Kelvin Probe Spectroscopy of a Two-Dimensional Electron Gas Below 300 mK

A scanning force microscope with a base temperature below 300 mK is used for measuring the local electron density of a two-dimensional electron gas embedded in an Ga[Al]As heterostructure. At different separations between AFM tip and sample, a dc-voltage is applied between the tip and the electron gas while simultaneously recording the frequency shift of the oscillating tip. Using a plate capacitor model the local electron density can be extracted from the data. The result coincides within 10% with the data obtained from transport measurements.Comment: 3 pages, 3 figure

Spatially resolved manipulation of single electrons in quantum dots using a scanned probe

The scanning metallic tip of a scanning force microscope was coupled capacitively to electrons confined in a lithographically defined gate-tunable quantum dot at a temperature of 300 mK. Single electrons were made to hop on or off the dot by moving the tip or by changing the tip bias voltage owing to the Coulomb-blockade effect. Spatial images of conductance resonances map the interaction potential between the tip and individual electronic quantum dot states. Under certain conditions this interaction is found to contain a tip-voltage induced and a tip-voltage independent contribution.Comment: 4 pages, 4 figure

Single electron charging of impurity sites visualized by scanning gate experiments on a quantum point contact

A quantum point contact (QPC) patterned on a two-dimensional electron gas is investigated with a scanning gate setup operated at a temperature of 300 mK. The conductance of the point contact is recorded while the local potential is modified by scanning the tip. Single electron charging of impurities induced by the local potential is observed as a stepwise conductance change of the constriction. By selectively changing the state of some of these impurities, it is possible to observe changes in transmission resonances of the QPC. The location of such impurities is determined, and their density is estimated to be below 50 per \mu m^2, corresponding to less than 1 % of the doping concentration

Unraveling quantum Hall breakdown in bilayer graphene with scanning gate microscopy

We use low-temperature scanning gate microscopy (SGM) to investigate the breakdown of the quantum Hall regime in an exfoliated bilayer graphene flake. SGM images captured during breakdown exhibit intricate patterns of "hotspots" where the conductance is strongly affected by the presence of the tip. Our results are well described by a model based on quantum percolation which relates the points of high responsivity to tip-induced scattering between localized Landau levels.Comment: 6 pages, 4 figure

Spatially highly resolved study of AFM scanning tip–quantum dot local interaction

Scanning-gate imaging of semiconductor quantum dots (QDs) promises access to probability distributions of quantum states. It could therefore be a novel tool for designing and optimizing tailored quantum states in such systems. A detailed study of a lithographically defined semiconductor QD in the Coulomb-blockade regime is presented, making use of the scanning-gate technique at a base temperature of 300 mK. The method allows a one-by-one manipulation of electrons in the structure. The obtained images interpreted with a suitable QD model guide the way to a local investigation of the electronic interior of the QD. Future perspectives of scanning-gate experiments on QDs are discussed

Scanning gate microscopy measurements on a superconducting single-electron transistor

We present measurements on a superconducting single-electron transistor (SET) in which the metallic tip of a low-temperature scanning force microscope is used as a movable gate. We characterize the SET through charge stability diagram measurements and compare them to scanning gate measurements taken in the normal conducting and the superconducting states. The tip-induced potential is found to have a rather complex shape. It consists of a gate voltage-dependent part and a part which is independent of gate voltage. Further scanning gate measurements reveal a dependence of the charging energy and the superconducting gap on the tip position and the voltage applied to it. We observe an unexpected correlation between the magnitude of the superconducting gap and the charging energy. The change in EC can be understood to be due to screening, however the origin of the observed variation in ? remains to be understood. Simulations of the electrostatic problem are in reasonable agreement with the measured capacitances.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin