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Measurements and a theoretical interpretation of the excitation spectrum of a two-electron quantum dot fabricated on a parabolic Ga[Al]As quantum well are reported. Experimentally, excited states are found beyond the well-known lowest singlet- and triplet states. These states can be reproduced in an exact diagonalization calculation of a parabolic dot with moderate in-plane anisotropy. The calculated spectra are in reasonable quantitative agreement with the measurement, and suggest that correlations between the electrons play a significant role in this system. Comparison of the exact results with the restricted Hartree-Fock and the generalized Heitler-London approach shows that the latter is more appropriate for this system because it can account for the spatial correlation of the electron states., Comment: ICPS28 (Vienna 2006) conference paper announcing early experimental results in semiconductor quantum dots that were successfully interpreted as a signature (strong suppression of the singlet-triplet gap) of Wigner-molecule formation, in accordance with previous theoretical work and predictions; see Ref. [7] in this paper

We have measured the current fluctuations in a quantum dot using a quantum point contact as a charge detector. By determining the distribution function for the charge transferred through the quantum dot, we get access not only to the shot noise but also to higher order moments. The noise in the quantum dot was found to be reduced compared to the single-barrier case. The reduction can be explained by an increase in temporal correlation due to the Coulomb blockade. Furthermore, we have used the time-resolved measurement techniques to investigate degenerate states in the quantum dot.

We present measurements and theoretical interpretation of the magnetic field dependent excitation spectra of a two-electron quantum dot. The quantum dot is based on an Al x Ga 1-x As parabolic quantum well with effective g⋆-factor close to zero. Results of tunneling spectroscopy of the four lowest states are compared to exact diagonalization calculations and a generalized Heitler–London approximation and good agreement is found. Electronic correlations, associated with the formation of an H 2-type Wigner molecule, turn out to be very important in this system.

We present an investigation of the influence of etching in hydrochloric acid on the topography and electronic properties of nanostructures defined by local anodic oxidation on shallow two-dimensional electron gases. The acid concentration and etching time necessary to remove the oxide lines were determined, yielding a process to create a flat surface suitable for scanning probe experiments. Changes in the low-temperature electronic properties induced by surface oxide removal include a moderate decrease of the total electron density and an increase of the breakdown voltages of the lines defined by local anodic oxidation. The obtained data indicate that the electronic width of the nanostructures decreases after the etching process.

While equilibrium properties of mesoscopic systems are well understood, many questions are still debated concerning the non-equilibrium properties, which govern nonlinear transport. Nonlinear transport measurements have been performed on two-terminal semiconductor quantum rings in the open regime, where the rings are used as electron interferometers and show the Aharonov–Bohm effect. We observe a magnetic field asymmetry of the nonlinear conductance, compatible with the non-validity of the Onsager–Casimir relations out-of-equilibrium. In particular, the voltage-antisymmetric part of the nonlinear conductance of these two-terminal devices is not phase rigid, as it is the case for the linear conductance. We show that this asymmetry is directly related to the electronic phase accumulated by the electrons along the arms of the ring and can be tuned using an electrostatic gate.

The Kondo effect has been widely studied in semiconductor quantum dots, in particular due to the ability to study the system out-of-equilibrium. However, the information obtained from a quantum dot connected to two leads is not complete, and a better understanding of this regime can be achieved by connecting the system to a third lead. We have performed transport measurements on a three-terminal quantum ring in the Kondo regime. With two leads strongly coupled to the ring, we have used the third weakly coupled lead as a tunnel probe in order to measure the Kondo density of states, and to study the decoherence of the Kondo state at large bias voltage. We also show that the three leads can be strongly coupled and all show a Kondo correlation with the ring, which is the first step towards observing a two-channel Kondo effect. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

By suitable design it is possible to achieve quasi-ballistic transport in semiconductor nanostructures over times up to the ps-range. Monte-Carlo simulations reveal that under these conditions phase-coherent real-space oscillations of an electron ensemble, generated by fs-pulses become possible in wide potential wells. Using a two-color pump-and-probe technique we have been able to observe this new phenomenon in excellent agreement with the theoretical predictions. Apart from its fundamental significance, ballistic transport in nanostructures can also be used for high-efficiency coherent THz-sources. The concept of these THz-emitters and its experimental confirmation will also be presented.

