Domains of doping in graphene on polycrystalline gold: first-principles and scanning tunneling spectroscopy studies

We have studied the graphene/gold interface by means of density functional theory (DFT) and scanning tunneling spectroscopy (STS). Weak interaction between graphene and the underlying gold surface leaves unperturbed Dirac cones in the band-structure, but they can be shifted with respect to the Fermi level of the whole system, which results in effective doping of graphene. DFT calculations revealed that the interface is extremely sensitive to the adsorption distance and to the structure of metal's surface, in particular strong variation in doping can be attributed to the specific rearrangements of substrate's atoms, such as the change in the crystallographic orientation, relaxation or other modifications of the surface. On the other hand, STS experiments have shown the presence of energetic heterogeneity in terms of the changes in the local density of states (LDOS) measured at different places on the sample. Randomly repeated regions of zero-doping and p-type doping have been identified from parabolic shape characteristics and from well defined Dirac points, respectively. The doping domains of graphene on gold seem to be related to the presence of various types of the surface structure across the sample. DFT simulations for graphene interacting with Au have shown large differences in doping induced by considered structures of substrate, in agreement with experimental findings. All these results demonstrate the possibility of engineering the electronic properties of graphene, especially tuning the doping across one flake which can be useful for applications of graphene in electronic devices

Structure and stability of the Si(331)-(12x1) surface reconstruction

We recently proposed a new structural model for the Si(331)-(12x1) surface reconstruction containing silicon pentamers as elementary structural building blocks. Using first-principles density-functional theory we here investigate the stability of a variety of adatom configurations and determine the lowest energy configuration. We also present a detailed comparison of the energetics between our model for Si(331)-(12x1) and the adatom-tetramer-interstitial model for Si(110)-(16x2), which shares the same structural building blocks

The evolution of cosmic string loops in Kerr-de Sitter spacetimes

The equation of cosmic string loops in Kerr-de Sitter spacetimes is derived. Having solved the equation numerically, we find that the loops can expand and exist except for too small ones.Comment: 8 page

Fragile charge order in the nonsuperconducting ground state of the underdoped high-temperature superconductors.

The normal state in the hole underdoped copper oxide superconductors has proven to be a source of mystery for decades. The measurement of a small Fermi surface by quantum oscillations on suppression of superconductivity by high applied magnetic fields, together with complementary spectroscopic measurements in the hole underdoped copper oxide superconductors, point to a nodal electron pocket from charge order in YBa2Cu3(6+δ). Here, we report quantum oscillation measurements in the closely related stoichiometric material YBa2Cu4O8, which reveals similar Fermi surface properties to YBa2Cu3(6+δ), despite the nonobservation of charge order signatures in the same spectroscopic techniques, such as X-ray diffraction, that revealed signatures of charge order in YBa2Cu3(6+δ). Fermi surface reconstruction in YBa2Cu4O8 is suggested to occur from magnetic field enhancement of charge order that is rendered fragile in zero magnetic fields because of its potential unconventional nature and/or its occurrence as a subsidiary to more robust underlying electronic correlations.B.T., A.S, and S.E.S. acknowledge support from the Royal Society, the Winton Programme for the Physics of Sustainability, and the European Research Council under the European Unions Seventh Framework Programme (grant number FP/2007-2013)/ ERC Grant Agreement number 337425. N.H., Z.Z., F.F.B., and B.J.R. acknowl- edge support for high-magnetic-field experiments from the US Department of Energy, Office of Science, BES- MSE `Science of 100 Tesla' programme. G.G.L. acknowl- edges support from EPSRC grant EP/K012894/1. Work at NIU was supported by The Institute for Nanoscience, Engineering, and Technology - InSET. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF co-operative agreement number DMR-0654118, the state of Florida, and the DOE. We are grateful for the experimental assis- tance provided by National High Magnetic Field Labora- tory personnel, including J. B. Betts, Y. Coulter, M. Gor- don, C. H. Mielke, A. Parish, R. McDonald, D. Rickel, and D. Roybal.This is the author accepted manuscript. The final version is available from the National Academy of Sciences via http://dx.doi.org/10.1073/pnas.150416411

Highly site-specific H2 adsorption on vicinal Si(001) surfaces

Experimental and theoretical results for the dissociative adsorption of H_2 on vicinal Si(001) surfaces are presented. Using optical second-harmonic generation, sticking probabilities at the step sites are found to exceed those on the terraces by up to six orders of magnitude. Density functional theory calculations indicate the presence of direct adsorption pathways for monohydride formation but with a dramatically lowered barrier for step adsorption due to an efficient rehybridization of dangling orbitals.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Lett. (1998). Other related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

Isotope Effect for the Penetration Depth in Superconductors

We show that various factors can lead to an isotopic dependence of the penetration depth $\delta$. Non-adiabaticity (Jahn-Teller crossing) leads to the isotope effect of the charge carrier concentration $n$ and, consequently, of $\delta$ in doped superconductors such as the cuprates. A general equation relating the isotope coefficients of $T_c$ and of $\delta$ is presented for London superconductors. We further show that the presence of magnetic impurities or a proximity contact also lead to an isotopic dependence of $\delta$; the isotope coefficient turns out to be temperature dependent, $\beta(T)$, in these cases. The existence of the isotope effect for the penetration depth is predicted for conventional as well as for high-temperature superconductors. Various experiments are proposed and/or discussed.Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev.

Suppression of HD-cooling in protogalactic gas clouds by Lyman-Werner radiation

It has been shown that HD molecules can form efficiently in metal-free gas collapsing into massive protogalactic halos at high redshift. The resulting radiative cooling by HD can lower the gas temperature to that of the cosmic microwave background, T_CMB=2.7(1+z)K, significantly below the temperature of a few 100 K achievable via H_2-cooling alone, and thus reduce the masses of the first generation of stars. Here we consider the suppression of HD-cooling by UV irradiation in the Lyman-Werner (LW) bands. We include photo-dissociation of both H_2 and HD, and explicitly compute the self-shielding and shielding of both molecules by neutral hydrogen as well as the shielding of HD by H_2. We use a simplified dynamical collapse model, and follow the chemical and thermal evolution of the gas, in the presence of a UV background. We find that a LW flux of J_crit = 1e-22 erg/cm^2/sr/s/Hz is able to suppress HD cooling and thus prevent collapsing primordial gas from reaching temperatures below 100 K. The main reason for the lack of HD cooling for J>J_crit is the partial photo-dissociation of H_2, which prevents the gas from reaching sufficiently low temperatures (T<150K) for HD to become the dominant coolant; direct HD photo-dissociation is unimportant except for a narrow range of fluxes and column densities. Since the prevention of HD-cooling requires only partial H_2 photo-dissociation, the critical flux J_crit is modest, and is below the UV background required to reionize the universe at redshift z=10-20. We conclude that HD-cooling can reduce the masses of typical stars only in rare halos forming well before the epoch of reionization.Comment: 14 pages with 9 figures, submitted to MNRA

The role of electronic correlation in the Si(100) reconstruction: a quantum Monte Carlo study

Recent low-temperature scanning tunneling experiments have challenged the generally accepted picture of buckled silicon dimers as the ground state reconstruction of the Si(100) surface. Together with the symmetric dimer model of the surface suggested by quantum chemistry calculations on small clusters, these findings question our general understanding of electronic correlations at surfaces and its proper description within density functional theory. We present quantum Monte Carlo calculations on large cluster models of the symmetric and buckled surface, and conclude that buckling remains energetically more favorable even when the present-day best treatment of electronic correlation is employed.Comment: 5 pages, Revtex, 10 figure