First-principles calculations for the adsorption of water molecules on the Cu(100) surface

First-principles density-functional theory and supercell models are employed to calculate the adsorption of water molecules on the Cu(100) surface. In agreement with the experimental observations, the calculations show that a H2O molecule prefers to bond at a one-fold on-top (T1) surface site with a tilted geometry. At low temperatures, rotational diffusion of the molecular axis of the water molecules around the surface normal is predicted to occur at much higher rates than lateral diffusion of the molecules. In addition, the calculated binding energy of an adsorbed water molecule on the surfaces is significantly smaller than the water sublimation energy, indicating a tendency for the formation of water clusters on the Cu(100) surface.Comment: 5 pages, 3 figures, submitted to Phys. Rev.

Polarization of tightly focused laser beams

The polarization properties of monochromatic light beams are studied. In contrast to the idealization of an electromagnetic plane wave, finite beams which are everywhere linearly polarized in the same direction do not exist. Neither do beams which are everywhere circularly polarized in a fixed plane. It is also shown that transversely finite beams cannot be purely transverse in both their electric and magnetic vectors, and that their electromagnetic energy travels at less than c. The electric and magnetic fields in an electromagnetic beam have different polarization properties in general, but there exists a class of steady beams in which the electric and magnetic polarizations are the same (and in which energy density and energy flux are independent of time). Examples are given of exactly and approximately linearly polarized beams, and of approximately circularly polarized beams.Comment: 9 pages, 6 figure

Maximally polarized states for quantum light fields

The degree of polarization of a quantum state can be defined as its Hilbert-Schmidt distance to the set of unpolarized states. We demonstrate that the states optimizing this degree for a fixed average number of photons $\bar{N}$ present a fairly symmetric, parabolic photon statistics, with a variance scaling as $\bar{N}^2$. Although no standard optical process yields such a statistics, we show that, to an excellent approximation, a highly squeezed vacuum can be considered as maximally polarized.Comment: 4 pages, 3 eps-color figure

Metallic microswimmers driven up the wall by gravity

Experiments on autophoretic bimetallic nanorods propelling within a fuel of hydrogen peroxide show that tail-heavy swimmers preferentially orient upwards and ascend along inclined planes. We show that such gravitaxis is strongly facilitated by interactions with solid boundaries, allowing even ultraheavy microswimmers to climb nearly vertical surfaces. Theory and simulations show that the buoyancy or gravitational torque that tends to align the rods is reinforced by a fore-aft drag asymmetry induced by hydrodynamic interactions with the wall.MRSEC Program of the National Science Foundation under Award DMR-1420073 NSF Grants DMS-RTG-1646339, DMS-1463962 and DMS-1620331. Tamkeen under the NYU Abu Dhabi Research Institute grant CG002 “la Caixa” Foundation (ID 100010434) fellowship LCF/BQ/PI20/11760014 European Union’s Horizon 2020 under the Marie Sklodowska-Curie grant agreement No 847648

A study of random resistor-capacitor-diode networks to assess the electromagnetic properties of carbon nanotube filled polymers

We determined the frequency dependent effective permittivity of a large ternary network of randomly positioned resistors, capacitors, and diodes. A linear circuit analysis of such systems is shown to match the experimental dielectric response of single-walled carbon nanotube (SWCNT) filled polymers. This modeling method is able to reproduce the two most important features of SWCNT filled composites, i.e. the low frequency dispersion and dipolar relaxation. As a result of the modeling important physical conclusion proved by the experimental data was done: the low frequency behavior of SWCNT-filled polymer composites is mostly caused by the fraction of semiconducting SWCNTs

Microwave and mechanical properties of quartz/graphene-based polymer nanocomposites

We report microwave spectroscopy studies of graphene-based polymer-matrix composite materials subject to uniaxial elongation. The samples were prepared via shear mixing under the same thermal processing conditions of amorphous styrene butadiene rubber (SBR) with quartz grains on the order of micrometers in size and/or graphene sheets with thickness 10-20 nm and average lateral size 200 mu m. An important result is the observation of a significant increase (up to 25%) in the effective microwave permittivity of hybridized nanocomposites comprising both quartz and graphene compared to the nanocomposites with quartz only. We suggest that the coating of quartz grains by graphene sheets is the most likely origin of this synergetic effect. In all cases, we also observe that the permittivity spectrum is unaffected by strain up to 8%. By examining the mechanical response, it is shown that the elasticity network of SBR polymer chains is significantly affected in the rubbery state by filling SBR with graphene and quartz particles. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793411

Dissipation and decoherence in photon interferometry

The propagation of polarized photons in optical media can be effectively modeled by means of quantum dynamical semigroups. These generalized time evolutions consistently describe phenomena leading to loss of phase coherence and dissipation originating from the interaction with a large, external environment. High sensitive experiments in the laboratory can provide stringent bounds on the fundamental energy scale that characterizes these non-standard effects.Comment: 14 pages, plain-Te