We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane (p x and p y ) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p z components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture (NA = 0.9). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p z components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p z components on and off easily. Experimental evidence of these features has been achieved by exploiting the selection rules for e-hh and e-lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption.

Few-electron quantum dots with integrated charge read-out have been fabricated by layered local anodic oxidation of a Ga[Al]As heterostructure and a thin Titanium top gate. The additional set of gates provided by the metallic film is used to tune the quantum dots into the few-electron regime. Current through the quantum dots and the quantum dot charge have been simultaneously measured for electron numbers varying between zero and two. The singlet–triplet splitting varies in two different samples between 0.5 and 1.5 meV. The Zeeman splitting of the first conductance resonance is observed in parallel magnetic field. The high tunability and straightforward implementation of these structures are promising for future nanostructure design.

We report the measurement of exceptionally bright, incoherent radiation in the THz region by frequency down-conversion of amplified spontaneous emission around 775-nm wavelength. The down-conversion technique is optical mixing in an interdigital photoconductive capacitor made from ultra fast ErAs:GaAs. The brightness temperature into a single spatial mode is approximately 1.1×105 K, making the new radiation at least 70 times more intense than common incandescent sources in the THz region.

A summary of ultrafast In0.53Ga0.47As photoconductors at λ = 1.55 µm is provided, with emphasis placed on recent ion-implanted and ErAs-nanoparticulate materials that have displayed response times

Conventional semiconductor-based photomixers are limited by the carrier transit-time-roll-off and the RC-roll-off of the device-capacitance. We have developed a THz-photomixer which can overcome both limitations and proves far superior to a conventional pin-photomixer.

An all-solid-state continuous-wave sweep oscillator has been developed that provides high-resolution, transmission measurements between ∼30 GHz and 3 THz. It is based on difference-frequency generation between two cw frequency-offset lasers driving an ultrafast photoconductive mixer (photomixer). The output power around 100 GHz is approximately 10 μW, falling to about 1 μW around 1 THz and 0.1 μW around 3 THz. The sweep oscillator is used with two types of detectors: a hot electron bolometer for high-sensitivity measurements of weak absorption features below 1 THz, and a room temperature Golay cell for coarse measurements anywhere between 30 GHz and 3 THz. The sweep oscillator facilitates the rapid characterization of a broad variety of materials including inorganic solids, biological materials, liquids, and gases with far greater resolution (∼10 MHz), frequency accuracy (∼0.1 GHz), and spectral density (∼1 μW/MHz) than competitive wideband instruments such as Fourier-transform or time-domain spectrometer...

Antidot lattices with a period of 150 nm were defined by AFM lithography on a wafer with a back-gate electrode and later covered with a TiAu top-gate. This layout enabled us to study the electronic properties of the lattice at different electron densities and as a function of top- and back-gate voltage at constant Fermi energy. Commensurability peaks and Aharonov–Bohm-type oscillations were observed in the magnetoresistance and characterized under continuously varying conditions. The small lattice constant brings the classical commensurability oscillations to magnetic fields B>1.5 T , where quantum effects become important.

Self-assembled ErAs islands were grown on GaAs between a two-dimensional electron gas (2DEG) and a backgate electrode by molecular-beam epitaxy. The islands have overlapping Schottky barriers that form an insulating potential barrier. A TiAu topgate was added by shadow mask evaporation. Thermal activation and charging experiments were employed to gain insight into the electronic properties of the ErAs island systems. In addition the 2DEG was characterized as a function of topgate and backgate voltage.

