424 research outputs found
Bills and Notes: The Impact of the Setoff and Assignment Statute Upon Negotiable Instruments Law
Vertical quantum wire realized with double cleaved-edge overgrowth
A quantum wire is fabricated on (001)-GaAs at the intersection of two
overgrown cleaves. The wire is contacted at each end to n+ GaAs layers via
two-dimensional (2D) leads. A sidegate controls the density of the wire
revealing conductance quantization. The step height is strongly reduced from
2e^2/h due to the 2D-lead series resistance. We characterize the 2D density and
mobility for both cleave facets with four-point measurements. The density on
the first facet is modulated by the substrate potential, depleting a 2um wide
strip that defines the wire length. Micro-photoluminescence shows an extra peak
consistent with 1D electron states at the corner.Comment: 4 pages, 4 figure
Two-dimensional electron-gas actuation and transduction for GaAs nanoelectromechanical systems
We have fabricated doubly clamped beams from GaAs/AlGaAs quantum-well heterostructures containing a high-mobility two-dimensional electron gas (2DEG). Applying an rf drive to in-plane side gates excites the beam's mechanical resonance through a dipole–dipole mechanism. Sensitive high-frequency displacement transduction is achieved by measuring the ac emf developed across the 2DEG in the presence of a constant dc sense current. The high mobility of the incorporated 2DEG provides low-noise, low-power, and high-gain electromechanical displacement sensing through combined piezoelectric and piezoresistive mechanisms
Urincholesterinbestimmung im Rahmen der Früherkennung und Verlaufskontrolle von Harnblasentumoren
Shock Waves in Nanomechanical Resonators
The dream of every surfer is an extremely steep wave propagating at the
highest speed possible. The best waves for this would be shock waves, but are
very hard to surf. In the nanoscopic world the same is true: the surfers in
this case are electrons riding through nanomechanical devices on acoustic waves
[1]. Naturally, this has a broad range of applications in sensor technology and
for communication electronics for which the combination of an electronic and a
mechanical degree of freedom is essential. But this is also of interest for
fundamental aspects of nano-electromechanical systems (NEMS), when it comes to
quantum limited displacement detection [2] and the control of phonon number
states [3]. Here, we study the formation of shock waves in a NEMS resonator
with an embedded two-dimensional electron gas using surface acoustic waves. The
mechanical displacement of the nano-resonator is read out via the induced
acoustoelectric current. Applying acoustical standing waves we are able to
determine the anomalous acoustocurrent. This current is only found in the
regime of shock wave formation. We ontain very good agreement with model
calculations.Comment: 14 Pages including 4 figure
Quantum Hall Ferromagnetism in a Two-Dimensional Electron System
Experiments on a nearly spin degenerate two-dimensional electron system
reveals unusual hysteretic and relaxational transport in the fractional quantum
Hall effect regime. The transition between the spin-polarized (with fill
fraction ) and spin-unpolarized () states is accompanied
by a complicated series of hysteresis loops reminiscent of a classical
ferromagnet. In correlation with the hysteresis, magnetoresistance can either
grow or decay logarithmically in time with remarkable persistence and does not
saturate. In contrast to the established models of relaxation, the relaxation
rate exhibits an anomalous divergence as temperature is reduced. These results
indicate the presence of novel two-dimensional ferromagnetism with a
complicated magnetic domain dynamic.Comment: 15 pages, 5 figure
Optically Probing Spin and Charge Interactions in an Tunable Artificial Molecule
We optically probe and electrically control a single artificial molecule
containing a well defined number of electrons. Charge and spin dependent
inter-dot quantum couplings are probed optically by adding a single
electron-hole pair and detecting the emission from negatively charged exciton
states. Coulomb and Pauli blockade effects are directly observed and
hybridization and electrostatic charging energies are independently measured.
The inter-dot quantum coupling is confirmed to be mediated predominantly by
electron tunneling. Our results are in excellent accord with calculations that
provide a complete picture of negative excitons and few electron states in
quantum dot molecules.Comment: shortened version: 6 pages, 3 figures, 1 table, to appear in Phys.
Rev. Let
Direct observation of acoustic phonon mediated relaxation between coupled exciton states in a single quantum dot molecule
We probe acoustic phonon mediated relaxation between tunnel coupled exciton
states in an individual quantum dot molecule in which the inter-dot quantum
coupling and energy separation between exciton states is continuously tuned
using static electric field. Time resolved and temperature dependent optical
spectroscopy are used to probe inter-level relaxation around the point of
maximum coupling. The radiative lifetimes of the coupled excitonic states can
be tuned from ~2 ns to ~10 ns as the spatially direct and indirect character of
the wavefunction is varied by detuning from resonance. Acoustic phonon mediated
inter-level relaxation is shown to proceed over timescales comparable to the
direct exciton radiative lifetime, indicative of a relaxation bottleneck for
level spacings in the range $\Delta E\$ ~3-6 meV.Comment: 6 pages, 4 figures, submitted for publicatio
Observation of extremely slow hole spin relaxation in self-assembled quantum dots
We report the measurement of extremely slow hole spin relaxation dynamics in
small ensembles of self-assembled InGaAs quantum dots. Individual spin
orientated holes are optically created in the lowest orbital state of each dot
and read out after a defined storage time using spin memory devices. The
resulting luminescence signal exhibits a pronounced polarization memory effect
that vanishes for long storage times. The hole spin relaxation dynamics are
measured as a function of external magnetic field and lattice temperature. We
show that hole spin relaxation can occur over remarkably long timescales in
strongly confined quantum dots (up to ~270 us), as predicted by recent theory.
Our findings are supported by calculations that reproduce both the observed
magnetic field and temperature dependencies. The results suggest that hole spin
relaxation in strongly confined quantum dots is due to spin orbit mediated
phonon scattering between Zeeman levels, in marked contrast to higher
dimensional nanostructures where it is limited by valence band mixing.Comment: Published by Physical Review
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