8,055 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
Suppression of decoherence in a graphene monolayer ring
The influence of high magnetic fields on coherent transport is investigated.
A monolayer graphene quantum ring is fabricated and the Aharonov-Bohm effect is
observed. For increased magnitude of the magnetic field higher harmonics
appear. This phenomenon is attributed to an increase of the phase coherence
length due to reduction of spin flip scattering
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
Parasitic pumping currents in an interacting quantum dot
We analyze the charge and spin pumping in an interacting dot within the
almost adiabatic limit. By using a non-equilibrium Green's function technique
within the time-dependent slave boson approximation, we analyze the pumped
current in terms of the dynamical constraints in the infinite-U regime. The
results show the presence of parasitic pumping currents due to the additional
phases of the constraints. The behavior of the pumped current through the
quantum dot is illustrated in the spin-insensitive and in the spin-sensitive
case relevant for spintronics applications
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
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
Berry Phase Transition in Twisted Bilayer Graphene
The electronic dispersion of a graphene bilayer is highly dependent on
rotational mismatch between layers and can be further manipulated by electrical
gating. This allows for an unprecedented control over electronic properties and
opens up the possibility of flexible band structure engineering. Here we
present novel magnetotransport data in a twisted bilayer, crossing the
energetic border between decoupled monolayers and coupled bilayer. In addition
a transition in Berry phase between pi and 2pi is observed at intermediate
magnetic fields. Analysis of Fermi velocities and gate induced charge carrier
densities suggests an important role of strong layer asymmetry for the observed
phenomena.Comment: 20 pages main paper + 10 pages supporting informatio
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
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
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