Bastian Marquardt

Using a two-dimensional electron gas to study nonequilibrium tunneling dynamics and charge storage in self-assembled quantum dots

We demonstrate a strong influence of charged self-assembled quantum dots (QD) on the conductance of a nearby two-dimensional electron gas (2DEG). A conductance measurement of the 2DEG allows us to probe the charge tunneling dynamics between the 2DEG and the QDs in nonequilibrium as well as close to equilibrium. Measurements of hysteresis curves with different sweep times and time-resolved conductance measurements enable us to unambiguously identify the transients as tunneling events between the 2DEG and QD states.

Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots

Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature.

Quantum dot electrons as controllable scattering centers in the vicinity of a two-dimensional electron gas

Self-assembled InAs quantum dots can be controllably charged with a defined number of electrons per dot. We report on conductivity measurements of Al x Ga1−x As/GaAs heterostructures, where such quantum dots are embedded in the direct vicinity of a two-dimensional electron gas (2DEG). We demonstrate the controlled enhancement of the scattering rate in the 2DEG induced by charging the quantum dots with additional electrons. The resulting transport lifetimes are in good agreement with theoretical values for Coulomb scattering in two dimensions.

The influence of charged InAs quantum dots on the conductance of a two-dimensional electron gas: Mobility vs. carrier concentration

Using time-resolved transport spectroscopy, we investigate the influence of charge-tunable InAs quantum dots (QDs) on the conductance of a nearby two-dimensional electron gas (2DEG). Loading successively electrons into the self-assembled QDs decreases the carrier concentration and mobility in the 2DEG. We are able to quantify how these transport properties change for each additional charge in the s- or p-shell. It is found that mobility and carrier concentration contribute equally to the overall change in conductance.

A two-dimensional electron gas as a sensitive detector for time-resolved tunneling measurements on self-assembled quantum dots.

A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients.

Momentum matching in the tunneling between 2-dimensional and 0-dimensional electron systems

We investigate the tunneling rates from a 2-dimensional electron gas (2DEG) into the ground state of self-assembled InGaAs quantum dots. These rates are strongly affected by a magnetic field perpendicular to the tunneling direction. Surprisingly, we find an increase in the rates for fields up to 4 T before they decrease again. This can be explained by a mismatch between the characteristic momentum of the quantum dot ground state and the Fermi momentum kF of the 2DEG. Calculations of the tunneling probability can account for the experimental data and allow us to determine the dot geometry as well as kF.

All‐electrical transport spectroscopy of non‐equilibrium many‐particle states in self‐assembled quantum dots

We demonstrate an all‐electrical prepartion and detection of many‐particle spin states in self‐assembled QDs. The excited states of the first three ‘QD elements’ are measured, i. e. the degenerate s‐, p‐ and d‐shells of QD‐Hydrogen (one electron), the many‐particle triplet and singlet states of QD‐Helium (two electrons) and the many‐particle states of QD‐Lithium (three electrons). Comparison with exact diagonalization calculations fully accounting for the effects of Coulomb interaction enables us to reveal unambiguously the different charge and spin configurations.