16,821 research outputs found
Electronic Green's functions in a T-shaped multi-quantum dot system
We developed a set of equations to calculate the electronic Green's functions
in a T-shaped multi-quantum dot system using the equation of motion method. We
model the system using a generalized Anderson Hamiltonian which accounts for
{\em finite} intradot on-site Coulomb interaction in all component dots as well
as for the interdot electron tunneling between adjacent quantum dots. Our
results are obtained within and beyond the Hartree-Fock approximation and
provide a path to evaluate all the electronic correlations in the multi-quantum
dot system in the Coulomb blockade regime. Both approximations provide
information on the physical effects related to the finite intradot on-site
Coulomb interaction. As a particular example for our generalized results, we
considered the simplest T-shaped system consisting of two dots and proved that
our approximation introduces important corrections in the detector and side
dots Green's functions, and implicitly in the evaluation of the system's
transport properties. The multi-quantum dot T-shaped setup may be of interest
for the practical realization of qubit states in quantum dots systems.Comment: 13 pages, 2 figure
Transmission from reverse reaction coordinate mappings
We point out that the transport properties of non-interacting fermionic
chains tunnel-coupled to two reservoirs at their ends can be mapped to those of
a single quantum dot that is tunnel-coupled to two transformed reservoirs. The
parameters of the chain are mapped to additional structure in the spectral
densities of the transformed reservoirs. For example, this enables the
calculation of the transmission of quantum dot chains by evaluating the known
transmission of a single quantum dot together with structured spectral
densities. We exemplify this analytically for short chains, which allows to
optimize the transmission. In addition, we also demonstrate that the mapping
can be performed numerically by computing the transmission of a
Su-Schrieffer-Heeger chain.Comment: 9+5 pages, 9 figures, invited contribution to the special issue
"Non-linear and Complex Dynamics in Semiconductors and Related Materials"
with the motto ''Complex Systems Science meets Matter and Materials'' in EPJ
Spin states of the first four holes in a silicon nanowire quantum dot
We report measurements on a silicon nanowire quantum dot with a clarity that
allows for a complete understanding of the spin states of the first four holes.
First, we show control of the hole number down to one. Detailed measurements at
perpendicular magnetic fields reveal the Zeeman splitting of a single hole in
silicon. We are able to determine the ground-state spin configuration for one
to four holes occupying the quantum dot and find a spin filling with
alternating spin-down and spin-up holes, which is confirmed by
magnetospectroscopy up to 9T. Additionally, a so far inexplicable feature in
single-charge quantum dots in many materials systems is analyzed in detail. We
observe excitations of the zero-hole ground-state energy of the quantum dot,
which cannot correspond to electronic or Zeeman states. We show that the most
likely explanation is acoustic phonon emission to a cavity between the two
contacts to the nanowire.Comment: 24 pages, 8 figures, both including supporting informatio
Non-invasive detection of molecular bonds in quantum dots
We performed charge detection on a lateral triple quantum dot with star-like
geometry. The setup allows us to interpret the results in terms of two double
dots with one common dot. One double dot features weak tunnel coupling and can
be understood with atom-like electronic states, the other one is strongly
coupled forming molecule-like states. In nonlinear measurements we identified
patterns that can be analyzed in terms of the symmetry of tunneling rates.
Those patterns strongly depend on the strength of interdot tunnel coupling and
are completely different for atomic- or molecule-like coupled quantum dots
allowing the non-invasive detection of molecular bonds.Comment: 4 pages, 4 figure
Effects of different geometries on the conductance, shot noise and tunnel magnetoresistance of double quantum dots
The spin-polarized transport through a coherent strongly coupled double
quantum dot (DQD) system is analyzed theoretically in the sequential and
cotunneling regimes. Using the real-time diagrammatic technique, we analyze the
current, differential conductance, shot noise and tunnel magnetoresistance
(TMR) as a function of both the bias and gate voltages for double quantum dots
coupled in series, in parallel as well as for T-shaped systems. For DQDs
coupled in series, we find a strong dependence of the TMR on the number of
electrons occupying the double dot, and super-Poissonian shot noise in the
Coulomb blockade regime. In addition, for asymmetric DQDs, we analyze transport
in the Pauli spin blockade regime and explain the existence of the leakage
current in terms of cotunneling and spin-flip cotunneling-assisted sequential
tunneling. For DQDs coupled in parallel, we show that the transport
characteristics in the weak coupling regime are qualitatively similar to those
of DQDs coupled in series. On the other hand, in the case of T-shaped quantum
dots we predict a large super-Poissonian shot noise and TMR enhanced above the
Julliere value due to increased occupation of the decoupled quantum dot. We
also discuss the possibility of determining the geometry of the double dot from
transport characteristics. Furthermore, where possible, we compare our results
with existing experimental data on nonmagnetic systems and find qualitative
agreement.Comment: 15 pages, 12 figures, accepted in Phys. Rev.
Quantum interference and phonon-mediated back-action in lateral quantum dot circuits
Spin qubits have been successfully realized in electrostatically defined,
lateral few-electron quantum dot circuits. Qubit readout typically involves
spin to charge information conversion, followed by a charge measurement made
using a nearby biased quantum point contact. It is critical to understand the
back-action disturbances resulting from such a measurement approach. Previous
studies have indicated that quantum point contact detectors emit phonons which
are then absorbed by nearby qubits. We report here the observation of a
pronounced back-action effect in multiple dot circuits where the absorption of
detector-generated phonons is strongly modified by a quantum interference
effect, and show that the phenomenon is well described by a theory
incorporating both the quantum point contact and coherent phonon absorption.
Our combined experimental and theoretical results suggest strategies to
suppress back-action during the qubit readout procedure.Comment: 25 pages, 8 figure
Landau-Zener-Stuckelberg-Majorana interferometry of a single hole
We perform Landau-Zener-Stuckelberg-Majorana (LZSM) spectroscopy on a system
with strong spin-orbit interaction (SOI), realized as a single hole confined in
a gated double quantum dot. In analogy to the electron systems, at magnetic
field B=0 and high modulation frequencies we observe the photon-assisted
tunneling (PAT) between dots, which smoothly evolves into the typical LZSM
funnel-shaped interference pattern as the frequency is decreased. In contrast
to electrons, the SOI enables an additional, efficient spin-flipping interdot
tunneling channel, introducing a distinct interference pattern at finite B.
Magneto-transport spectra at low-frequency LZSM driving show the two channels
to be equally coherent. High-frequency LZSM driving reveals complex
photon-assisted tunneling pathways, both spin-conserving and spin-flipping,
which form closed loops at critical magnetic fields. In one such loop an
arbitrary hole spin state is inverted, opening the way toward its
all-electrical manipulation.Comment: 6 pages, 4 figures, and supplementary materia
Kondo effect in side coupled double quantum-dot molecule
Electron tunneling through a double quantum dot molecule side attached to a
quantum wire, in the Kondo regime, is studied. The mean-field finite-U
slave-boson formalism is used to obtain the solution of the problem. We found
conductance cancelations when the molecular energies of the side attached
double quantum-dot cross the Fermi energy. We investigate the many body
molecular Kondo states as a function of the parameters of the system.Comment: 12 pages, 7 figures. Submitted to Solid State Com
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