We report the growth by molecular beam epitaxy of composite epitaxial materials consisting of layers of semimetallic ErAs particles embedded in an In 0.53 Ga 0.47 As matrix. These structures are grown on (1 0 0)-oriented semiinsulating InP:Fe substrates at 490°C. Cross-sectional and plan view transmission electron microscopy images show that the ErAs forms 4 monolayer high islands and range in size from 30 nm×4 nm to 2×nm×2 nm. Epitaxial overgrowth of these islands is possible by first depositing 2.5 nm of In 0.52 Al 0.48 As before overgrowing with 37.5 nm of In 0.53 Ga 0.47 As. Samples grown without the In 0.52 Al 0.48 As smoothing layer have a much higher surface roughness. An increase in the In 0.53 Ga 0.47 As growth rate (and a consequential increase in the III/V flux ratio) is found to enhance the surface morphology of samples grown without In 0.52 Al 0.48 As smoothing layers. The temperature dependence of the surface morphology is also investigated.

We present a scheme that enables gigahertz-bandwidth three-dimensional control of electron spins in a semiconductor heterostructure with the use of a single voltage signal. Microwave modulation of the Landé g tensor produces frequency-modulated electron spin precession. Driving at the Larmor frequency results in g-tensor modulation resonance, which is functionally equivalent to electron spin resonance but without the use of time-dependent magnetic fields. These results provide proof of the concept that quantum spin information can be locally manipulated with the use of high-speed electrical circuits.

Electrical tuning of coherent electron spin dynamics is demonstrated in a specially designed AlxGa1−xAs quantum well in which the Al concentration x is parabolically varied across the structure. Application of an electric bias leads to a continuous displacement of an unperturbed wave function into regions with different Al concentration. Using time-resolved optical techniques, we directly observe gate-voltage mediated tunability of electron spin precession over a 13GHz frequency range at low temperatures and at a fixed magnetic field of 6T. Such control of spin coherence persists up to room temperature. Furthermore, complete suppression of spin precession and reversal of the sign of g is demonstrated.

We report the growth by molecular-beam epitaxy of a composite epitaxial material consisting of layers of semimetallic ErAs particles embedded in an In 0.53 Ga 0.47 As matrix. The ErAs particles were found to be 14±3 A high (∼4 monolayers) and varied in area depending on amount of ErAs deposited. The material was found to be strongly n-type for depositions less than one-half of a monolayer. As the deposition of ErAs increases, the lateral size and density of the particles also increases, resulting in a more rapid freeze out of the electrons and reduction of the free electron concentration at 300 K . We find the free electron concentration can be further reduced by compensation with beryllium, leading to more resistive material.

The processing of quantum information based on the electron spin degree of freedom1,2 requires fast and coherent manipulation of local spins. One approach is to provide spatially selective tuning of the spin splitting—which depends on the g-factor—by using magnetic fields3, but this requires their precise control at reduced length scales. Alternative proposals employ electrical gating1 and spin engineering in semiconductor heterostructures involving materials with different g-factors. Here we show that spin coherence can be controlled in a specially designed AlxGa1-xAs quantum well in which the Al concentration x is gradually varied across the structure. Application of an electric field leads to a displacement of the electron wavefunction within the quantum well, and because the electron g-factor varies strongly with x, the spin splitting is therefore also changed. Using time-resolved optical techniques, we demonstrate gate-voltage-mediated control of coherent spin precession over a 13-GHz frequency range in a fixed magnetic field of 6 T, including complete suppression of precession, reversal of the sign of g, and operation up to room temperature.

This letter reports the fabrication and demonstration of an ErAs:GaAs interdigitated photomixer as a tunable THz source ranging from ∼20GHzto∼2THz, with 12μW maximum power typically around ∼90GHz. Each photomixer is coupled to a composite dipole-spiral planar antenna that emits a Gaussian-type beam into free space. The beam switches from dipole to spiral antenna behavior as the frequency increases. A distributed Bragg reflector is embedded in the device beneath the photomixer to increase its external quantum efficiency. The photomixer has a 900A thick silicon nitride coating which serves as an antireflection and passivation layer, and also improves the reliability and heat tolerance of the device.

We demonstrate subpicosecond photocarrier lifetimes at 1.55μm in GaSb∕ErSb nanoparticle superlattices grown by molecular beam epitaxy. Pump–probe measurements were made with a 1.55μm mode-locked laser in transmission geometry to determine the photocarrier lifetime. The lifetime is found to be dependent on the size of the ErSb particles, amount of ErSb, and the distance between layers of particles. Through manipulation of these three parameters the photocarrier lifetime can be tuned down to less than 300fs, the temporal limit of our experiment.

