Imaging a Coupled Quantum Dot - Quantum Point Contact System

We performed measurements on a quantum dot and a capacitively coupled quantum point contact by using the sharp metallic tip of a low-temperature scanning force microscope as a scanned gate. The quantum point contact served as a detector for charges on the dot or nearby. It allowed us to distinguish single electron charging events in several charge traps from charging events on the dot. We analyzed the tip-induced potential quantitatively and found its shape to be independent of the voltage applied to the tip within a certain range of parameters. We estimate that the trap density is below 0.1% of the doping density and that the interaction energy between the quantum dot and a trap is a significant portion of the dot's charging energy. Possibly, such charge traps are the reason for frequently observed parametric charge rearrangements.Comment: 6 pages, 5 figure

Novel techniques to cool and rotate Bose-Einstein condensates in time-averaged adiabatic potentials

We report two novel techniques for cooling and rotating Bose-Einstein condensates in a dilute rubidium vapour that highlight the control and versatility afforded over cold atom systems by time-averaged adiabatic potentials (TAAPs). The intrinsic loss channel of the TAAP has been successfully employed to evaporatively cool a sample of trapped atoms to quantum degeneracy. The speed and efficiency of this process compares well with that of conventional forced rf-evaporation. In an independent experiment, we imparted angular momentum to a cloud of atoms forming a Bose-Einstein condensate by introducing a rotating elliptical deformation to the TAAP geometry. Triangular lattices of up to 60 vortices were created. All findings reported herein result from straightforward adjustments of the magnetic fields that give rise to the TAAP.Comment: The first two authors contributed equally to this wor

Trapping Ultracold Atoms in a Time-Averaged Adiabatic Potential

We report the first experimental realization of ultracold atoms confined in a time-averaged, adiabatic potential (TAAP). This novel trapping technique involves using a slowly oscillating ($\sim$ kHz) bias field to time-average the instantaneous potential given by dressing a bare magnetic potential with a high frequency ($\sim$ MHz) magnetic field. The resultant potentials provide a convenient route to a variety of trapping geometries with tunable parameters. We demonstrate the TAAP trap in a standard time-averaged orbiting potential trap with additional Helmholtz coils for the introduction of the radio frequency dressing field. We have evaporatively cooled 5 $\times 10^4$ atoms of $^{87}$Rb to quantum degeneracy and observed condensate lifetimes of over \unit[3]{s}.-Comment: 4 pages, 6 figure

In Situ Treatment of a Scanning Gate Microscopy Tip

In scanning gate microscopy, where the tip of a scanning force microscope is used as a movable gate to study electronic transport in nanostructures, the shape and magnitude of the tip-induced potential are important for the resolution and interpretation of the measurements. Contaminations picked up during topography scans may significantly alter this potential. We present an in situ high-field treatment of the tip that improves the tip-induced potential. A quantum dot was used to measure the tip-induced potential.Comment: 3 pages, 1 figure, minor changes to fit published versio

Measurement of the Tip-Induced Potential in Scanning Gate Experiments

We present a detailed experimental study on the electrostatic interaction between a quantum dot and the metallic tip of a scanning force microscope. Our method allowed us to quantitatively map the tip-induced potential and to determine the spatial dependence of the tip's lever arm with high resolution. We find that two parts of the tip-induced potential can be distinguished, one that depends on the voltage applied to the tip and one that is independent of this voltage. The first part is due to the metallic tip while we interpret the second part as the effect of a charged dielectric particle on the tip. In the measurements of the lever arm we find fine structure that depends on which quantum state we study. The results are discussed in view of scanning gate experiments where the tip is used as a movable gate to study nanostructures.Comment: 7 pages, 5 figures, minor changes to fit published versio

