The dispersion and the damping of the sheet plasmon in a graphene monolayer grown on Pt(111) have been studied by using angle-resolved electron energy loss spectroscopy. We found that the dispersion relation of the plasmon mode confined in the graphene sheet is linear, as a consequence of the screening by the metal substrate. Present results demonstrate that the presence of an underlying metal substrate could have striking consequences on the plasmon propagation even in the case of a system which exhibits a weak graphene-substrate interaction. Moreover, we found that Landau damping essentially occurs via interband excitations starting above the Fermi wave vector. On the contrary, intraband transitions do not have a significant influence on the collective mode. Low-energy collective excitations in graphene are attracting much interest in recent years 1-8 as they influence many of the peculiar properties of graphene samples. In particular, the dispersion and damping of plasmons in epitaxial graphene have recently been studied for the case of graphene deposited on SiC(0001) (Refs. 2 and 4) and Ir(111). 6 The understanding of plasmonic excitations of graphene plays a key role in tailoring the properties of novel graphene-based devices. 9Among graphene systems, the epitaxial growth of monolayer graphene (MLG) on Pt(111) is particularly interesting 10-14 as a consequence of the weak graphene-Pt interaction, 11 in contrast with MLG grown on other transition-metal substrates. 15,16 In fact, the graphene-Pt distance (3.30Å) lies close to the c-axis spacing in graphite. The electronic structure of MLG on Pt(111) resembles that of isolated graphene. 10 In particular, the linear dispersion of π bands in the so-called Dirac cones, which gives rise to many manifestations of massless Dirac fermions, is preserved. Angle-resolved photoemission spectroscopy (ARPES) experiments 10 do not show any remarkable hybridization of graphene π states with metal d states. They just represent a superposition of graphene and metal-derived states, with minimal interaction between them. The MLG on Pt(111) is hole doped by charge transfer to the Pt substrate. Epitaxial graphene on Pt(111) thus behaves as an ideal 2D system, sustaining a purely 2D electron gas (2DEG) system whose collective excitations (plasmon modes) are able to propagate along the sheet. The dielectric response of the 2DEG system is determined by plasmon dispersion, which could be measured by high-resolution electron energy loss spectroscopy (HREELS).The 2D plasmon, characterized by its square-root-like dispersion, has been predicted 17 and observed in metal layers on semiconductors.18 On the other hand, the acoustic surface plasmon (ASP) with a linear dispersion was demonstrated to exist on semiconductor quantum wells with two interacting quantum well minibands.19 Successively, ASP has been experimentally revealed on Be(0001) (Ref. 20) and on noble-metal surfaces. 21,22 The acoustic-like dispersion is a consequence of the combination of the nonlocality of the 3D response and t...

Kidney collecting-duct cells swell in response to changes in medulla osmolality caused by the transition from antidiuresis to diuresis. Regulatory volume decrease (RVD) mechanisms must be activated to face this hypotonic stress. In Aquaporin-2 (AQP2)-expressing renal CD8 cells, hypotonicity decreased cell surface expression of AQP2 and increased the amount of AQP2 localized intracellularly, whereas the total amount of AQP2 phosphorylated at ser-256 decreased. Analysis of cAMP dynamics using fluorescence resonance energy transfer (FRET) showed that hypotonicity causes a reduction of cAMP, consistent with a decrease in phospho-AQP2. Moreover, hypotonicity caused a profound actin reorganization, associated with the loss of stress fibers and formation of F-actin patches (microspikes) at the cell border. Those changes were regulated by the monomeric GTPase Cdc42. Interestingly, expression of the dominant-negative Cdc42 (N17-Cdc42) prevented the hypotonicity-induced microspike formation and the generation of Cl(-) currents. Hypotonicity also caused the relocation from the cytosol to the plasma membrane and increase in interaction with actin of ICln (nucleotide-sensitive chloride current protein), which is essential for the generation of ion currents activated during RVD. Together, the profound actin remodeling, internalization of AQP2 and translocation of ICln to the plasma membrane during hypotonicity may contribute to RVD after cell swelling in renal medulla.

Studying the interaction of CH3SH, methanethiol, with Cu(111) as a model system, we demonstrate the ability of chemical-shift normal incidence x-ray standing wave field measurements to identify the local adsorption geometries of coadsorbed reaction products at different temperatures, a technical problem of broad chemical significance. In the present case the local geometries of four distinct S-containing adsorbate species (intact CH3SH, two thiolate (CH3S-) reaction intermediates and atomic S) are determined.