We demonstrate the growth by molecular beam epitaxy of a metal/semiconductor composite consisting of epitaxial semimetallic ErSb particles in a GaSb matrix. The ErSb nucleates in an island growth mode leading to the spontaneous formation of nanometer-sized particles. These particles are found to preferentially grow along a [011] direction on a (100) GaSb surface. The particles can be overgrown with GaSb to form an epitaxial superlattice consisting of ErSb particles between GaSb spacer layers. The size of the ErSb particles increases monotonically with the deposition. The carrier concentrations in the superlattices are found to be dependent on both the size and density of the ErSb particles. Smaller particles and closer layer spacings reduce the hole concentration in the film.

This letter reports the measurement of photocarrier lifetime in ErAs:InGaAs epitaxial material grown on InP substrates. The measurement technique is ultrafast time-resolved phototransmission using a 1.55 μm mode-locked erbium-doped fiber laser. A lifetime of 3.56 ps is found in a sample containing an InAlAs smoothing layer, compared to 0.96 ps in a sample without any InAlAs. The difference is explained using a model of ambipolar diffusion of photocarriers.

We report electrical conductivity studies of highly-doped GaAs pn diodes containing a strongly n-doped low-temperature-grown (LT)–GaAs layer and pn junctions containing an approximately one monolayer thick ErAs layer. At room temperature, current densities of 1 kA/cm2 for the n-LT–GaAs samples and 6 kA/cm2 for the ErAs samples at 1 V forward bias have been measured. The I–V characteristics under forward bias for the n-LT–GaAs and ErAs samples exhibit significantly different behavior. At low temperatures, the n-LT–GaAs samples reveal a shoulder in the I–V characteristics, which can be explained by a model taking into account tunneling of carriers into LT midgap states. A similar model was able to explain the current transport in the ErAs diodes as tunneling of carriers into metallic regions inside the pn junction.

A sampling setup for a cryogenic environment has been developed using fiber-illuminated photoconductive switches fabricated from a material composed of equidistant layers of self-assembled ErAs nanoislands in a GaAs matrix. The setup includes dispersion compensation of the fibers and exhibits a time resolution better than 2.2 ps, which is insensitive to temperature and applied magnetic field and limited only by the properties of the coplanar waveguide circuitry. In cryogenic photocurrent autocorrelation measurements on single switches, a resolution exceeding 850 fs could be achieved.

We report here the fabrication and demonstration of the photomixers made from In0.53Ga0.47As epitaxial material lattice-matched to InP. The material consists of layers of ErAs nanoparticles separated by InGaAs and compensated with Be to reduce the photocarrier lifetime to picosecond levels and to increase the resistivity to ∼100 Ω cm. Interdigitated-electrode and planar-antenna structures were fabricated by e-beam lithography and tested for dc electrical characteristics, 1.55-μm optical responsivity, and difference-frequency photomixing. The measured responsivity of 8 mA/W and photomixer output of >0.1 μW beyond 100 GHz are already comparable to GaAs photomixers and suggest that coherent THz generation is now feasible using the abundant 1.55-μm-semiconductor-laser and optical-fiber technologies.

Self-assembled ErAs islands on GaAs embedded between a backgate electrode and a two-dimensional electron gas (2DEG) were grown by molecular-beam epitaxy. The nanometer-sized islands form Schottky barriers with overlapping depletion regions, which insulate the backgate from the 2DEG. From temperature-dependent measurements and charging experiments the effective barrier height between the islands and the Schottky barrier height onto the islands could be determined. In addition, the effects of illumination were studied.

We describe a radio frequency mechanical resonator that includes a quantum point contact, defined using electrostatic top gates. We can mechanically actuate the resonator using either electrostatic or magnetomotive forces. We demonstrate the use of the quantum point contact as a displacement sensor, operating as a radio frequency mixer at the mechanical resonance frequency of 1.5 MHz. We calculate a displacement sensitivity of about 3×10−12 m/Hz1/2. This device will potentially permit quantum-limited displacement sensing of nanometer-scale resonators, allowing the quantum entanglement of the electronic and mechanical degrees of freedom of a nanoscale system.

