P. Studerus

Counting statistics of single electron transport in a quantum dot.

We have measured the full counting statistics of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact. This method gives direct access to the distribution function of current fluctuations. Suppression of the second moment (related to the shot noise) and the third moment (related to the asymmetry of the distribution) in a tunable semiconductor QD is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels.

Semiconductor quantum point contact fabricated by lithography with an atomic force microscope

We report on the experimental realization of a quantum point contact in a semiconductor heterostructure by lithography with an atomic force microscope (AFM). A thin, homogeneous titanium film on top of the chip surface was patterned by local anodic oxidation, induced by a current applied to an n-doped AFM tip. We demonstrate that self-aligned gate structures in the sub-micron regime can be fabricated with this technique.

Detailed analysis of forces influencing lateral resolution for Q-control and tapping mode

The recently introduced active quality factor control for dynamic force microscopy (Q-control) in liquids promises higher sensitivity, and thus, smaller interaction forces compared to normal tapping (intermittent contact) mode. By a careful analysis of force distance curves, we show that the lateral resolution obtained on fragile biological membranes is actually comparable for both modes if scan parameters are optimized accordingly. In addition, Q-control is limited to small scan rates only.

Counting statistics and super-Poissonian noise in a quantum dot : Time-resolved measurements of electron transport

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 the full counting statistics (FCS) 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.

Local oxidation of Ga[Al]As heterostructures with modulated tip-sample voltages

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 the constant and modulated voltages applied to the conductive tip of the scanning force microscope. All the lithography has been performed in noncontact 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.

Quantum capacitance and density of states of graphene

We report capacitance measurements in top-gated graphene sheets as a function of charge carrier density. A measurement method using an LC-circuit provides high sensitivity to small capacitance changes and hence allows the observation of the quantum part of the capacitance. The extracted density of states has a finite value of 1◊10 17 m 2 eV 1 in the vicinity of the Dirac point, which is in contrast to the theoretical prediction for ideal graphene. We attribute this discrepancy to fluctuations of the electrostatic potential with a typical amplitude of 100meV in our device.

An in situ tunable radio-frequency quantum point contact

Incorporating a variable capacitance diode into a radio-frequency (rf) matching circuit allows us to in situ tune the resonance frequency of a rf quantum point contact, increasing the versatility of the latter as a fast charge sensor of a proximal quantum circuit. The performance of this method is compared in detail to conventional low-frequency charge detection. The approach is also applicable to other rf-detection schemes, such as rf single electron transistor circuits.

Counting statistics of single electron transport in a quantum dot

We have measured the full counting statistics of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact. This method gives direct access to the distribution function of current fluctuations. Suppression of the second moment (related to the shot noise) and the third moment (related to the asymmetry of the distribution) in a tunable semiconductor QD is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels.