Reversible Graphene decoupling by NaCl photo-dissociation

We describe the reversible intercalation of Na under graphene on Ir(111) by photo-dissociation of a previously adsorbed NaCl overlayer. After room temperature evaporation, NaCl adsorbs on top of graphene forming a bilayer. With a combination of electron diffraction and photoemission techniques we demonstrate that the NaCl overlayer dissociates upon a short exposure to an X-ray beam. As a result, chlorine desorbs while sodium intercalates under the graphene, inducing an electronic decoupling from the underlying metal. Low energy electron diffraction shows the disappearance of the moir\'e pattern when Na intercalates between graphene and iridium. Analysis of the Na 2p core-level by X-ray photoelectron spectroscopy shows a chemical change from NaCl to metallic buried Na at the graphene/Ir interface. The intercalation-decoupling process leads to a n-doped graphene due to the charge transfer from the Na, as revealed by constant energy angle resolved X-ray photoemission maps. Moreover, the process is reversible by a mild annealing of the samples without damaging the graphene

Coordination networks incorporating halogen-bond donor sites and azobenzene groups

Two Zn coordination networks, [Zn(1)(Py)2]2(2-propanol)n (3) and [Zn(1)2(Bipy)2](DMF)2n (4), incorporating halogen-bond (XB) donor sites and azobenzene groups have been synthesized and fully characterized. Obtaining 3 and 4 confirms that it is possible to use a ligand wherein its coordination bond acceptor sites and XB donor sites are on the same molecular scaffold (i.e., an aromatic ring) without interfering with each other. We demonstrate that XBs play a fundamental role in the architectures and properties of the obtained coordination networks. In 3, XBs promote the formation of 2D supramolecular layers, which, by overlapping each other, allow the incorporation of 2-propanol as a guest molecule. In 4, XBs support the connection of the layers and are essential to firmly pin DMF solvent molecules through I⋯O contacts, thus increasing the stability of the solvated systems

Sensitivity for tau neutrinos at PeV energies and beyond with the MAGIC telescopes

The MAGIC telescopes, located at the Roque de los Muchachos Observatory (2200 a.s.l.) in the Canary Island of La Palma, are placed on the top of a mountain, from where a window of visibility of about 5 deg in zenith and 80 deg in azimuth is open in the direction of the surrounding ocean. This permits to search for a signature of particle showers induced by earth-skimming cosmic tau neutrinos in the PeV to EeV energy range arising from the ocean. We have studied the response of MAGIC to such events, employing Monte Carlo simulations of upward-going tau neutrino showers. The analysis of the shower images shows that air showers induced by tau neutrinos can be discriminated from the hadronic background coming from a similar direction. We have calculated the point source acceptance and the expected event rates, assuming an incoming tau neutrino flux consistent with IceCube measurements, and for a sample of generic neutrino fluxes from photo-hadronic interactions in AGNs. The analysis of about 30 hours of data taken toward the sea leads to a point source sensitivity for tau neutrinos at the level of the down-going point source analysis of the Pierre Auger Observatory.Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC 2017), Bexco, Busan, Korea,(arXiv:1708.05153

Halogen bonding stabilizes a cis-azobenzene derivative in the solid state: A crystallographic study

Crystals of trans- and cis-isomers of a fluorinated azobenzene derivative have been prepared and characterized by single-crystal X-ray diffraction. The presence of F atoms on the aromatic core of the azobenzene increases the lifetime of the metastable cis-isomer, allowing single crystals of the cis-azobenzene to be grown. Structural analysis on the cis-azobenzene, complemented with density functional theory calculations, highlights the active role of the halogen-bond contact (N...I synthon) in promoting the stabilization of the cis-isomer. The presence of a long aliphatic chain on the azobenzene unit induces a phase segregation that stabilizes the molecular arrangement for both the trans- and cis-isomers. Due to the rarity of cis-azobenzene crystal structures in the literature, our paper makes a step towards understanding the role of non-covalent interactions in driving the packing of metastable azobenzene isomers. This is expected to be important in the future rational design of solid-state, photoresponsive materials based on halogen bonding. We show by single-crystal X-ray diffraction studies and computational analysis that halogen bonding can stabilize a metastable cis-azobenzene derivative in the solid state

Pressure-induced structural phase transitions in the AMnF4 series (A=Cs, Rb, K) studied by synchrotron x-ray powder diffraction: Correlation between hydrostatic and chemical pressure