We have grown epitaxial superlattice structures of layers of semimetallic ErAs particles embedded in an InGaAs matrix on (001) Fe-doped InP substrates. Temperature-dependent Hall measurements, x-ray diffraction, and transmission electron microscopy were performed on the materials. The carrier mobility and the temperature dependence of the charge density imply conduction in the InGaAs matrix. We calculate an offset between the conduction-band minimum of the InGaAs matrix and the Fermi level of the ErAs particles that is strongly dependent on the amount of ErAs deposited. As the size of the ErAs particles increases, the Fermi level decreases from ∼0.01 eV above the InGaAs conduction-band edge to ∼0.2 eV below the InGaAs conduction-band edge and the electrical conduction properties change from metallic to semiconducting.

We experimentally study the thickness dependence of the terahertz (THz) response in {110}-oriented GaAs crystals for free space electro-optic sampling at 1.55 microm. The THz response bandwidths are analyzed and simulated under phase-matching condition with a model frequency response function. The results indicate that the detection bandwidth increases from 2 THz to 3 THz when the thickness of GaAs is reduced from 2 mm to 1 mm. Below 1 mm, the detected bandwidth is increasingly limited by the emitter characteristics and the finite probe pulse duration. The broadest bandwidth in experiment reaches 3.3 THz when using a 0.2 mm thick crystal, while it exceeds 5 THz in theory. The THz response sensitivity was studied experimentally and modeled taking into account the absorption of the THz radiation in the GaAs crystal. While absorption was found to be negligible for the crystal thickness range studied here, strong saturation is predicted theoretically for crystal thicknesses exceeding 5 mm.

ErAs:In 0.53 Ga 0.47 As superlattice THz emitters are fabricated and characterized at an excitation wavelength of 1.55 µm. Photocurrent-voltage characteristics, carrier lifetimes and bandwidth of the THz output are discussed as a function of the superlattice period.

The ion yield from dc discharges in the fluorobromomethanes CF4−xBrx (x=0 to x=3) and CF3Cl has been observed. The ion yield variation with pressure and power is presented. Results for the bromine substituted methanes, and CF3Cl have been compared to examine the effects of molecular symmetry (chemical substitution), and of the electronegativity of the halogen substituent. Input power was varied in order to observe approximate discharge energy dependance, and cell pressure was varied to explore the importance of ion molecule collisions.

We use time-resolved charge detection techniques to investigate single-electron tunneling in semiconductor quantum dots. The ability to detect individual charges in real-time makes it possible to count electrons one-by-one as they pass through the structure. The setup can thus be used as a high-precision current meter for measuring ultra-low currents, with resolution several orders of magnitude better than that of conventional current meters. In addition to measuring the average current, the counting procedure also makes it possible to investigate correlations between charge carriers. Electron correlations are conventionally probed in noise measurements, which are technically challenging due to the difficulty to exclude the influence of external noise sources in the experimental setup. Using real-time charge detection techniques, we circumvent the problem by studying the electron correlation directly from the counting statistics of the tunneling electrons. In quantum dots, we find that the strong Coulomb interaction makes electrons try to avoid each other. This leads to electron anti-bunching, giving stronger correlations and reduced noise compared to a current carried by statistically independent electrons. The charge detector is implemented by monitoring changes in conductance in a nearby capacitively coupled quantum point contact. We find that the quantum point contact not only serves as a detector but also causes a back-action onto the measured device. Electron scattering in the quantum point contact leads to emission of microwave radiation. The radiation is found to induce an electronic transition between two quantum dots, similar to the absorption of light in real atoms and molecules. Using a charge detector to probe the electron transitions, we can relate a single-electron tunneling event to the absorption of a single photon. Moreover, since the energy levels of the double quantum dot can be tuned by external gate voltages, we use the device as a frequency-selective single-photon detector operating at microwave energies. The ability to put an on-chip microwave detector close to a quantum conductor opens up the possibility to investigate radiation emitted from mesoscopic structures and gives a deeper understanding of the role of electron–photon interactions in quantum conductors. A central concept of quantum mechanics is the wave–particle duality; matter exhibits both wave- and particle-like properties and cannot be described by either formalism alone. To investigate the wave properties of the electrons, we perform experiments on a structure containing a double quantum dot embedded in the Aharonov–Bohm ring interferometer. Aharonov–Bohm rings are traditionally used to study interference of electron waves traversing different arms of the ring, in a similar way to the double-slit setup used for investigating interference of light waves. In our case, we use the time-resolved charge detection techniques to detect electrons one-by-one as they pass through the interferometer. We find that the individual particles indeed self-interfere and give rise to a strong interference pattern as a function of external magnetic field. The high level of control in the system together with the ability to detect single electrons enables us to make direct observations of non-intuitive fundamental quantum phenomena like single-particle interference or time–energy uncertainty relations.

