Plasmons in a layered strange metal using the gauge-gravity duality

In an attempt to understand the density-density response of the cuprate superconductors, we study plasmons in a layered strange metal using the Gubser-Rocha model. The latter is a well-known bottom-up holographic model for a strange metal that is used here to describe the strongly repulsive on-site interactions between the electrons in each copper-oxide (CuO2) layer, whereas the long-range Coulomb interactions are incorporated by a so-called double-trace deformation. To be able to model the bilayer cuprates more realistically, we consider in particular the case of two closely spaced CuO2 layers per unit cell. In the response we then obtain for vanishing out-of-plane momentum both an optical and an acoustic plasmon, whereas for nonvanishing out-of-plane momentum there are two acoustic plasmon modes. We present the full density-density spectral functions with parameters typical for cuprates and discuss both the dispersion and the lifetime of these plasmon excitations. Moreover, we compute the conductivity after introducing disorder into the system. Finally, we also compute the loss function to facilitate a comparison with experimental results from electron energy-loss spectroscopy

Origin of room-temperature ferromagnetism in hydrogenated epitaxial graphene on silicon carbide

\u3cp\u3eThe discovery of room-temperature ferromagnetism of hydrogenated epitaxial graphene on silicon carbide challenges for a fundamental understanding of this long-range phenomenon. Carbon allotropes with their dispersive electron states at the Fermi level and a small spin-orbit coupling are not an obvious candidate for ferromagnetism. Here we show that the origin of ferromagnetism in hydrogenated epitaxial graphene with a relatively high Curie temperature (>300 K) lies in the formation of curved specific carbon site regions in the graphene layer, induced by the underlying Si-dangling bonds and by the hydrogen bonding. Hydrogen adsorption is therefore more favourable at only one sublattice site, resulting in a localized state at the Fermi energy that can be attributed to a pseudo-Landau level splitting. This n = 0 level forms a spin-polarized narrow band at the Fermi energy leading to a high Curie temperature and larger magnetic moment can be achieved due to the presence of Si dangling bonds underneath the hydrogenated graphene layer.\u3c/p\u3

Surface phonon scattering in epitaxial graphene on 6H-SiC

We show the growth of high-quality epitaxial graphene on 6H-SiC with Raman signatures comparable to exfoliated flakes. We ascribe the remaining low-quality transport properties to the strong electron-phonon coupling to two low-energy phonon modes at 70 and 16 meV. The coupling of these modes is enhanced by the defects present in the SiC substrate and buffer layer. Measurements of the mobility versus carrier concentration show a square-root dependence, corroborating the importance of surface phonon scattering in the limited mobility of graphene on SiC

Modeling realistic tip structures : scanning tunneling microscopy of NO adsorption on Rh(111)

We have performed a joint experimental and theoretical scanning tunneling microscopy (STM) study of NO adsorbed on Rh(111). The experimental STM images showed a strong sensitivity to the tip conditions that could be altered by dipping the tip into the sample or by application of voltage pulses. Only via STM simulations performed over an exhaustive range of tip-apex terminations, including several contaminants in different adsorption geometries and with different spatial orientations, we have been able to reproduce the rich variety of measured images. From the analysis, we are able to infer a realistic structural model for the ultimate tip-apex structure involving apex geometries considerably more complex than those typically employed in STM modeling

Role of silicon dangling bonds in the electronic properties of epitaxial graphene on silicon carbide

\u3cp\u3eIn this paper, we study the electronic properties of epitaxial graphene (EG) on silicon carbide by means of ab initio calculations based on the local spin density approximation + U method taking into account the Coulomb interaction between Si localized electrons. We show that this interaction is not completely suppressed but is screened by carbon layers grown on-top of silicon carbide. This finding leads to a good qualitative understanding of the experimental results reported on EG on silicon carbide. Our results highlight the presence of the Si localized states and might explain the anomalous Hanle curve and the high values of spin relaxation time in EG.\u3c/p\u3