The effect of applying hydrostatic pressure in the layered-perovskite AMnF4 (A=Cs, Rb, K) series has been studied using energy-dispersive synchrotron x-ray powder diffraction at pressures between ambient and 20 GPa. At ambient pressure CsMnF4 is tetragonal with space group P4/n, RbMnF4 is orthorhombic with space group Pmab and KMnF4 is monoclinic with space group P21/a. CsMnF4 was found to undergo a first-order structural phase transition, from tetragonal to orthorhombic symmetry at Pc1=1.4±0.2 GPa. At pressures in excess of Pc2=6.3±1 GPa, for the Cs derivative, and Pc3.=4.5±1 GPa, for the Rb derivative, the symmetry appears to be monoclinic. Moreover, the critical unit-cell volumes associated with Pc1, Pc2, and Pc3 are slightly higher than the ambient pressure unit-cell volumes of RbMnF4 for Pc1 and KMnF4 for Pc2 and Pc3. Hydrostatic pressure has been found to have a similar effect on the crystal symmetry of the series as the decreasing of the radius of the alkaline ion from Cs to Rb and K. A correlation between hydrostatic and chemical pressure can therefore be established from the structural point of view for the AMnF4 series. The tetragonal to orthorhombic transition of CsMnF4 has been found to be inhibited when NaCl is used as an internal pressure calibrant. The partial substitution of Cs by Na in CsMnF4 at Pc1 has been shown to be a likely explanation for this behavior. The anisotropic broadening of the Bragg peaks for pressures higher than Pc1 has been analyzed in terms of microstrain affecting the CsMnF4 lattice due to Na incorporation. A substitutional reaction has been shown to be a competitive process, versus a structural phase transition, that enables the system to return to equilibrium after applying pressure on it. Finally, the equation of state associated with the different high-pressure phases has been calculated including compressibilities.We would like to thank the C.I.C.Y.T. for Grants No. MAT91-681, MAT94-43, and MAT95-1490-E, and U.K. Science & Engineering Research Council for providing Synchrotron Radiation beam time under the E.C. Large Scale Facilities Programme.Peer Reviewe

Fermi surface instabilities in CeRh2Si2 at high magnetic field and pressure

We present thermoelectric power (TEP) studies under pressure and high magnetic field in the antiferromagnet CeRh2Si2 at low temperature. Under magnetic field, large quantum oscillations are observed in the TEP, S(H), in the antiferromagnetic phase. They suddenly disappear when entering in the polarized paramagnetic (PPM) state at Hc pointing out an important reconstruction of the Fermi surface (FS). Under pressure, S/T increases strongly of at low temperature near the critical pressure Pc, where the AF order is suppressed, implying the interplay of a FS change and low energy excitations driven by spin and valence fluctuations. The difference between the TEP signal in the PPM state above Hc and in the paramagnetic state (PM) above Pc can be explained by different FS. Band structure calculations at P = 0 stress that in the AF phase the 4f contribution at the Fermi level (EF) is weak while it is the main contribution in the PM domain. By analogy to previous work on CeRu2Si2, in the PPM phase of CeRh2Si2 the 4f contribution at EF will drop.Comment: 10 pages, 13 figure

Experimental Analysis of a Flat Plate Solar Collector with Integrated Latent Heat Thermal Storage

In the present paper, an experimental analysis of a solar water heating collector with an integrated latent heat storage unit is presented. With the purpose to determine the performance of a device on a lab scale, but with commercial features, a flat plate solar collector with phase change material (PCM) containers under the absorber plate was constructed and tested. PCM used was a commercial semi-refined light paraffin with a melting point of 60°C. Tests were carried out in outdoor conditions from October 2016 to March 2017 starting at 7:00 AM until the collector does not transfer heat to the water after sunset. Performance variables as water inlet temperature, outlet temperature, mass flow and solar radiation were measured in order to determine a useful heat and the collector efficiency. Furthermore, operating temperatures of the glass cover, air gap, absorber plate, and PCM containers are presented. Other external variables as ambient temperature, humidity and wind speed were measured with a weather station located next to the collector. The developed prototype reached an average thermal efficiency of 24.11% and a maximum outlet temperature of 50°C. Results indicate that the absorber plate reached the PCM melting point in few cases, this suggests that the use of a PCM with a lower melting point could be a potential strategy to increase thermal storage. A thermal analysis and conclusions of the device performance are discussed