We show by angle-resolved photoemission spectroscopy that a tunable gap in quasi-free-standing monolayer graphene on Au can be induced by hydrogenation. The size of the gap can be controlled via hydrogen loading and reaches approximately 1.0 eV for a hydrogen coverage of 8%. The local rehybridization from sp(2) to sp(3) in the chemical bonding is observed by X-ray photoelectron spectroscopy and X-ray absorption and allows for a determination of the amount of chemisorbed hydrogen. The hydrogen induced gap formation is completely reversible by annealing without damaging the graphene. Calculations of the hydrogen loading dependent core level binding energies and the spectral function of graphene are in excellent agreement with photoemission experiments. Hydrogenation of graphene gives access to tunable electronic and optical properties and thereby provides a model system to study hydrogen storage in carbon materials.

Localized and delocalized plasmon excitations in single wall carbon nanotubes Pichler, T.; Knupfer, M.; Golden, M.S.; Fink, J.; Rinzler, A.; Smalley, R.E.

No abstract

The distribution of valence electrons in metals usually follows the symmetry of an ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron Calculations of the electronic structure of the new pnictide superconductors unanimously predict a Fermi surface (FS) consisting of hole-like pocket in the centre (Γ point) of the Brillouin zone (BZ) and electron-like ones at the corners (X point) of the BZ. A shift by the (π, π) vector would result in a significant overlap of these FSs. Such an electronic structure is highly unstable since any interaction allowing an electron to gain a (π, π) momentum would favour a density wave order, which then results in aforementioned shift and a concomitant opening of the gaps, thus strongly reducing the electronic kinetic energy. It is surprising that ARPES data are reported to be in general, and sometimes in very detailed [9], agreement with the calculations giving a potentially unstable solution [5,6,7]. Even in the parent compound, where the spin density wave transition is clearly seen by other techniques [16,17], no evidence for the expected energy gap has been detected by photoemission experiments [7,8]. In fact, no consensus exists regarding the overall FS topology. According to Refs. 6 and 5, there is a single electron-like FS pocket around the X point, while Ref. 18 reports two intensity spots without any discernible signature for the electron pocket in the normal state. Intensity spots near the X point can also be found in Refs. 6, 7 and 9, but those are interpreted as parts of electron-like pockets. Obviously, such substantial variations in the photoemission signal preclude unambiguous assignment of the observed features to the calculated FS, leaving the electronic structure of the arsenides unclear.In Fig. 1 we show experimental FS map of Ba 1−x K x Fe 2 As 2 (BKFA) measured in superconducting state. To eliminate possible effects of photoemission matrix elements, as well as to cut the electronic structure at different k z values, we have done measurements at several excitation energies (Fig. 1a-b) and polarizations ( Fig. 1c-d). Although there are obvious changes in the intensities of the features, no signatures indicating k z dispersion can be concluded. With this in mind, the apparently different intensity distributions at neighboring Γ points appear unusual. While in the first BZ the two concentric contours are broadly consistent with

The precise momentum dependence of the superconducting gap in the iron-arsenide superconductor Ba 1−x K x Fe 2 As 2 ͑BKFA͒ with T c = 32 K was determined from angle-resolved photoemission spectroscopy ͑ARPES͒ via fitting the distribution of the quasiparticle density to a model. The model incorporates finite lifetime and experimental resolution effects, as well as accounts for peculiarities of BKFA electronic structure. We have found that the value of the superconducting gap is practically the same for the inner ⌫ barrel, X pocket, and "blade" pocket, and equals 9 meV, while the gap on the outer ⌫ barrel is estimated to be less than 4 meV, resulting in 2⌬ / k B T c = 6.8 for the large gap and 2⌬ / k B T c Ͻ 3 for the small gap. A large ͑77Ϯ 3 %͒ nonsuperconducting component in the photoemission signal is observed below T c . Details of gap extraction from ARPES data are discussed in Appendixes A and B.