review Letter
Non-Markovian quantum state diffusion for an open quantum system in fermionic environments
Non-Markovian quantum state diffusion (NMQSD) provides a powerful approach to
the dynamics of an open quantum system in bosonic environments. Here we develop
an NMQSD method to study the open quantum system in fermionic environments.
This problem involves anticommutative noise functions (i.e., Grassmann
variables) that are intrinsically different from the noise functions of bosonic
baths. We obtain the NMQSD equation for quantum states of the system and the
non-Markovian master equation. Moreover, we apply this NMQSD method to single
and double quantum-dot systems.Comment: 9 pages, 1 figur
Semiconductor Microstructure in a Squeezed Vacuum: Electron-Hole Plasma Luminescence
We consider a semiconductor quantum-well placed in a wave guide microcavity
and interacting with the broadband squeezed vacuum radiation, which fills one
mode of the wave guide with a large average occupation. The wave guide modifies
the optical density of states so that the quantum well interacts mostly with
the squeezed vacuum. The vacuum is squeezed around the externally controlled
central frequency \om_0, which is tuned above the electron-hole gap ,
and induces fluctuations in the interband polarization of the quantum-well. The
power spectrum of scattered light exhibits a peak around \om_0, which is
moreover non-Lorentzian and is a result of both the squeezing and the
particle-hole continuum. The squeezing spectrum is qualitatively different from
the atomic case. We discuss the possibility to observe the above phenomena in
the presence of additional non-radiative (e-e, phonon) dephasing.Comment: 6 pages, 3 figure
Nonequilibrium Green's function theory for nonadiabatic effects in quantum electron transport
We develop nonequilibribrium Green's function based transport theory, which
includes effects of nonadiabatic nuclear motion in the calculation of the
electric current in molecular junctions. Our approach is based on the
separation of slow and fast timescales in the equations of motion for the
Green's functions by means of the Wigner representation. Time derivatives with
respect to central time serves as a small parameter in the perturbative
expansion enabling the computation of nonadiabatic corrections to molecular
Green's functions. Consequently, we produce series of analytic expressions for
non-adiabatic electronic Green's functions (up to the second order in the
central time derivatives); which depend not solely on instantaneous molecular
geometry but likewise on nuclear velocities and accelerations. Extended formula
for electric current is derived which accounts for the non-adiabatic
corrections. This theory is concisely illustrated by the calculations on a
model molecular junction
Adiabatic pumping through a quantum dot in the Kondo regime: Exact results at the Toulouse limit
Transport properties of ultrasmall quantum dots with a single unpaired
electron are commonly modeled by the nonequilibrium Kondo model, describing the
exchange interaction of a spin-1/2 local moment with two leads of
noninteracting electrons. Remarkably, the model possesses an exact solution
when tuned to a special manifold in its parameter space known as the Toulouse
limit. We use the Toulouse limit to exactly calculate the adiabatically pumped
spin current in the Kondo regime. In the absence of both potential scattering
and a voltage bias, the instantaneous charge current is strictly zero for a
generic Kondo model. However, a nonzero spin current can be pumped through the
system in the presence of a finite magnetic field, provided the spin couples
asymmetrically to the two leads. Tunneling through a Kondo impurity thus offers
a natural mechanism for generating a pure spin current. We show, in particular,
that one can devise pumping cycles along which the average spin pumped per
cycle is closely equal to . By analogy with Brouwer's formula for
noninteracting systems with two driven parameters, the pumped spin current is
expressed as a geometrical property of a scattering matrix. However, the
relevant %Alex: I replaced topological with geometrical in the sentence above
scattering matrix that enters the formulation pertains to the Majorana fermions
that appear at the Toulouse limit rather than the physical electrons that carry
the current. These results are obtained by combining the nonequilibrium Keldysh
Green function technique with a systematic gradient expansion, explicitly
exposing the small parameter controlling the adiabatic limit.Comment: 14 pages, 3 figures, revised versio
Revivals, collapses and magnetic-pulse generation in quantum rings
Using a microscopic theory based on the density matrix formalism we
investigate quantum revivals and collapses of the charge polarization and
charge current dynamics in mesoscopic rings driven by short asymmetric
electromagnetic pulses. The collapsed state is utilized for sub-picosecond
switching of the current and associated magnetization, enabling thus the
generation of pulsed magnetic fields with a tunable time structure and shape
asymmetry which provides a new tool to study ultrafast spin-dynamics and
ratchet-based effects.Comment: 4 pages, 2 figure
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
Non-Markovian Dynamics of Charge Carriers in Quantum Dots
We have investigated the dynamics of bound particles in multilevel
current-carrying quantum dots. We look specifically in the regime of resonant
tunnelling transport, where several channels are available for transport.
Through a non-Markovian formalism under the Born approximation, we investigate
the real-time evolution of the confined particles including transport-induced
decoherence and relaxation. In the case of a coherent superposition between
states with different particle number, we find that a Fock-space coherence may
be preserved even in the presence of tunneling into and out of the dot.
Real-time results are presented for various asymmetries of tunneling rates into
different orbitals.Comment: 9 pages, 3 figures, International Workshop on Physics-Based
Mathematical Models for Low-Dimensional Semiconductor Nanostructures. BIRS,
November 18-23, 200
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
Kinetics of four-wave mixing for a 2D magneto-plasma in strong magnetic fields
We investigate the femtosecond kinetics of an optically excited 2D
magneto-plasma at intermediate and high densities under a strong magnetic field
perpendicular to the quantum well (QW). We assume an additional weak lateral
confinement which lifts the degeneracy of the Landau levels partially. We
calculate the femtosecond dephasing and relaxation kinetics of the laser pulse
excited magneto-plasma due to bare Coulomb potential scattering, because
screening is under these conditions of minor importance. In particular the
time-resolved and time-integrated four-wave mixing (FWM) signals are calculated
by taking into account three Landau subbands in both the valance and the
conduction band assuming an electron-hole symmetry. The FWM signals exhibit
quantum beats mainly with twice the cyclotron frequency. Contrary to general
expectations, we find no pronounced slowing down of the dephasing with
increasing magnetic field. On the contrary, one obtains a decreasing dephasing
time because of the increase of the Coulomb matrix elements and the number of
states in a given Landau subband. In the situation when the loss of scattering
channels exceeds these increasing effects, one gets a slight increase at the
dephasing time. However, details of the strongly modulated scattering kinetics
depend sensitively on the detuning, the plasma density, and the spectral pulse
width relative to the cyclotron frequency.Comment: 13 pages, in RevTex format, 10 figures, Phys. Rev B in pres
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