117 research outputs found
Imaging correlated wave functions of few-electron quantum dots: Theory and scanning tunneling spectroscopy experiments
We show both theoretically and experimentally that scanning tunneling
spectroscopy (STS) images of semiconductor quantum dots may display clear
signatures of electron-electron correlation. We apply many-body tunneling
theory to a realistic model which fully takes into account correlation effects
and dot anisotropy. Comparing measured STS images of freestanding InAs quantum
dots with those calculated by the full configuration interaction method, we
explain the wave function sequence in terms of images of one- and two-electron
states. The STS map corresponding to double charging is significantly distorted
by electron correlation with respect to the non-interacting case.Comment: RevTeX 4.0, 5 pages, 3 B/W figures, 1 table. This paper is based on
an invited talk presented by the authors at the 28th International Conference
on the Physics of Semiconductors, which was held 24-28 July 2006, in Vienna,
Austri
Local scale-invariance in ageing phenomena
Many materials quenched into their ordered phase undergo ageing and there
show dynamical scaling. For any given dynamical exponent z, this can be
extended to a new form of local scale-invariance which acts as a dynamical
symmetry. The scaling functions of the two-time correlation and response
functions of ferromagnets with a non-conserved order parameter are determined.
These results are in agreement with analytical and numerical studies of various
models, especially the kinetic Glauber-Ising model in 2 and 3 dimensions.Comment: Invited talk; spring meeting of the german physical society,
Regensburg the 8th of March 2004, 12 pages, style file
Charged Excitons in the Quantum Hall Regime
We review our recent optical experiments on two-dimensional electron systems at temperatures below 1 K and under high magnetic fields. The two-dimensional electron systems are realized in modulation-doped GaAs-AlGaAs single quantum wells. Via gate electrodes the carrier density of the two-dimensional electron systems can be tuned in a quite broad range between about 1×10^{10} cm^{-2} and 2×10^{11} cm^{-2}. In dilute two-dimensional electron systems, at very low electron densities, we observe the formation of negatively charged excitons in photoluminescence experiments. In this contribution we report about the observation of a dark triplet exciton, which is observable at temperatures below 1 K and for electron filling factors <1/3, i.e., in the fractional quantum Hall regime only. In experiments where we have increased the density of the two-dimensional electron systems so that a uniform two-dimensional electron system starts to form, we have found a strong energy anomaly of the charged excitons in the vicinity of filling factor 1/3. This anomaly was found to exist in a very narrow parameter range of the density and temperature, only. We propose a model where we assume that localized charged excitons and a uniform Laughlin liquid coexist. The localized charged exciton in close proximity to the Laughlin liquid leads to the creation of a fractionally-charged quasihole in the liquid, which can account for the experimentally observed anomaly
Electrical and terahertz magnetospectroscopy studies of laser-patterned micro- and nanostructures on InAs-based heterostructures
Nanostructures fabricated from narrow-gap semiconductors with strong
spin-orbit interaction (SOI), such as InAs, can be used to filter momentum
modes of electrons and offer the possibility to create and detect
spin-polarized currents entirely by electric fields. Here, we present
magnetotransport and THz magnetospectroscopy investigations of Hall-bars with
back-gates made from in InGaAs/InAlAs quantum well structures with a strained 4
nm InAs inserted channel. The two-dimensional electron gas is at 53 nm depth
and has a carrier density of about cm and mobility of
about cm/Vs, after illumination. Electrical and THz optical
transport measurements at low temperatures and in high magnetic fields reveal
an effective mass of 0.038 and an anisotropic -factor of up to 20,
larger than for bulk InAs or InAs-based heterostructures. We demonstrate that
quasi-one-dimensional channels can be formed by micro-laser lithography. The
population of subbands is controlled by in-plane gates. Contrary to previous
reports symmetric and asymmetric in-plane gate voltages applied to quasi-one
dimensional channels did not show indications of SOI-induced anomalies in the
conductance.Comment: v1 did not contain references due to filename mix-up; v3 is revision
following referee report; v4 is corrected version following acceptance; v5 is
the published versio
Thermally controlled widening of droplet etched nanoholes
We describe a method to control the shape of nanoholes in GaAs (001) which combines the technique of local droplet etching using Ga droplets with long-time thermal annealing. The cone-like shape of inverted nanoholes formed by droplet etching is transformed during long-time annealing into widened holes with flat bottoms and reduced depth. This is qualitatively understood using a simplified model of mass transport incorporating surface diffusion and evaporation. The hole diameter can be thermally controlled by varying the annealing time or annealing temperature which provides a method for tuning template morphology for subsequent nanostructure nucleation. We also demonstrate the integration of the combined droplet/thermal etching process with heteroepitaxy by the thermal control of hole depth in AlGaAs layers
Dynamics of mass transport during nanohole drilling by local droplet etching
Local droplet etching (LDE) utilizes metal droplets during molecular beam epitaxy for the self-assembled drilling of nanoholes into III/V semiconductor surfaces. An essential process during LDE is the removal of the deposited droplet material from its initial position during post-growth annealing. This paper studies the droplet material removal experimentally and discusses the results in terms of a simple model. The first set of experiments demonstrates that the droplet material is removed by detachment of atoms and spreading over the substrate surface. Further experiments establish that droplet etching requires a small arsenic background pressure to inhibit re-attachment of the detached atoms. Surfaces processed under completely minimized As pressure show no hole formation but instead a conservation of the initial droplets. Under consideration of these results, a simple kinetic scaling model of the etching process is proposed that quantitatively reproduces experimental data on the hole depth as a function of the process temperature and deposited amount of droplet material. Furthermore, the depth dependence of the hole side-facet angle is analyzed
Enhanced Transmission in Rolled-up Hyperlenses utilizing Fabry-Pe\'rot Resonances
We experimentally demonstrate that the transmission though rolled-up
metal/semiconductor hyperlenses can be enhanced at desired frequencies
utilizing Fabry-P\'erot resonances. By means of finite difference time domain
simulations we prove that hyperlensing occurs at frequencies of high
transmission.Comment: 3 pages, 3 figure
- …