We performed a transport measurement in a ballistic Aharonov-Bohm ring and a Fabry-Perot type interferometer. In both cases we found that the interference signal is reversed at a certain bias voltage and that the visibility decays exponentially as a function of temperature, being in a strong analogy with recent reports on the electronic Mach-Zehnder interferometers. By analyzing the data including those in the previous works, the energy scales that characterize the dephasing are found to be dominantly dependent on the interferometer size, implying the presence of a universal behavior in ballistic interferometers in both linear and non-linear transport regimes., Comment: 4 pages, 4 figures, accepted for publication in PRB rapid comm

We study the tunability of the spin-orbit interaction in a two-dimensional electron gas with a front and a back gate electrode by monitoring the spin precession frequency of drifting electrons using time-resolved Kerr rotation. The Rashba spin splitting can be tuned by the gate biases, while we find a small Dresselhaus splitting that depends only weakly on the gating. We determine the absolute values and signs of the two components and show that for zero Rashba spin splitting the anisotropy of the spin-dephasing rate vanishes., Comment: 5 pages, 3 figures

We use time-resolved charge detection methods to detect single photons absorbed by a double quantum dot (DQD). With the ability to tune the energy levels in the DQD, the frequency of the detected photon can be continuously varied between 10 and 80 GHz. The working principle of this device is demonstrated by measuring the radiation emitted by a nearby quantum point contact driven out-of-equilibrium at large bias voltage.

We investigate interference of electrons in an Aharonov–Bohm geometry with two embedded quantum dots. The interfering electrons are detected using time-resolved charge detection methods. The Aharonov–Bohm oscillations have a visibility higher than 90%, demonstrating a high degree of phase coherence in the system. We use the technique to probe phase shifts for excited states in one of the embedded quantum dot.

We demonstrate real-time detection of self-interfering electrons in a double quantum dot embedded in an Aharonov-Bohm interferometer, with visibility approaching unity. We use a quantum point contact as a charge detector to perform time-resolved measurements of single-electron tunneling. With increased bias voltage, the quantum point contact exerts a back-action on the interferometer leading to decoherence. We attribute this to emission of radiation from the quantum point contact, which drives non-coherent electronic transitions in the quantum dots., Comment: Supporting Information Available: http://www.nanophys.ethz.ch/gallery/movies/SingleElectronAB/index.htm contains a movie showing how the interference pattern builds up in real-time as electrons pass through the structure one-by-one

We investigate experimentally the influence of current flow through two independent quantum point contacts to a nearby double quantum dot realized in a GaAs-AlGaAs heterostructure. The observed current through the double quantum dot can be explained in terms of coupling to a bosonic bath. The temperature of the bath depends on the power generated by the current flow through the quantum point contact. We identify the dominant absorption and emission mechanisms in a double quantum dot as an interaction with acoustic phonons. The experiment excludes coupling of a double quantum dot to shot noise generated by quantum point contact as the dominant mechanism., Comment: 9 pages, 7 figures

We use time-resolved charge-detection techniques to probe virtual tunneling processes in a double quantum dot. The process involves an energetically forbidden state separated by an energy $\ensuremath{\delta}$ from the Fermi energy in the leads. The nonzero tunneling probability can be interpreted as cotunneling, which occurs as a direct consequence of time-energy uncertainty. For small energy separation the electrons in the quantum dots delocalize and form molecular states. In this regime we establish the experimental equivalence between cotunneling and sequential tunneling into molecular states for electron transport in a double quantum dot. Finally, we investigate inelastic cotunneling processes involving excited states of the quantum dots. Using the time-resolved charge-detection techniques, we are able to extract the shot noise of the current in the cotunneling regime.

