We experimentally investigate electrical transport properties of graphene, which is a two dimensional (2D) conductor with relativistic energy dispersion relation. By investigating single-and bi-layer graphene devices with different aspect ratios, we confirm experimentally that the minimum conductivity in wide and short graphene strips approaches the universal value of 4e 2 /πh. At low temperatures, quantum interference of multiply-reflected waves of electrons and holes in graphene give rise to periodic conductance oscillations with bias and gate voltages. Thus graphene acts as a quantum billiard, a 2D ballistic, phase coherent electron system with long phase coherence length that exceeds 5 µm. Additional features in differential conductance emerge when graphene is coupled to superconducting electrodes. We observe proximity-induced enhanced conductance at low bias, and conductance dips at energy scales far above the superconducting gap of the electrodes. The latter provides preliminary evidence for a novel superconducting material that consists of graphene coated with metallic atoms. *To whom correspondence should be addressed. Email: lau@physics.ucr.edu Graphene, a two-dimensional (2D) a honey-comb lattice of carbon atoms, exhibits rather unusual energy dispersion relations -the low-lying electrons in single layer graphene behave like massless relativistic Dirac fermions with vanishing density of states at the Dirac point, and bilayer's band structure resembles that of a zero band gap semiconductor (Fig. 1a). Since recent experimental isolation and measurement of graphene [1][2][3], it has attracted tremendous attention, as the special band structures in single and bi-layer graphenes yield novel aspects to the physics of two-dimensional electron systems. The Dirac spectrum in graphene is predicted to give rise to a number of phenomena, such as quantum spin hall effects [4][5][6], enhanced Coulomb interaction [7][8][9][10][11], and suppression of weak localization [12,13]. Technologically, graphene is an attractive material for nanoscale electronics engineering. As a two-dimensional (2D) relative of carbon nanotubes, it manifests high mobility, extraordinary thermal conductivity and atomic perfection; but in contrast to nanotubes, traditional lithographic techniques can potentially be employed for tailoring of transport properties and device synthesis [14].In the past two years, much progress has been made on theoretical understanding of the novel electronic properties that may emerge in graphene. In contrast, experimental measurements of graphene have been relatively scarce.Here we demonstrate experimentally that single layer (SLG) and bi-layer (BLG) graphene can act as a quantum billiard, i.e. a 2D ballistic system where scattering only occur at boundaries, with a phase coherence length that exceeds 5 µm. The ballistic transport for charge carriers, coupled with phase coherent multiple-reflection at the electrodes, give rise to quantum interference of waves of the charge carriers, thus realizing a quant...
The prototype TIGRE Tracking and Imaging Gamma-Ray Experiment is being prepared for a scientific balloon flight in fall, 2006. TIGRE is a Compton telescope for 0.5-10 MeV gamma rays and a pair telescope for 10-100 MeV gammas. It uses multiple layers of thin silicon strip detectors as both the Compton and pair converter and the charged particle tracker. The event coincidence requirement is completed with arrays of CsI(Tl)-photodiode detectors surrounding the converter/tracker and large Na(Tl)-PMT detectors below. The purpose of this flight is to demonstrate the background suppression capabilities of the TIGRE instrument with Compton recoil electron tracking and the improved angular resolution for pairs with silicon as the converter material. Details of the control and readout of the detectors will be described. Calibration results using laboratory radioisotopes will likewise be presented.
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