6,497 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
Excitation Induced Dephasing in Semiconductor Quantum Dots
A quantum kinetic theory is used to compute excitation induced dephasing in
semiconductor quantum dots due to the Coulomb interaction with a continuum of
states, such as a quantum well or a wetting layer. It is shown that a frequency
dependent broadening together with nonlinear resonance shifts are needed for a
microscopic explanation of the excitation induced dephasing in such a system,
and that excitation induced dephasing for a quantum-dot excitonic resonance is
different from quantum-well and bulk excitons.Comment: 6 pages, 4 figures. Extensively revised text, two figures change
Bimodal Counting Statistics in Single Electron Tunneling through a Quantum Dot
We explore the full counting statistics of single electron tunneling through
a quantum dot using a quantum point contact as non-invasive high bandwidth
charge detector. The distribution of counted tunneling events is measured as a
function of gate and source-drain-voltage for several consecutive electron
numbers on the quantum dot. For certain configurations we observe
super-Poissonian statistics for bias voltages at which excited states become
accessible. The associated counting distributions interestingly show a bimodal
characteristic. Analyzing the time dependence of the number of electron counts
we relate this to a slow switching between different electron configurations on
the quantum dot
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
Photon heat transport in low-dimensional nanostructures
At low temperatures when the phonon modes are effectively frozen, photon
transport is the dominating mechanism of thermal relaxation in metallic
systems. Starting from a microscopic many-body Hamiltonian, we develop a
nonequilibrium Green's function method to study energy transport by photons in
nanostructures. A formally exact expression for the energy current between a
metallic island and a one-dimensional electromagnetic field is obtained. From
this expression we derive the quantized thermal conductance as well as show how
the results can be generalized to nonequilibrium situations. Generally, the
frequency-dependent current noise of the island electrons determines the energy
transfer rate.Comment: 4 pages, 3 Fig
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
Self-consistent ac quantum transport using nonequilibrium Green functions
We develop an approach for self-consistent ac quantum transport in the
presence of time-dependent potentials at non-transport terminals. We apply the
approach to calculate the high-frequency characteristics of a nanotube
transistor with the ac signal applied at the gate terminal. We show that the
self-consistent feedback between the ac charge and potential is essential to
properly capture the transport properties of the system. In the on-state, this
feedback leads to the excitation of plasmons, which appear as pronounced
divergent peaks in the dynamic conductance at terahertz frequencies. In the
off-state, these collective features vanish, and the conductance exhibits
smooth oscillations, a signature of single-particle excitations. The proposed
approach is general and will allow the study of the high-frequency
characteristics of many other low-dimensional nanoscale materials such as
nanowires and graphene-based systems, which are attractive for terahertz
devices, including those that exploit plasmonic excitations.Comment: 11 pages, 5 figures, accepted in Physical Review
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