Tracing ultrafast modifications of the Franz-Keldysh absorption spectrum of biased AlxGa1−xAs heterostructure diodes, we directly analyze the dynamical build up of a nonequilibrium carrier avalanche due to impact ionization for electric fields F > 350 kV/cm. The timescale of the carrier multiplication is found to be in the order of 10 ps depending on the applied bias. Monte Carlo simulations in a simplified band structure agree well with the experiment.

By using a quantum point contact as a charge detector, we show the measurement of current fluctuations in a semiconductor quantum dot by counting electrons tunneling through the system one by one. This method gives direct access to the full counting statistics of current fluctuations. In the sequential tunneling regime, we show the suppression of the noise compared to its classical Poissonian value, which is expected due to Coulomb blockade.

We use a double quantum dot as a frequency-tunable on-chip microwave detector to investigate the radiation from electron shot-noise in a near-by quantum point contact. The device is realized by monitoring the inelastic tunneling of electrons between the quantum dots due to photon absorption. The frequency of the absorbed radiation is set by the energy separation between the dots, which is easily tuned with gate voltages. Using time-resolved charge detection techniques, we can directly relate the detection of a tunneling electron to the absorption of a single photon.

In scanning gate microscopy, where the tip of a scanning force microscope is used as a movable gate to study electronic transport in nanostructures, the shape and magnitude of the tip-induced potential are important for the resolution and interpretation of the measurements. Contaminations picked up during topography scans may significantly alter this potential. We present an in situ high-field treatment of the tip that improves the tip-induced potential. A quantum dot was used to measure the tip-induced potential., Comment: 3 pages, 1 figure, minor changes to fit published version

We present a detailed experimental study on the electrostatic interaction between a quantum dot and the metallic tip of a scanning force microscope. Our method allowed us to quantitatively map the tip-induced potential and to determine the spatial dependence of the tip's lever arm with high resolution. We find that two parts of the tip-induced potential can be distinguished, one that depends on the voltage applied to the tip and one that is independent of this voltage. The first part is due to the metallic tip while we interpret the second part as the effect of a charged dielectric particle on the tip. In the measurements of the lever arm we find fine structure that depends on which quantum state we study. The results are discussed in view of scanning gate experiments where the tip is used as a movable gate to study nanostructures., Comment: 7 pages, 5 figures, minor changes to fit published version

We present time-resolved measurements of electron transport through a quantum dot. The measurements were performed using a nearby quantum point contact as a charge detector. The rates for tunneling through the two barriers connecting the dot to source and drain contacts could be determined individually. In the high bias regime, the method was used to probe excited states of the dot. Furthermore, we have detected bunching of electrons, leading to super-Poissonian noise. We have used the framework of full counting statistics to model the experimental data. The existence of super-Poissonian noise suggests a long relaxation time for the involved excited state, which could be related to the spin relaxation time.

We present measurements of counting statistics for electron transport in a quantum dot. By counting the electrons one by one using a quantum point contact as a charge detector, we are able to generate the distribution function of the current fluctuations. From the distribution function, higher order noise correlations can be determined. We measure a reduction of both the second moment (variance, related to shot noise) and the third moment (asymmetry) of the current distribution compared to a Poisson distribution. The reduction of noise can be explained by the increase in electron correlation due to Coulomb blockade.

The low-temperature magnetoresistance of parabolic quantum wells displays pronounced minima between integer filling factors. Concomitantly the Hall effect exhibits overshoots and plateau-like features next to well-defined ordinary quantum Hall plateaus. These effects set in with the occupation of the second subband. We discuss our observations in the context of single-particle Landau fan charts of a two-subband system empirically extended by a density dependent subband separation and an enhanced spin-splitting g*., 5 pages, submitted

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. Phys