Y. Zhang

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 theoretical value of 4e^2/\pi\h. 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 microns. 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.

Total-energy responses of a 76 mm × 200 mm BGO detector were used to deduce energy distributions of quasi-monoenergetic gamma-rays generated at the Shanghai Laser Electron Gamma Source (SLEGS) of the Shanghai Synchrotron Radiation Facility. Responses of the BGO detector to monoenergetic gamma-rays produced in (p,γ) reactions on LiF, 27Al, and 13C targets were measured at the China Institute of Atomic Energy. A direct unfolding of BGO total-energy responses into gamma-ray energy distributions was iteratively performed by solving a set of linear equations with the monoenergetic response of the BGO detector being the matrix element. We present resultant energy distributions of gamma-ray beams produced in the slant-scattering at SLEGS.

Basing on the slant Compton scattering of 10640 nm photons from a 100 W CO2 laser on 3.5 GeV electrons from the Shanghai Light Source, the Shanghai Laser Electron Gamma Source (SLEGS) produces gamma-ray in the energy range of 0.66 MeV–21.7 MeV and flux of 105–107 photons/s. A Gamma Modulation System (GMS) composed of the collimators, attenuator, beam spot imaging(MiniPIX) and flux monitor(LaBr3 detector) was developed to ensure producing bandwidth-variable quasi-monochromatic gamma-ray with a variable-flux at same time. Using the GMS, variable-flux quasi-monochromatic gamma-ray beams with a best bandwidth of 10% in the 90∘ slant-scattering and 4% in back-scattering were obtained respectively. We report on the performance of the GMS and results of gamma-ray beam modulating measurement with GMS at SLEGS.

The presence of one-dimensional trigonal arrangements of chromium atoms (Cr3) in the structure of TiCrIr2B2 leads to a magnetic transition near ambient temperature. Herein we report an investigation of the nature of electronic and magnetic properties of TiCrIr2B2 via ab initio calculations together with 11B NMR Knight shift (K) and spin-lattice relaxation rate (1/T1) analysis. The presence of a characteristic rectangular powder pattern below 280 K, absence of a Korringa relation, strong enhancement of 1/T1.T at low temperatures and weak temperature dependence of K indicate competition between antiferromagnetic and ferromagnetic spin fluctuations in the itinerant d-band electrons, in agreement with ab initio calculations. One-dimensional trigonal arrangements of magnetically active elements in intermetallic compounds, as is found in TiCrIr2B2, are quite rare and could lead to exotic phenomena such as spin-chirality and quantum criticality in low-dimensional frustrated lattices.