803 research outputs found
Multiple transitions of the spin configuration in quantum dots
Single electron tunneling is studied in a many electron quantum dot in high
magnetic fields. For such a system multiple transitions of the spin
configuration are theoretically predicted. With a combination of spin blockade
and Kondo effect we are able to detect five regions with different spin
configurations. Transitions are induced with changing electron numbers.Comment: 4 pages, 5 figure
Spin Blockade in Capacitively Coupled Quantum Dots
We present transport measurements on a lateral double dot produced by
combining local anodic oxidation and electron beam lithography. We investigate
the tunability of our device and demonstrate, that we can switch between
capacitive and tunnel coupling. In the regime of capacitive coupling we observe
the phenomenon of spin blockade in a magnetic field and analyze the influence
of capacitive interdot coupling on this effect.Comment: 4 pages, 3 figure
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
Interaction-Induced Spin Polarization in Quantum Dots
The electronic states of lateral many electron quantum dots in high magnetic
fields are analyzed in terms of energy and spin. In a regime with two Landau
levels in the dot, several Coulomb blockade peaks are measured. A zig-zag
pattern is found as it is known from the Fock-Darwin spectrum. However, only
data from Landau level 0 show the typical spin-induced bimodality, whereas
features from Landau level 1 cannot be explained with the Fock-Darwin picture.
Instead, by including the interaction effects within spin-density-functional
theory a good agreement between experiment and theory is obtained. The absence
of bimodality on Landau level 1 is found to be due to strong spin polarization.Comment: 4 pages, 5 figure
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
Towards visualisation of central-cell-effects in scanning-tunnelling-microscope images of subsurface dopant qubits in silicon
Atomic-scale understanding of phosphorous donor wave functions underpins the
design and optimisation of silicon based quantum devices. The accuracy of
large-scale theoretical methods to compute donor wave functions is dependent on
descriptions of central-cell-corrections, which are empirically fitted to match
experimental binding energies, or other quantities associated with the global
properties of the wave function. Direct approaches to understanding such
effects in donor wave functions are of great interest. Here, we apply a
comprehensive atomistic theoretical framework to compute scanning tunnelling
microscopy (STM) images of subsurface donor wave functions with two
central-cell-correction formalisms previously employed in the literature. The
comparison between central-cell models based on real-space image features and
the Fourier transform profiles indicate that the central-cell effects are
visible in the simulated STM images up to ten monolayers below the silicon
surface. Our study motivates a future experimental investigation of the
central-cell effects via STM imaging technique with potential of fine tuning
theoretical models, which could play a vital role in the design of donor-based
quantum systems in scalable quantum computer architectures.Comment: Nanoscale 201
Combined atomic force microscope and electron-beam lithography used for the fabrication of variable-coupling quantum dots
We have combined direct nanofabrication by local anodic oxidation with
conventional electron-beam lithography to produce a parallel double quantum dot
based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography
methods allows to fabricate robust in-plane gates and Cr/Au top gate electrodes
on the same device for optimal controllability. This is illustrated by the
tunability of the interdot coupling in our device. We describe our fabrication
and alignment scheme in detail and demonstrate the tunability in
low-temperature transport measurements.Comment: 4 pages, 3 figure
Non-invasive detection of charge-rearrangement in a quantum dot in high magnetic fields
We demonstrate electron redistribution caused by magnetic field on a single
quantum dot measured by means of a quantum point contact as non-invasive
detector. Our device which is fabricated by local anodic oxidation allows to
control independently the quantum point contact and all tunnelling barriers of
the quantum dot. Thus we are able to measure both the change of the quantum dot
charge and also changes of the electron configuration at constant number of
electrons on the quantum dot. We use these features to exploit the quantum dot
in a high magnetic field where transport through the quantum dot displays the
effects of Landau shells and spin blockade. We confirm the internal
rearrangement of electrons as function of the magnetic field for a fixed number
of electrons on the quantum dot.Comment: 4 pages, 5 figure
Two path transport measurements on a triple quantum dot
We present an advanced lateral triple quantum dot made by local anodic
oxidation. Three dots are coupled in a starlike geometry with one lead attached
to each dot thus allowing for multiple path transport measurements with two
dots per path. In addition charge detection is implemented using a quantum
point contact. Both in charge measurements as well as in transport we observe
clear signatures of states from each dot. Resonances of two dots can be
established allowing for serial transport via the corresponding path. Quadruple
points with all three dots in resonance are prepared for different electron
numbers and analyzed concerning the interplay of the simultaneously measured
transport along both paths.Comment: 4 pages, 4 figure
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