509 research outputs found
Electron spin interferometry using a semiconductor ring structure
A ring structure fabricated from GaAs is used to achieve interference of the
net spin polarization of conduction band electrons. Optically polarized spins
are split into two packets by passing through two arms of the ring in the
diffusive transport regime. Optical pumping with circularly polarized light on
one arm establishes dynamic nuclear polarization which acts as a local
effective magnetic field on electron spins due to the hyperfine interaction.
This local field causes one spin packet to precess faster than the other,
thereby controlling the spin interference when the two packets are combined.Comment: 4 pages, 2 figure
Antiferromagnetic s-d exchange coupling in GaMnAs
Measurements of coherent electron spin dynamics in
Ga(1-x)Mn(x)As/Al(0.4)Ga(0.6)As quantum wells with 0.0006% < x < 0.03% show an
antiferromagnetic (negative) exchange bewteen s-like conduction band electrons
and electrons localized in the d-shell of the Mn2+ impurities. The magnitude of
the s-d exchange parameter, N0 alpha, varies as a function of well width
indicative of a large and negative contribution due to kinetic exchange. In the
limit of no quantum confinement, N0 alpha extrapolates to -0.09 +/- 0.03 eV
indicating that antiferromagnetic s-d exchange is a bulk property of GaMnAs.
Measurements of the polarization-resolved photoluminescence show strong
discrepancy from a simple model of the exchange enhanced Zeeman splitting,
indicative of additional complexity in the exchange split valence band.Comment: 5 pages, 4 figures and one action figur
Fast Single-Charge Sensing with an rf Quantum Point Contact
We report high-bandwidth charge sensing measurements using a GaAs quantum
point contact embedded in a radio frequency impedance matching circuit
(rf-QPC). With the rf-QPC biased near pinch-off where it is most sensitive to
charge, we demonstrate a conductance sensitivity of 5x10^(-6) e^(2)/h Hz^(-1/2)
with a bandwidth of 8 MHz. Single-shot readout of a proximal few-electron
double quantum dot is investigated in a mode where the rf-QPC back-action is
rapidly switched.Comment: related papers available at http://marcuslab.harvard.ed
Local oxidation of Ga[Al]As heterostructures with modulated tip-sample voltages
Nanolithography based on local oxidation with a scanning force microscope has
been performed on an undoped GaAs wafer and a Ga[Al]As heterostructure with an
undoped GaAs cap layer and a shallow two-dimensional electron gas. The oxide
growth and the resulting electronic properties of the patterned structures are
compared for constant and modulated voltage applied to the conductive tip of
the scanning force microscope. All the lithography has been performed in
non-contact mode. Modulating the applied voltage enhances the aspect ratio of
the oxide lines, which significantly strengthens the insulating properties of
the lines on GaAs. In addition, the oxidation process is found to be more
reliable and reproducible. Using this technique, a quantum point contact and a
quantum wire have been defined and the electronic stability, the confinement
potential and the electrical tunability are demonstrated to be similar to the
oxidation with constant voltage.Comment: 7 pages, 7 figures, accepted by J. Appl. Phy
Kinetics of Exciton Emission Patterns and Carrier Transport
We report on the measurements of the kinetics of expanding and collapsing
rings in the exciton emission pattern. The rings are found to preserve their
integrity during expansion and collapse, indicating that the observed kinetics
is controlled by charge carrier transport rather than by a much faster process
of exciton production and decay. The relation between ring kinetics and carrier
transport, revealed by our experiment and confirmed by comparison with a
theoretical model, is used to determine electron and hole transport
characteristics in a contactless fashion.Comment: 6 pages, 4 figure
Multiple layer local oxidation for fabricating semiconductor nanostructures
Coupled semiconductor nanostructures with a high degree of tunability are
fabricated using local oxidation with a scanning force microscope. Direct
oxidation of the GaAs surface of a Ga[Al]As heterostructure containing a
shallow two-dimensional electron gas is combined with the local oxidation of a
thin titanium film evaporated on top. A four-terminal quantum dot and a double
quantum dot system with integrated charge readout are realized. The structures
are tunable via in-plane gates formed by isolated regions in the electron gas
and by mutually isolated regions of the Ti film acting as top gates. Coulomb
blockade experiments demonstrate the high quality of this fabrication process.Comment: 3 pages, 3 figure
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