R. J. Haug

Edge-state transport and its experimental consequences in high magnetic fields

Many magnetotransport experiments in two-dimensional electron systems have been explained in recent years within the so-called edge-state model. Starting from a historical context, the model is described. Experiments in Hall-bar geometries with and without gate barriers are reviewed and discussed in terms of the edge-state picture.

Photon‐assisted tunneling through a quantum dot at high microwave frequencies

We investigate the influence of high frequency microwave radiation on single electron tunneling through a quantum dot. Effective coupling of the radiation to the quantum dot is achieved by an on‐chip integrated broadband antenna. Simulations of the current distribution on the antenna are shown. We find that radiation with a frequency of ν=155 GHz, which corresponds to half of the bare charging energy Ec/2 results in an additional conductance peak within the Coulomb blockade regime. This additional resonance is attributed to photon‐assisted tunneling.

Tilted magnetic field studies of spin- and valley-splittings in Si/Si1-xGex heterostructures

Abstract Magnetotransport measurements on the 2DEG of a Si/Si 1 − x Ge x heterostructure with an electron mobility of 32 m 2 /Vs are presented. The coincidence method in a tilted magnetic field is used to determine the effective g factor. In the filling factor range 16 ≤ v ≤ 28 an almost constant effective g factor of g * = 3.39 is found. For lower filling factors, g * increases. Activation energy experiments at different tilting angles show that the size of the valley splitting depends only on the normal component of the magnetic field. Around filling factor v = 3 in a tilted field, an overshoot occurs in the Hall resistance, which is attributed to level crossings as a consequence of the exchange enhancement effects of spin- and valley-splitting.

Influence of edge currents on shubnikov-deHaas oscillations in two-dimensional systems

Magnetotransport measurements on two-dimensional systems show that the experimentally determined resistivity ρxx may depend on the length and the width of the device. This result is explained on the basis of a combination of edge and bulk currents if a non-equilibrium exists between the one-dimensional edge currents. The experiments demonstrate that such a nonequilibrium between adjacent edge channels may be present not only if quantum point contacts are used for a selective population of edge currents but also in standard resistivity measurements with ohmic contacts.

Peak‐to‐valley ratio of small resonant‐tunneling diodes with various barrier‐thickness asymmetries

Resonant tunneling through small double‐barrier heterostructures is investigated in dependence of the charge accumulation in the quantum well and the device diameter. The study comprises a series of resonant‐tunneling diodes with four different barrier‐thickness ratios and diameters between 300 nm and 10 μm. Special emphasis lies on the peak‐to‐valley ratio of the resonant‐tunneling current peaks, which drops drastically with decreasing device diameter for weak electron accumulation, while it is size independent in the strong‐charging case.

Variable range hopping transport in the tails of the conductivity peaks between quantum Hall plateaus

We measure the electrical conductivity in the dissipative regime between adjacent integral quantum Hall plateaus. Variable-range-hopping transport is found, in accordance with a recent theory. The characteristic temperature for hopping conduction follows a power-law dependence on filling factor. Conductivity-peak broadening with increasing current is studied and a power-law dependence is observed.

Single-electron tunnelling transistor as a current rectifier with potential-controlled current polarity

The asymmetry of the current-voltage characteristic of a single-electron tunnelling transistor causes a current rectification when applying a low-frequency voltage signal. The polarity of the direct current depends on the voltage applied to a gate electrode. The characteristic change in current polarity with increasing gate voltage can be used to detect a difference in the low frequency excitation, and also the high frequency excitation of the two electron reservoirs coupled to the electronic island.

Realization of an in‐plane‐gate single‐electron transistor

By etching narrow trenches in an AlGaAs/GaAs heterostructure and defining a channel and six in‐plane gates in the two‐dimensional electron gas, a single‐electron transistor is produced in a simple way. As expected from charging effects, the conductivity of the transistor oscillates with the voltage on each gate. However, in contrast to metallic devices or heterostructure devices with metallic top gates, the voltage dependent depletion of the gates themselves leads to changes in the period of the oscillations.

Spectroscopy of Local Density of States Fluctuations in a Disordered Conductor

The local density of states of a degenerate semiconductor is investigated at low magnetic fields. In order to realize this experiment, we designed a strongly asymmetric double-barrier heterostructure with heavily doped contacts and study the resonant tunneling transport through the lowest localized (zero-dimensional) level in the quantum well. Fine structure in the tunneling current as a function of bias voltage and magnetic field images mesoscopic fluctuations of the local density of states in the emitter contact. Our quantitative analysis demonstrates that the fluctuation pattern originates from quantum interference of diffusive electron waves in the three-dimensional disordered system on the length scale of the mean free path. Since the fluctuations reflect the properties of the electron states below the Fermi level, the observed mesoscopic effect is temperature insensitive.

Transport characterization of in‐plane gate devices fabricated by direct epitaxial growth on patterned substrates

Direct molecular‐beam epitaxial growth of GaAs/AlxGa1−xAs heterostructures on bow‐tie shaped constrictions, prepatterned on GaAs substrates is used to fabricate in‐plane gate transistors. The fabricated devices can be tuned by applying voltages to in‐plane gates, which are also realized during the epitaxial growth. In this way, complete in‐plane gate transistors are fabricated in a single growth process. Transport measurements of the devices at 1.3 K show conductance quantization or Coulomb blockade depending on the width of the constriction. The Coulomb blockade effect in the narrowest structures is caused by the formation of a self‐assembled quantum dot in the center of the constriction.