404 research outputs found
Kondo Effect in a Many-Electron Quantum Ring
The Kondo effect is investigated in a many-electron quantum ring as a
function of magnetic field. For fields applied perpendicular to the plane of
the ring a modulation of the Kondo effect with the Aharonov-Bohm period is
observed. This effect is discussed in terms of the energy spectrum of the ring
and the parametrically changing tunnel coupling. In addition, we use gate
voltages to modify the ground-state spin of the ring. The observed splitting of
the Kondo-related zero-bias anomaly in this configuration is tuned with an
in-plane magnetic field.Comment: 4 pages, 4 figure
Singlet-Triplet Transition Tuned by Asymmetric Gate Voltages in a Quantum Ring
Wavefunction and interaction effects in the addition spectrum of a Coulomb
blockaded many electron quantum ring are investigated as a function of
asymmetrically applied gate voltages and magnetic field. Hartree and exchange
contributions to the interaction are quantitatively evaluated at a crossing
between states extended around the ring and states which are more localized in
one arm of the ring. A gate tunable singlet-triplet transition of the two
uppermost levels of this many electron ring is identified at zero magnetic
field.Comment: 4 page
Towards electron transport measurements in chemically modified graphene: The effect of a solvent
Chemical functionalization of graphene modifies the local electron density of
the carbon atoms and hence electron transport. Measuring these changes allows
for a closer understanding of the chemical interaction and the influence of
functionalization on the graphene lattice. However, not only chemistry, in this
case diazonium chemistry, has an effect on the electron transport. Latter is
also influenced by defects and dopants resulting from different processing
steps. Here, we show that solvents used in the chemical reaction process change
the transport properties. In more detail, the investigated combination of
isopropanol and heating treatment reduces the doping concentration and
significantly increases the mobility of graphene. Furthermore, the isopropanol
treatment alone increases the concentration of dopants and introduces an
asymmetry between electron and hole transport which might be difficult to
distinguish from the effect of functionalization. The results shown in this
work demand a closer look on the influence of solvents used for chemical
modification in order to understand their influence
In-plane gate single-electron transistor in Ga[Al]As fabricated by scanning probe lithography
A single-electron transistor has been realized in a Ga[Al]As heterostructure
by oxidizing lines in the GaAs cap layer with an atomic force microscope. The
oxide lines define the boundaries of the quantum dot, the in-plane gate
electrodes, and the contacts of the dot to source and drain. Both the number of
electrons in the dot as well as its coupling to the leads can be tuned with an
additional, homogeneous top gate electrode. Pronounced Coulomb blockade
oscillations are observed as a function of voltages applied to different gates.
We find that, for positive top-gate voltages, the lithographic pattern is
transferred with high accuracy to the electron gas. Furthermore, the dot shape
does not change significantly when in-plane voltages are tuned.Comment: 4 pages, 3 figure
Optical polarization of localized hole spins in p-doped quantum wells
The initialization of spin polarization in localized hole states is
investigated using time-resolved Kerr rotation. We find that the sign of the
polarization depends on the magnetic field, and the power and the wavelength of
the circularly polarized pump pulse. An analysis of the spin dynamics and the
spin-initialization process shows that two mechanisms are responsible for spin
polarization with opposite sign: The difference of the g factor between the
localized holes and the trions, as well as the capturing process of dark
excitons by the localized hole states.Comment: 4 pages, 2 figure
Transmission Phase Through Two Quantum Dots Embedded in a Four-Terminal Quantum Ring
We use the Aharonov-Bohm effect in a four-terminal ring based on a Ga[Al]As
heterostructure for the measurement of the relative transmission phase. In each
of the two interfering paths we induce a quantum dot. The number of electrons
in the two dots can be controlled independently. The transmission phase is
measured as electrons are added to or taken away from the individual quantum
dots.Comment: 3 pages, 4 figure
Transport properties of quantum dots with hard walls
Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation
with an atomic force microscope. This technique, in combination with top gate
voltages, allows us to generate steep walls at the confining edges and small
lateral depletion lengths. The confinement is characterized by low-temperature
magnetotransport measurements, from which the dots' energy spectrum is
reconstructed. We find that in small dots, the addition spectrum can
qualitatively be described within a Fock-Darwin model. For a quantitative
analysis, however, a hard-wall confinement has to be considered. In large dots,
the energy level spectrum deviates even qualitatively from a Fock-Darwin model.
The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure
Transport properties of quantum dots with hard walls
Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation
with an atomic force microscope. This technique, in combination with top gate
voltages, allows us to generate steep walls at the confining edges and small
lateral depletion lengths. The confinement is characterized by low-temperature
magnetotransport measurements, from which the dots' energy spectrum is
reconstructed. We find that in small dots, the addition spectrum can
qualitatively be described within a Fock-Darwin model. For a quantitative
analysis, however, a hard-wall confinement has to be considered. In large dots,
the energy level spectrum deviates even qualitatively from a Fock-Darwin model.
The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure
Density dependence of microwave induced magneto-resistance oscillations in a two-dimensional electron gas
We have measured the magneto-resistance of a two-dimensional electron gas
(2DEG) under continuous microwave irradiation as a function of electron density
and mobility tuned with a metallic top-gate. In the entire range of density and
mobility we have investigated, we observe microwave induced oscillations of
large amplitude that are B-periodic. These B-periodic oscillations are
reminiscent of the ones reported by Kukushkin \textit{et al}[1] and which were
attributed to the presence of edge-magneto-plasmons. We have found that the
B-periodicity does not increase linearly with the density in our sample but
shows a plateau in the range (2.4-3) 10^{11}\rm cm^{-2} $. In this regime, the
phase of the B-periodic oscillations is found to shift continuously by two
periods.Comment: 5 pages, 4 figure
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