Electrically tunable transverse magnetic focusing in graphene

Author's final manuscript January 9, 2013Electrons in a periodic lattice can propagate without scattering for macroscopic distances despite the presence of the non-uniform Coulomb potential due to the nuclei. Such ballistic motion of electrons allows the use of a transverse magnetic field to focus electrons. This phenomenon, known as transverse magnetic focusing (TMF), has been used to study the Fermi surface of metals and semiconductor heterostructures, as well as to investigate Andreev reflection and spin–orbit interaction, and to detect composite fermions. Here we report on the experimental observation of TMF in high-mobility mono-, bi- and tri-layer graphene devices. The ability to tune the graphene carrier density enables us to investigate TMF continuously from the hole to the electron regime and analyse the resulting focusing fan. Moreover, by applying a transverse electric field to tri-layer graphene, we use TMF as a ballistic electron spectroscopy method to investigate controlled changes in the electronic structure of a material. Finally, we demonstrate that TMF survives in graphene up to 300 K, by far the highest temperature reported for any system, opening the door to new room-temperature applications based on electron-optics.National Science Foundation (U.S.) (CAREER Award DMR-0845287)United States. Office of Naval Research. GATE MURI Projec

Scanning-Probe Electronic Imaging of Lithographically Patterned Quantum Rings

Quantum rings patterned from two-dimensional semiconductor heterostructures exhibit a wealth of quantum transport phenomena at low temperature and in a magnetic field that can be mapped in real space thanks to dedicated scanning probe techniques. Here, we summarize our studies of GaInAs- and graphene-based quantum rings by means of scanning-gate microscopy both at low magnetic field, where Aharonov-Bohm interferences and the electronic local density-of-states are imaged, and at high magnetic field and very low temperatures, where the scanning probe can image Coulomb islands in the quantum Hall regime. This allows decrypting the apparent complexity of the magneto-resistance of a mesoscopic system in this regime. Beyond imaging and beyond a strict annular shape of the nanostructure, we show that this scanning-probe technique can also be used to unravel a new counter-intuitive behavior of branched-out rectangular quantum rings, which turns out to be a mesoscopic analog of the Braess paradox, previously known for road or other classical networks only

The LHCb Upgrade I

Abstract The LHCb upgrade represents a major change of the experiment. The detectors have been almost completely renewed to allow running at an instantaneous luminosity five times larger than that of the previous running periods. Readout of all detectors into an all-software trigger is central to the new design, facilitating the reconstruction of events at the maximum LHC interaction rate, and their selection in real time. The experiment's tracking system has been completely upgraded with a new pixel vertex detector, a silicon tracker upstream of the dipole magnet and three scintillating fibre tracking stations downstream of the magnet. The whole photon detection system of the RICH detectors has been renewed and the readout electronics of the calorimeter and muon systems have been fully overhauled. The first stage of the all-software trigger is implemented on a GPU farm. The output of the trigger provides a combination of totally reconstructed physics objects, such as tracks and vertices, ready for final analysis, and of entire events which need further offline reprocessing. This scheme required a complete revision of the computing model and rewriting of the experiment's software.</jats:p