851 research outputs found
Surface Acoustic Wave induced Transport in a Double Quantum Dot
We report on non-adiabatic transport through a double quantum dot under
irradiation of surface acoustic waves generated on-chip. At low excitation
powers, absorption and emission of single and multiple phonons is observed. At
higher power, sequential phonon assisted tunneling processes excite the double
dot in a highly non-equilibrium state. The present system is attractive for
studying electron-phonon interaction with piezoelectric coupling.Comment: 4 pages, 3 figure
Negative differential conductance and magnetoresistance oscillations due to spin accumulation in ferromagnetic double-island devices
Spin-dependent electronic transport in magnetic double-island devices is
considered theoretically in the sequential tunneling regime. Electric current
and tunnel magnetoresistance are analyzed as a function of the bias voltage and
spin relaxation time in the islands. It is shown that the interplay of spin
accumulation on the islands and charging effects leads to periodic modification
of the differential conductance and tunnel magnetoresistance. For a
sufficiently long spin relaxation time, the modulations are associated with
periodic oscillations of the sign of both the tunnel magnetoresistance and
differential conductance
Non-Markovian dynamics of double quantum dot charge qubits due to acoustic phonons
We investigate the dynamics of a double quantum dot charge qubit which is
coupled to piezoelectric acoustic phonons, appropriate for GaAs
heterostructures. At low temperatures, the phonon bath induces a non-Markovian
dynamical behavior of the oscillations between the two charge states of the
double quantum dot. Upon applying the numerically exact quasiadiabatic
propagator path-integral scheme, the reduced density matrix of the charge qubit
is calculated, thereby avoiding the Born-Markov approximation. This allows a
systematic study of the dependence of the Q-factor on the lattice temperature,
on the size of the quantum dots, as well as on the interdot coupling. We
calculate the Q-factor for a recently realized experimental setup and find that
it is two orders of magnitudes larger than the measured value, indicating that
the decoherence due to phonons is a subordinate mechanism.Comment: 5 pages, 7 figures, replaced with the version to appear in Phys. Rev.
Transient regime in non-linear transport through many-level quantum dots
We investigate the nonstationary electronic transport in noninteracting
nanostructures driven by a finite bias and time-dependent signals applied at
their contacts to the leads. The systems are modelled by a tight-binding
Hamiltonian and the transient currents are computed from the non-equilibrium
Green-Keldysh formalism. The numerical implementation is not restricted to weak
coupling to the leads and does not imply the wide-band limit assumption for the
spectral width of the leads. As an application of the method we study in detail
the transient behavior and the charge dynamics in single and double quantum
dots connected to leads by a step-like potential, but the method allows as well
the consideration of non-periodic potentials or short pulses. We show that when
the higher energy levels of the isolated system are located within the bias
window of the leads the transient current approaches the steady state in a
non-oscillatory smooth fashion. At moderate coupling to the leads and fixed
bias the transient acquires a step-like structure, the length of the steps
increasing with the system size. The number of levels inside a finite bias
window can be tuned by a constant gate potential. We find also that the
transient behavior depends on the specific way of coupling the leads to the
mesoscopic system.Comment: RevTeX, 12 pages, 11 include .eps figure
Gate-tunable band structure of the LaAlO-SrTiO interface
The 2-dimensional electron system at the interface between LaAlO and
SrTiO has several unique properties that can be tuned by an externally
applied gate voltage. In this work, we show that this gate-tunability extends
to the effective band structure of the system. We combine a magnetotransport
study on top-gated Hall bars with self-consistent Schr\"odinger-Poisson
calculations and observe a Lifshitz transition at a density of
cm. Above the transition, the carrier density of one
of the conducting bands decreases with increasing gate voltage. This surprising
decrease is accurately reproduced in the calculations if electronic
correlations are included. These results provide a clear, intuitive picture of
the physics governing the electronic structure at complex oxide interfaces.Comment: 14 pages, 4 figure
Direct control of the tunnel splitting in a one-electron double quantum dot
Quasi-static transport measurements are employed on a laterally defined
tunnel-coupled double quantum dot. A nearby quantum point contact allows us to
track the charge as added to the device. If charged with only up to one
electron, the low-energy spectrum of the double quantum dot is characterized by
its quantum mechanical interdot tunnel splitting. We directly measure its
magnitude by utilizing particular anticrossing features in the stability
diagram at finite source-drain bias. By modification of gate voltages defining
the confinement potential as well as by variation of a perpendicular magnetic
field we demonstrate the tunability of the coherent tunnel coupling.Comment: High resolution pdf file available at
http://www2.nano.physik.uni-muenchen.de/~huettel/research/anticrossing.pd
Molecular states in carbon nanotube double quantum dots
We report electrical transport measurements through a semiconducting
single-walled carbon nanotube (SWNT) with three additional top-gates. At low
temperatures the system acts as a double quantum dot with large inter-dot
tunnel coupling allowing for the observation of tunnel-coupled molecular states
extending over the whole double-dot system. We precisely extract the tunnel
coupling and identify the molecular states by the sequential-tunneling line
shape of the resonances in differential conductance.Comment: 5 pages, 4 figure
Transport through a Strongly Correlated Quantum-Dot with Fano Interference
We present the transport properties of a strongly correlated quantum dot
attached to two leads with a side coupled non-interacting quantum dot.
Transport properties are analyzed using the slave boson mean field theory which
is reliable in the zero temperature and low bias regime. It is found that the
transport properties are determined by the interplay of two fundamental
physical phenomena,i.e. the Kondo effects and the Fano interference. The linear
conductance will depart from the unitary limit and the zero bias anomaly will
be suppressed in the presence of interdot coupling. The zero bias shot noise
Fano factor increases with the interdot coupling and tends to the Poisson
value. The shot noise Fano factor shows a non-monotonic behavior as a function
of the interdot coupling for various side dot energy levels
The JCMT Spectral Legacy Survey: physical structure of the molecular envelope of the high-mass protostar AFGL2591
The understanding of the formation process of massive stars (>8 Msun) is
limited, due to theoretical complications and observational challenges.
We investigate the physical structure of the large-scale (~10^4-10^5 AU)
molecular envelope of the high-mass protostar AFGL2591 using spectral imaging
in the 330-373 GHz regime from the JCMT Spectral Legacy Survey. Out of ~160
spectral features, this paper uses the 35 that are spatially resolved.
The observed spatial distributions of a selection of six species are compared
with radiative transfer models based on a static spherically symmetric
structure, a dynamic spherical structure, and a static flattened structure. The
maps of CO and its isotopic variations exhibit elongated geometries on scales
of ~100", and smaller scale substructure is found in maps of N2H+, o-H2CO, CS,
SO2, CCH, and methanol lines. A velocity gradient is apparent in maps of all
molecular lines presented here, except SO, SO2, and H2CO. We find two emission
peaks in warm (Eup~200K) methanol separated by 12", indicative of a secondary
heating source in the envelope.
The spherical models are able to explain the distribution of emission for the
optically thin H13CO+ and C34S, but not for the optically thick HCN, HCO+, and
CS, nor for the optically thin C17O. The introduction of velocity structure
mitigates the optical depth effects, but does not fully explain the
observations, especially in the spectral dimension. A static flattened envelope
viewed at a small inclination angle does slightly better.
We conclude that a geometry of the envelope other than an isotropic static
sphere is needed to circumvent line optical depth effects. We propose that this
could be achieved in envelope models with an outflow cavity and/or
inhomogeneous structure at scales smaller than ~10^4 AU. The picture of
inhomogeneity is supported by observed substructure in at least six species.Comment: 17 pages; accepted for publication in A&
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