Kohn anomaly and interplay of electron-electron and electron-phonon interactions in epitaxial graphene

The interplay of electron-phonon (el-ph) and electron-electron (el-el) interactions in epitaxial graphene is studied by directly probing its electronic structure. We found a strong coupling of electrons to the soft part of the A1g phonon evident by a kink at 150+/-15 meV, while the coupling of electrons to another expected phonon E2g at 195 meV can only be barely detected. The possible role of the el-el interaction to account for the enhanced coupling of electrons to the A1g phonon, and the contribution of el-ph interaction to the linear imaginary part of the self energy at high binding energy are also discussed. Our results reveal the dominant role of the A1g phonon in the el-ph interaction in graphene, and highlight the important interplay of el-el and el-ph interactions in the self energy of graphene.Comment: accepted to Phys. Rev.

Polarization control at the microscopic and electronic structure observatory

The new Microscopic and Electronic Structure Observatory (MAESTRO) at the Advanced Light Source (ALS) in Berkeley provides X-rays of variable polarization, produced by an elliptically polarized undulator (EPU), for angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM) experiments. The interpretation of photoemission data, in particular of dichroism effects in ARPES, requires the precise knowledge of the exact polarization state. Numerical simulations show that the first harmonics of the EPU at MAESTRO provides soft X-rays of almost 100% on axis polarization. However, the higher harmonics as well as the downstream optical elements of the beamline, have a considerable impact on the polarization of the light delivered to the experimental end-station. Employing a simple reflective polarimeter, the polarization is characterized for variable EPU and beamline settings and the overall degree of polarization in the MAESTRO end-stations is estimated to be on the order of 83%

Syndecan-2 is a novel target of insulin-like growth factor binding protein-3 and is over-expressed in fibrosis

Extracellular matrix deposition and tissue scarring characterize the process of fibrosis. Transforming growth factor beta (TGFβ) and Insulin-like growth factor binding protein-3 (IGFBP-3) have been implicated in the pathogenesis of fibrosis in various tissues by inducing mesenchymal cell proliferation and extracellular matrix deposition. We identified Syndecan-2 (SDC2) as a gene induced by TGFβ in an IGFBP-3-dependent manner. TGFβ induction of SDC2 mRNA and protein required IGFBP-3. IGFBP-3 independently induced production of SDC2 in primary fibroblasts. Using an ex-vivo model of human skin in organ culture expressing IGFBP-3, we demonstrate that IGFBP-3 induces SDC2 ex vivo in human tissue. We also identified Mitogen-activated protein kinase-interacting kinase (Mknk2) as a gene induced by IGFBP-3. IGFBP-3 triggered Mknk2 phosphorylation resulting in its activation. Mknk2 independently induced SDC2 in human skin. Since IGFBP-3 is over-expressed in fibrotic tissues, we examined SDC2 levels in skin and lung tissues of patients with systemic sclerosis (SSc) and lung tissues of patients with idiopathic pulmonary fibrosis (IPF). SDC2 levels were increased in fibrotic dermal and lung tissues of patients with SSc and in lung tissues of patients with IPF. This is the first report describing elevated levels of SDC2 in fibrosis. Increased SDC2 expression is due, at least in part, to the activity of two pro-fibrotic factors, TGFβ and IGFBP-3. © 2012 Ruiz et al

Quasiparticle Transformation During a Metal-Insulator Transition in Graphene

Here we show, with simultaneous transport and photoemission measurements, that the graphene terminated SiC(0001) surface undergoes a metal-insulator transition (MIT) upon dosingwith small amounts of atomic hydrogen. We find the room temperature resistance increases by about 4 orders of magnitude, a transition accompanied by anomalies in the momentum-resolved spectral function including a non-Fermi Liquid behaviour and a breakdown of the quasiparticle picture. These effects are discussed in terms of a possible transition to a strongly (Anderson) localized ground state.Comment: 11 pages, 4 figure

Penetration of alkali atoms throughout graphene membrane: theoretical modeling

Theoretical studies of penetration of various alkali atoms (Li, Na, Rb, Cs) throughout graphene membrane grown on silicon carbide substrate are reported and compared with recent experimental results. Results of first principles modeling demonstrate rather low (about 0.8 eV) energy barrier for the formation of temporary defects in carbon layer required for the penetration of Li at high concentration of adatoms, higher (about 2 eV) barrier for Na, and barriers above 4 eV for Rb and Cs. Experiments prove migration of lithium adatoms from graphene surface to the buffer layer and SiC substrate at room temperature, sodium at 100{\deg}C and impenetrability of graphene membrane for Rb and Cs. Differences between epitaxial and free standing graphene for the penetration of alkali ions are also discussed.Comment: 16 pages, 3 figure, accepted to Nanoscal

Dirac dispersion and non-trivial Berry's phase in three-dimensional semimetal RhSb3

We report observations of magnetoresistance, quantum oscillations and angle-resolved photoemission in RhSb$_3$, a unfilled skutterudite semimetal with low carrier density. The calculated electronic band structure of RhSb$_3$ entails a $Z_2$ quantum number $\nu_0=0,\nu_1=\nu_2=\nu_3=1$ in analogy to strong topological insulators, and inverted linear valence/conduction bands that touch at discrete points close to the Fermi level, in agreement with angle-resolved photoemission results. Transport experiments reveal an unsaturated linear magnetoresistance that approaches a factor of 200 at 60 T magnetic fields, and quantum oscillations observable up to 150~K that are consistent with a large Fermi velocity ($\sim 1.3\times 10^6$ ms$^{-1}$), high carrier mobility ($\sim 14$ $m^2$/Vs), and small three dimensional hole pockets with nontrivial Berry phase. A very small, sample-dependent effective mass that falls as low as $0.015(7)$ bare masses scales with Fermi velocity, suggesting RhSb$_3$ is a new class of zero-gap three-dimensional Dirac semimetal.Comment: 9 pages, 4 figure

Angle-Resolved Photoemission Spectroscopy of Tetragonal CuO: Evidence for Intralayer Coupling Between Cupratelike Sublattices

We investigate by angle-resolved photoemission the electronic structure of in situ grown tetragonal CuO, a synthetic quasi-two-dimensional edge-sharing cuprate. We show that, in spite of the very different nature of the copper oxide layers, with twice as many Cu in the CuO layers of tetragonal CuO as compared to the CuO2 layers of the high-T-c cuprates, the low-energy electronic excitations are surprisingly similar, with a Zhang-Rice singlet dispersing on weakly coupled cupratelike sublattices. This system should thus be considered as a member of the high-T-c cuprate family, with, however, interesting differences due to the intralayer coupling between the cupratelike sublattices.open1199sciescopu