5,393 research outputs found
Modulation of the dephasing time for a magnetoplasma in a quantum well
We investigate the femtosecond kinetics of optically excited 2D
magneto-plasma. 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. By
taking into account four Landau subbands in both the conduction band and the
valence band, we are now able to extend our earlier study [Phys. Rev. B {\bf
58}, 1998,in print (see also cond-mat/9808073] to lower magnetic fields. We can
also fix the magnetic field and change the detuning to further investigate the
carrier density-dependence of the dephasing time. For both cases, we predict
strong modulation in the dephasing time.Comment: RevTex, 3 figures, to be published in Solid. Stat. Commu
Optical response of graphene under intense terahertz fields
Optical responses of graphene in the presence of intense circularly and
linearly polarized terahertz fields are investigated based on the Floquet
theory. We examine the energy spectrum and density of states. It is found that
gaps open in the quasi-energy spectrum due to the single-photon/multi-photon
resonances. These quasi-energy gaps are pronounced at small momentum, but
decrease dramatically with the increase of momentum and finally tend to be
closed when the momentum is large enough. Due to the contribution from the
states at large momentum, the gaps in the density of states are effectively
closed, in contrast to the prediction in the previous work by Oka and Aoki
[Phys. Rev. B {\bf 79}, 081406(R) (2009)]. We also investigate the optical
conductivity for different field strengths and Fermi energies, and show the
main features of the dynamical Franz-Keldysh effect in graphene. It is
discovered that the optical conductivity exhibits a multi-step-like structure
due to the sideband-modulated optical transition. It is also shown that dips
appear at frequencies being the integer numbers of the applied terahertz field
frequency in the case of low Fermi energy, originating from the quasi-energy
gaps at small momentums. Moreover, under a circularly polarized terahertz
field, we predict peaks in the middle of the "steps" and peaks induced by the
contribution from the states around zero momentum in the optical conductivity.Comment: 15 pages, 10 figure
Competition between Kondo screening and quantum Hall edge reconstruction
We report on a Kondo correlated quantum dot connected to two-dimensional
leads where we demonstrate the renormalization of the g-factor in the pure
Zeeman case i.e, for magnetic fields parallel to the plane of the quantum dot.
For the same system we study the influence of orbital effects by investigating
the quantum Hall regime i.e. a perpendicular magnetic field is applied. In this
case an unusual behaviour of the suppression of the Kondo effect and of the
split zero-bias anomaly is observed. The splitting decreases with magnetic
field and shows discontinuous changes which are attributed to the intricate
interplay between Kondo screening and the quantum Hall edge structure
originating from electrostatic screening. This edge structure made up of
compressible and incompressible stripes strongly affects the Kondo temperature
of the quantum dot and thereby influences the renormalized g-factor
Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots
The time evolution of optically excited carriers in semiconductor quantum
wells and quantum dots is analyzed for their interaction with LO-phonons. Both
the full two-time Green's function formalism and the one-time approximation
provided by the generalized Kadanoff-Baym ansatz are considered, in order to
compare their description of relaxation processes. It is shown that the
two-time quantum kinetics leads to thermalization in all the examined cases,
which is not the case for the one-time approach in the intermediate-coupling
regime, even though it provides convergence to a steady state. The
thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.
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
A 150-million-year-old crab larva and its implications for the early rise of brachyuran crabs
True crabs (Brachyura) are the most successful group of decapod crustaceans. This success is most likely coupled to their life history, including two specialised larval forms, zoea and megalopa. The group is comparably young, starting to diversify only about 100 million years ago (mya),with a dramatic increase in species richness beginning approximately 50 mya. Early evolution of crabs is still very incompletely known. Here, we report a fossil crab larva, 150 mya, documented with up-to-date imaging techniques. It is only the second find of any fossil crab larva, but the first complete one, the first megalopa, and the oldest one (other fossil ca. 110 mya). Despite its age, the new fossil possesses a very modern morphology, being indistinguishable from many extant crab larvae. Hence, modern morphologies must have been present significantly earlier than formerly anticipated. We briefly discuss the impact of this find on our understanding of early crab evolution
Quantum master equation scheme of time-dependent density functional theory to time-dependent transport in nano-electronic devices
In this work a practical scheme is developed for the first-principles study
of time-dependent quantum transport. The basic idea is to combine the transport
master-equation with the well-known time-dependent density functional theory.
The key ingredients of this paper include: (i) the partitioning-free initial
condition and the consideration of the time-dependent bias voltages which base
our treatment on the Runge-Gross existence theorem; (ii) the non-Markovian
master equation for the reduced (many-body) central system (i.e. the device);
and (iii) the construction of Kohn-Sham master equation for the reduced
single-particle density matrix, where a number of auxiliary functions are
introduced and their equations of motion (EOM) are established based on the
technique of spectral decomposition. As a result, starting with a well-defined
initial state, the time-dependent transport current can be calculated
simultaneously along the propagation of the Kohn-Sham master equation and the
EOM of the auxiliary functions.Comment: 9 pages, no figure
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
Probing a Kondo correlated quantum dot with spin spectroscopy
We investigate Kondo effect and spin blockade observed on a many-electron
quantum dot and study the magnetic field dependence. At lower fields a
pronounced Kondo effect is found which is replaced by spin blockade at higher
fields. In an intermediate regime both effects are visible. We make use of this
combined effect to gain information about the internal spin configuration of
our quantum dot. We find that the data cannot be explained assuming regular
filling of electronic orbitals. Instead spin polarized filling seems to be
probable.Comment: 4 pages, 5 figure
Electron spin relaxation in n-type InAs quantum wires
We investigate the electron spin relaxation of -type InAs quantum wires by
numerically solving the fully microscopic kinetic spin Bloch equations with the
relevant scattering explicitly included. We find that the quantum-wire size and
the growth direction influence the spin relaxation time by modulating the
spin-orbit coupling. Due to inter-subband scattering in connection with the
spin-orbit interaction, spin-relaxation in quantum wires can show different
characteristics from those in bulk or quantum wells and can be effectively
manipulated by various means.Comment: 8 pages, 6 figure
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