Lever arm of a metallic tip in scanning gate experiments

Abstract We present a scanning gate experiment on the electrostatic interaction between a semiconductor quantum dot and the metallic tip of a scanning force microscope. With the help of a feedback mechanism we can map the lever arm of the tip, using the quantum dot in a given quantum state as a sensitive electrometer. Besides the geometrically expected shape at length scales of hundreds of nanometers, we observe fine structure on much shorter length scales. r 2007 Elsevier B.V. All rights reserved. There are relatively few studies about an important factor common to all scanning gate experiments, namely the electrostatic potential that the tip induces in the sample. The importance of the tip potential for the interpretation of scanning gate measurements has been mentioned in some of the first studies [5] but only recently a technique was demonstrated allowing to determine it with high precision. In Ref. [6] we used a quantum dot as a very sensitive potentiometer to study the tip-induced potential. We demonstrated how, with the help of a feedback mechanism, one can map the tip potential with high spatial and energetical resolution. Additionally, we showed how the tip's lever arm on the quantum dot can be mapped and used to better understand the properties of the tip potential. In these measurements we found fine structure which illustrates how the scanning gate technique may yield local information about the quantum dot. Here we show a measurement reproducing the main findings. The measurement conditions were identical as in Ref. We used a quantum dot prepared on a GaAs/AlGaAs heterostructure with a two-dimensional electron gas residing 34 nm below the surface. The quantum dot was patterned by local anodic oxidation of the GaAs surface at room temperature. Subsequently, we evaporated a thin Ti film on the sample surface and this film was again patterned by local anodic oxidation We scanned the SFM tip at a constant height of about 200 nm over the sample surface. The dot was tuned into the Coulomb blockade regime, and we used a feedback mechanism to apply a voltage to a plunger gate such that one of the quantized energy levels of the dot always stayed in resonance with the chemical potential of the source and drain leads. The voltage on the plunger gate corresponds to the tip potential and with this technique we could ensure ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ -see front matter

Scanning probe with tuning fork sensor, microfabricated silicon cantilever and conductive tip for microscopy at cryogenic temperature

A quartz tuning-fork (TF)-based scanning probe is presented for local electrical transport measurements on quantum devices below the liquid 4 He temperature. The TF is utilized to drive and sense the mechanical oscillation of an attached, microfabricated cantilever featuring a conductive tip made of platinum silicide. The microfabricated structure allows the application of an external voltage to the tip, while the cantilever is electrically grounded. The probe was characterized at room temperature, 70 K, and 2 K. It was found that spatial sensitivity decreased with temperature. Imaging a gold surface at 2 K was successfully performed. A number of probes can be batch-fabricated, thus shortening the lead time for conducting experiments in cryogenic scanning force microscopy

Hadron Energy Reconstruction for the ATLAS Calorimetry in the Framework of the Non-parametrical Method

This paper discusses hadron energy reconstruction for the ATLAS barrel prototype combined calorimeter (consisting of a lead-liquid argon electromagnetic part and an iron-scintillator hadronic part) in the framework of the non-parametrical method. The non-parametrical method utilizes only the known $e/h$ ratios and the electron calibration constants and does not require the determination of any parameters by a minimization technique. Thus, this technique lends itself to an easy use in a first level trigger. The reconstructed mean values of the hadron energies are within $\pm 1$ of the true values and the fractional energy resolution is $[(58\pm3)/\sqrt{E}+(2.5\pm0.3)$. The value of the $e/h$ ratio obtained for the electromagnetic compartment of the combined calorimeter is $1.74\pm0.04$ and agrees with the prediction that $e/h > 1.7$ for this electromagnetic calorimeter. Results of a study of the longitudinal hadronic shower development are also presented. The data have been taken in the H8 beam line of the CERN SPS using pions of energies from 10 to 300 GeV.Comment: 33 pages, 13 figures, Will be published in NIM

Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC

Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H →γ γ, H → Z Z∗ →4l and H →W W∗ →lνlν. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV, corresponding to an integrated luminosity of about 25 fb−1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson

Standalone vertex finding in the ATLAS muon spectrometer

A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bbar b final states, and pp collision data at √s = 7 TeV collected with the ATLAS detector at the LHC during 2011