[show abstract][hide abstract] ABSTRACT: We report on measurements of the quantum capacitance in graphene as a function of charge carrier density. A resonant LC-circuit giving high sensitivity to small capacitance changes is employed. The density of states, which is directly proportional to the quantum capacitance, is found to be significantly larger than zero at and around the charge neutrality point. This finding is interpreted to be a result of potential fluctuations with amplitudes of the order of 100 meV in good agreement with scanning single-electron transistor measurements on bulk graphene and transport studies on nanoribbons.
[show abstract][hide abstract] ABSTRACT: By using a quantum point contact as a charge detector, we show the measurement of current fluctuations in a semiconductor
quantum dot by counting electrons tunneling through the system one by one. This method gives direct access to the full counting
statistics of current fluctuations. In the sequential tunneling regime, we show the suppression of the noise compared to its
classical Poissonian value, which is expected due to Coulomb blockade.
[show abstract][hide abstract] ABSTRACT: We present a scanning force microscope that operates in a dilution refrigerator at temperatures of about 100 mK. We use tuning fork sensors for scanning gate experiments on mesoscopic semiconductor nanostructures. Slip-stick motors allow sample coarse-positioning at base temperature. The construction, thermal anchoring, and a procedure to optimize the settings of the phase-locked loop that we use for sensor control are discussed in detail. We present low-temperature topographic and scanning gate images as examples of successful operation.
Review of Scientific Instruments 02/2007; 78(1):013704. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: We report on the design of loop-gap resonators (LGR) operating in the frequency range 34-36 GHz with the goal to achieve single electron spin resonance (ESR) in quantum dot nanostructures. We present a comprehensive study of the magnetic field strength and the spatial distribution of the electric and magnetic fields in the resonator by means of experiments and numerical simulations. Comment: 25 pages, submitted
Review of Scientific Instruments 02/2006; · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 constant and modulated voltage applied to the conductive tip of the scanning force microscope. All the lithography has been performed in non-contact 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. Comment: 7 pages, 7 figures, accepted by J. Appl. Phys
Journal of Applied Physics 01/2006; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The implementation of a tuning fork sensor in a scanning force microscope operational at 300 mK is described and the harmonic oscillator model of the sensor is motivated. These sensors exhibit very high quality factors at low temperatures. The nested feedback comprising the sensor, a phase locked loop and a conventional $z$-feedback is analyzed in terms of linear control theory and the dominant noise source of the system is identified. It is shown that the nested feedback has a low pass response and that the optimum feedback parameters for the phase-locked loop and the $z$-feedback can be determined from the knowledge of the tuning fork resonance alone regardless of the tip shape. The advantages of this system compared to pure phase control are discussed.
[show abstract][hide abstract] ABSTRACT: Piezoelectric quartz tuning forks have been employed as the force sensor in a dynamic mode scanning force microscope operating at temperatures down to 1.7 K at He-gas pressures of typically 5 mbar. An electrochemically etched tungsten tip glued to one of the tuning fork prongs acts as the local force sensor. Its oscillation amplitude can be tuned between a few angstroms and tens of nanometers. Quality factors of up to 120 000 allow a very accurate measurement of small frequency shifts. Three calibration procedures are compared which allow the determination of the proportionality constant between frequency shift and local force gradient based on the harmonic oscillator model and on electrostatic forces. The calibrated sensor is then used for a study of the interaction between the tip and a highly oriented pyrolytic graphite (HOPG) substrate. Force gradient and dissipated power can be recorded simultaneously. It is found that during approaching the tip to the sample considerable power starts to be dissipated although the force gradient is still negative, i.e., the tip is still in the attractive regime. This observation concurs with experiments with true atomic resolution, which seem to require the same tip–sample separation.
[show abstract][hide abstract] ABSTRACT: Piezoelectric quartz tuning forks are investigated in view of their use as force sensors in dynamic mode scanning probe microscopy at temperatures down to 1.5 K and in magnetic fields up to 8 T. The mechanical properties of the forks are extracted from the frequency dependent admittance and simultaneous interferometric measurements. The performance of the forks in a cryogenic environment is investigated. Force-distance studies performed with these sensors at low temperatures are presented.
Review of Scientific Instruments 07/1999; · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: A scanning force microscope was implemented operating at temperatures below 4.2K and in magnetic fields up to 8T. Piezoelectric quartz tuning forks were employed for non optical tip-sample distance control in the dynamic operation mode. Fast response was achieved by using a phase-locked loop for driving the mechanical oscillator. Possible applications of this setup for various scanning probe techniques are discussed. Comment: 5 pages, 5 figures, submitted to "Review of Scientific Instruments"
Review of Scientific Instruments 01/1999; · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: We use an atomic force microscope (AFM) to pattern metal surfaces in the nanometer range. Our technique is based on an electrochemical process called anodic oxidation. By applying a voltage between the AFM-tip and the sample substrate an electrochemical reaction is induced. With this technique several metals and semiconductors can be oxidized locally, i.e. in close vicinity of the tip scanning over the surface. We show how the formation of these oxide structures depends on key parameters, such as humidity and writing speed. Instead of a voltage source, we are using a constant current source to drive the oxidation. By means of this method, deficiencies related to the voltage source technique can be avoided. As a result, we are able to write structures on thin titanium films with excellent electronic properties. We focus on the patterning of titanium, since titanium is suited as a gate material on Ga[Al]As-heterostructures.
Physica E: Low-dimensional Systems and Nanostructures. 01/1998;
[show abstract][hide abstract] ABSTRACT: 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. (C) 1997 American Institute of Physics. [S0003-6951(97)02642-9].