A computer graphics display and data compression technique

The computer program discussed is intended for the graphical presentation of a general dependent variable X that is a function of two independent variables, U and V. The required input to the program is the variation of the dependent variable with one of the independent variables for various fixed values of the other. The computer program is named CRP, and the output is provided by the SD 4060 plotter. Program CRP is an extremely flexible program that offers the user a wide variety of options. The dependent variable may be presented in either a linear or a logarithmic manner. Automatic centering of the plot is provided in the ordinate direction, and the abscissa is scaled automatically for a logarithmic plot. A description of the carpet plot technique is given along with the coordinates system used in the program. Various aspects of the program logic are discussed and detailed documentation of the data card format is presented

Observation of Fermi-energy dependent unitary impurity resonances in a strong topological insulator Bi_2Se_3 with scanning tunneling spectroscopy

Scanning tunneling spectroscopic studies of Bi_2Se_3 epitaxial films on Si (111) substrates reveal highly localized unitary impurity resonances associated with non-magnetic quantum impurities. The strength of the resonances depends on the energy difference between the Fermi level (E_F) and the Dirac point (E_D) and diverges as E_F approaches E_D. The Dirac-cone surface state of the host recovers within ~ 2Å spatial distance from impurities, suggesting robust topological protection of the surface state of topological insulators against high-density impurities that preserve time reversal symmetry

Observation of vortices and hidden pseudogap from scanning tunneling spectroscopic studies of electron-doped cuprate superconductor Sr0.9La0.1CuO2Sr_{0.9}La_{0.1}CuO_2

We present the first demonstration of vortices in an electron-type cuprate superconductor, the highest $T_c$ (= 43 K) electron-type cuprate $Sr_{0.9}La_{0.1}CuO_2$. Our spatially resolved quasiparticle tunneling spectra reveal a hidden low-energy pseudogap inside the vortex core and unconventional spectral evolution with temperature and magnetic field. These results cannot be easily explained by the scenario of pure superconductivity in the ground state of high-$T_c$ superconductivity.Comment: 6 pages, 4 figures. Two new graphs have been added into Figure 2. Accepted for publication in Europhysics Letters. Corresponding author: Nai-Chang Yeh (E-mail: [email protected]

Scanning Tunneling Spectroscopic Studies of the Effects of Dielectrics and Metallic Substrates on the Local Electronic Characteristics of Graphene

Atomically resolved imaging and spectroscopic characteristics of graphene grown by chemical vapor deposition (CVD) on copper foils are investigated and compared with those of mechanical exfoliated graphene on SiO_2. For exfoliated graphene, the local spectral deviations from ideal behavior may be attributed to strain induced by the SiO_2 substrate. For CVD grown graphene, the lattice structure appears strongly distorted by the underlying copper, with regions in direct contact with copper showing nearly square lattices whereas suspended regions from thermal relaxation exhibiting nearly honeycomb or hexagonal lattice structures. The electronic density of states (DOS) correlates closely with the atomic arrangements of carbon, showing excess zero-bias tunneling conductance and nearly energy-independent DOS for strongly distorted graphene, in contrast to the linearly dispersive DOS for suspended graphene. These results suggest that graphene can interact strongly with both metallic and dielectric materials in close proximity, leading to non-negligible modifications to the electronic properties

Scanning Tunneling Spectroscopic Studies of the Low-Energy Quasiparticle Excitations in Cuprate Superconductors

We report scanning tunneling spectroscopic (STS) studies of the low-energy quasiparticle excitations of cuprate superconductors as a function of magnetic field and doping level. Our studies suggest that the origin of the pseudogap (PG) is associated with competing orders (COs), and that the occurrence (absence) of PG above the superconducting (SC) transition T_c is associated with a CO energy Δ_(CO) larger (smaller) than the SC gap Δ_(SC). Moreover, the spatial homogeneity of Δ_(SC) and Δ_(CO) depends on the type of disorder in different cuprates: For optimally and under-doped YBa_2Cu_3O_(7−δ) (Y-123), we find that Δ_(SC) < Δ_(CO) and that both Δ_(SC) and Δ(CO) exhibit long-range spatial homogeneity, in contrast to the highly inhomogeneous STS in Bi_2Sr_2CaCu_2O_(8+x) (Bi-2212). We attribute this contrast to the stoichiometric cations and ordered apical oxygen in Y-123, which differs from the non-stoichiometric Bi-to-Sr ratio in Bi-2212 with disordered Sr and apical oxygen in the SrO planes. For Ca-doped Y-123, the substitution of Y by Ca contributes to excess holes and disorder in the CuO_2 planes, giving rise to increasing inhomogeneity, decreasing Δ_(SC) and Δ_(CO), and a suppressed vortex-solid phase. For electron-type cuprate Sr_(0.9)La_(0.1)CuO_2 (La-112), the homogeneous Δ_(SC) and Δ_(CO) distributions may be attributed to stoichiometric cations and the absence of apical oxygen, with Δ_(CO) < Δ_(SC) revealed only inside the vortex cores. Finally, the vortex-core radius (ξ_(halo)) in electron-type cuprates is comparable to the SC coherence length ξ_(SC), whereas ξ_(halo) ∼ 10ξ_(SC) in hole-type cuprates, suggesting that ξ_(halo) may be correlated with the CO strength. The vortex-state irreversibility line in the magnetic field versus temperature phase diagram also reveals doping dependence, indicating the relevance of competing orders to vortex pinning

Scanning Tunnelling Spectroscopic Studies of Dirac Fermions in Graphene and Topological Insulators

We report novel properties derived from scanning tunnelling spectroscopic (STS) studies of Dirac fermions in graphene and the surface state (SS) of a strong topological insulator (STI), Bi_2Se_3. For mono-layer graphene grown on Cu by chemical vapour deposition (CVD), strain-induced scalar and gauge potentials are manifested by the charging effects and the tunnelling conductance peaks at quantized energies, respectively. Additionally, spontaneous time-reversal symmetry breaking is evidenced by the alternating anti-localization and localization spectra associated with the zero-mode of two sublattices while global time-reversal symmetry is preserved under the presence of pseudo-magnetic fields. For Bi_2Se_3 epitaxial films grown on Si(111) by molecular beam epitaxy (MBE), spatially localized unitary impurity resonances with sensitive dependence on the energy difference between the Fermi level and the Dirac point are observed for samples thicker than 6 quintuple layers (QL). These findings are characteristic of the SS of a STI and are direct manifestation of strong topological protection against impurities. For samples thinner than 6-QL, STS studies reveal the openup of an energy gap in the SS due to overlaps of wave functions between the surface and interface layers. Additionally, spin-preserving quasiparticle interference wave-vectors are observed, which are consistent with the Rashba-like spin-orbit splitting