Anneli Löfgren

Symmetry of Two-Terminal Nonlinear Electric Conduction

The well-established symmetry relations for linear transport phenomena cannot, in general, be applied in the nonlinear regime. Here we propose a set of symmetry relations with respect to bias voltage and magnetic field for the nonlinear conductance of two-terminal electric conductors. We experimentally confirm these relations using phase-coherent, semiconductor quantum dots.

Quantum behavior in nanoscale ballistic rectifiers and artificial materials

Low-temperature experiments are performed on nanoscale nonlinear devices (ballistic rectifiers) as well as nanostructured artificial materials, fabricated from an InP/InGaAs quantum well wafer. A dc output is generated between the lower and upper contacts of these devices, when an ac voltage is applied between the left and right contacts. As the temperature is lowered from room temperature, the dc output voltage of the ballistic rectifiers gradually changes from negative to positive. Since the negative output at high temperatures has been well understood in the framework of the classical ballistic electron transport, our results indicate that the electron transport comes into a different physical regime at low temperatures. Furthermore, we find that at even lower temperatures, the devices generate a pronounced oscillatory output as a function of the applied bias. Very similar phenomena are observed in the artificial nanomaterials, suggesting the existence of a common mechanism. We present a simple model based on quantum transport, which explains the key phenomena that we have observed at low temperatures. (Less)

Epitaxially overgrown, stable W-GaAs Schottky contacts with sizes down to 50 nm

A processing scheme for the fabrication of embedded W-GaAs contacts has been established and the resulting contact characteristics have been evaluated. The main advantage of these contacts is that they are stable during high-temperature epitaxial overgrowth. The fabrication scheme is based on a liftoff process with electron beam evaporation of tungsten and subsequent epitaxial overgrowth using metalorganic vapor phase epitaxy. Various methods were used to characterize the buried contacts. First, the structural properties of GaAs surrounding embedded W features, with widths down to 50 nm, were characterized by high-resolution transmission electron microscopy. Measurements of the conductivity in individual, buried wires were performed in order to study the influence of the overgrowth process on the properties of the tungsten. We also evaluated the current-voltage characteristics for macroscopic contacts, which revealed a clear dependence on processing parameters. Optimized processing conditions could thus be established under which limited contact degradation occurred during overgrowth. Finally, we used the overgrowth technique to perform a detailed investigation of the electrical and optical properties of floating-potential embedded nano-Schottky contacts by space-charge spectroscopy.

Voltage and temperature limits for the operation of a quantum dot ratchet

Abstract Open quantum dots without spatial inversion symmetry can partially rectify a small AC voltage because of the asymmetric effect of an electric field on the electron interference inside the dot. Quantum dots can therefore be viewed as ratchets, that is devices in which directed particle flow is induced by non-equilibrium fluctuations, in the absence of (time-averaged) external net forces or gradients. Here we discuss thermal energy averaging and phase breaking as the parameters that limit the voltage- and temperature range where quantum dot ratchets can operate.

Electron quantum ratchets

Ratchets are spatially asymmetric devices in which directed particle flow can occur due to the rectification of fluctuations, in the absence of external net forces or gradients. In this paper we point out that mesoscopic semiconductor structures are ideally suited systems for the study of quantum ratchet effects. As an example, we present experimental and theoretical data showing that phase coherent, asymmetric electron cavities can partially rectify an applied AC voltage.

Preserved symmetries in nonlinear electric conduction

For the general case of a mesoscopic, two‐terminal device with no geometrical symmetry, conductance symmetries break down in the non‐linear regime. Using basic symmetry arguments, we predict and experimentally confirm a set of symmetry relations that are preserved for electric conductors with respect to bias voltage, V, and B in the non‐linear regime.

Triangular ballistic quantum dots: classical, semiclassical and wave mechanical electron dynamics

The relation between classical electron orbits and quantum mechanical wavefunctions is discussed in two ways, using the example of triangular ballistic quantum dots. Firstly, the frequency of conductance fluctuations observed in the magnetoresistance is linked, in the spirit of the semiclassical periodic orbit theory, to a specific classical electron trajectory that is also important for ballistic commensurability effects. Secondly, we present initial results of a method to construct semiclassical particle density distributions inside the dot and compare the result with those of the classical and the quantum mechanical calculations of particle density distributions.