Graphene has shown great application potential as the host material for next-generation electronic devices. However, despite its intriguing properties, one of the biggest hurdles for graphene to be useful as an electronic material is the lack of an energy gap in its electronic spectra. This, for example, prevents the use of graphene in making transistors. Although several proposals have been made to open a gap in graphene's electronic spectra, they all require complex engineering of the graphene layer. Here, we show that when graphene is epitaxially grown on SiC substrate, a gap of approximately 0.26 eV is produced. This gap decreases as the sample thickness increases and eventually approaches zero when the number of layers exceeds four. We propose that the origin of this gap is the breaking of sublattice symmetry owing to the graphene-substrate interaction. We believe that our results highlight a promising direction for bandgap engineering of graphene.

Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity 1 . The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high transition temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle resolved photoemission spectroscopy to probe electron dynamicsvelocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity. We investigated the electronic quasiparticle dispersions in three different families of hole-doped cuprates, Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) and Pb doped Pb-Bi2212, Pb-doped Bi 2 Sr 2 CuO 6 (Pb-Bi2201) and La 2-x Sr x CuO 4 (LSCO). Except for the Bi2201 (overdoped, T c =7K) data and that in Fig. 3b, recorded at the beam-line 5.4 of the Stanford Synchrotron Radiation Laboratory (SSRL), all the data were recorded at the Advanced Light Source (ALS), as detailed elsewhere 2 . The top panels of figure 1 report the momentum distribution curve (MDC) derived dispersions along the (0, 0)-(π, π) direction for LSCO (panel a) and Bi2212 (panel b) superconducting state and for Pb-Bi2201 normal state (panel c) vs the rescaled momentum, k ' , defined by normalizing to one the momentum k relative to the Fermi momentum k F , (k-k F ), at the binding energy E=170meV. A "kink" in the dispersion around 50-80meV, highlighted by thick arrows in the figure, is the many-body effect of

Angle-resolved photoemission and X-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(0001) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films cause adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K-point. Each cone corresponds to an individual macro-scale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.

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The measurement of the Cu-O distances by a local and fast probe, polarized Cu K-edge extended x-ray absorption fine structure (EXAFS) in La 1.85 Sr 0.15 CuO 4 crystal shows two different conformations of the CuO 6 octahedra below 100 K assigned to two types of stripes with different lattice. This experiment supports a model of "two components" spatially separated in a superlattice of quantum stripes for the anomalous properties of cuprate superconductors. [S0031-9007(96)00119-6] PACS numbers: 74.72. Dn, 61.10.Ht, 78.70.Dm Experimental methods probing the local structure have shown that the structure of the metallic CuO 2 plane in high T c cuprate superconductors is not homogeneous at a mesoscopic scale length [1][2][3][4]. It has been proposed that an anharmonic 1D modulation of the CuO 2 plane is a key feature for the mechanism of high T c superconductivity [5]. A superstructure of the type q pb ‫ء‬ 1 ͑1͞n͒c ‫ء‬ , in the orthorhombic notation, seems to be a common feature of the superconducting cuprates close to the optimum doping. It has been observed in Bi 2 Sr 2 CaCu 2 O 81d (Bi2212) [6,7] and in Bi 2 Sr 2 Cu 2 O 61d (Bi2201) [8] with p ϳ 0.21 and n 2 considering doubling of the c axis; in La 2 CuO 4.1 (LCO) with p 0.22 and n 3 [9] and with p 0.2 and n 3 [10]; in La 22x Sr x Cu 2 O 4 (LSCO) for x ϳ 0.075 with p ϳ 0.16 and n ϳ 2.5 [11] and a similar superstructure for x 0.1, 0.15 but much weaker in intensity for the overdoped sample, i.e., x 0.2 [12]; in Tl 2 Ba 2 CaCu 2 O 8 (Tl2212) with p ϳ 0.2 [13] and a similar one in Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl2223) [14]. This superstructure is difficult to identify in some of the compounds (for example, in the case of LSCO it could be identified only after about 9 yr of the discovery of high T c superconductivity in this material), and it is more clear at temperatures lower than 100-200 K (e.g., in Tl2212, Tl2223, LCO, LSCO). On the other hand, the superstructure is stable even at high temperatures in Bi2212. The c-axis modulation, different from sample to sample, is due to ordering of dopants in the rock-salt block layers as it is clear in the isostructural compounds, e.g., La 2 NiO 41d . The long wavelength incommensurate modulation of the CuO 2 plane along the 45 ± direction from the Cu-Cu direction, involving ϳ10 Cu sites, appears to be a common feature of cuprate superconductors at optimum doping.A "two-component" model has been proposed [5] where at optimum doping (0.2 hole per Cu sites) a first component with hole density d i ϳ 1 1 0.16 coexists with a second component of impurity states, with hole density d ᐉ ϳ 0.04, spatially separated in two different types of stripes forming a superlattice of quantum wires. A com-mensurate superstructure with lower period (4 Cu sites) was predicted [5] where all doped holes form a single electronic component, a pinned Wigner polaronic charge density wave (CDW), that will suppress superconductivity, and it has been observed at the 1͞8 critical doping and in the nickelates [15].In the case of Bi2212 we have shown [5] that th...

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Quasiparticle dispersion in Bi2Sr2CaCu2O8 is investigated with improved angular resolution as a function of temperature and doping. Unlike the linear dispersion predicted by the band calculation, the data show a sharp break in dispersion at 50+/-15 meV binding energy where the velocity changes by a factor of 2 or more. This change provides an energy scale in the quasiparticle self-energy. This break in dispersion is evident at and away from the d-wave node line, but the magnitude of the dispersion change decreases with temperature and with increasing doping.

The Fermi velocity, vF, is one of the key concepts in the study of a material, as it bears information on a variety of fundamental properties. Upon increasing demand on the device applications, graphene is viewed as a prototypical system for engineering vF. Indeed, several efforts have succeeded in modifying vF by varying charge carrier concentration, n. Here we present a powerful but simple new way to engineer vF while holding n constant. We find that when the environment embedding graphene is modified, the vF of graphene is (i) inversely proportional to its dielectric constant, reaching vF ≈2.5×10 6 m/s, the highest value for graphene on any substrate studied so far and (ii) clearly distinguished from an ordinary Fermi liquid. The method demonstrated here provides a new route toward Fermi velocity engineering in a variety of two-dimensional electron systems including topological insulators.