1,053 research outputs found
Spin Effects in a Quantum Ring
Recent experiments are reviewed that explore the spin states of a ring-shaped
many-electron quantum dot. Coulomb-blockade spectroscopy is used to access the
spin degree of freedom. The Zeeman effect observed for states with successive
electron number allows to select possible sequences of spin ground states of
the ring. Spin-paired orbital levels can be identified by probing their
response to magnetic fields normal to the plane of the ring and electric fields
caused by suitable gate voltages. This narrows down the choice of ground-state
spin sequences. A gate-controlled singlet--triplet transition is identified and
the size of the exchange interaction matrix element is determined.Comment: 13 pages, 3 figures, Proceedings of the QD2004 conference in Banf
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
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
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
Magnetic field dependent transmission phase of a double dot system in a quantum ring
The Aharonov-Bohm effect is measured in a four-terminal open ring geometry
based on a Ga[Al]As heterostructure. Two quantum dots are embedded in the
structure, one in each of the two interfering paths. 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. Although the measured phase shifts are in qualitative agreement with
theoretical predictions, the phase evolution exhibits unexpected dependence on
the magnetic field. For example, phase lapses are found only in certain ranges
of magnetic field.Comment: 5 pages, 4 figure
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