3,330 research outputs found
Resonant hot charge-transfer excitations in fullerene-porphyrin complexes: a many-body Bethe-Salpeter study
We study within the many-body Green's function GW and Bethe-Salpeter
approaches the neutral singlet excitations of the zinctetraphenylporphyrin and
C70 fullerene donor-acceptor complex. The lowest transition is a
charge-transfer excitation between the donor and the acceptor with an energy in
excellent agreement with recent constrained density functional theory
calculations. Beyond the lowest charge-transfer state, of which the energy can
be determined with simple electrostatic models that we validate, the
Bethe-Salpeter approach provides the full excitation spectrum. We evidence the
existence of hot electron-hole states which are resonant in energy with the
lowest donor intramolecular excitation and show an hybrid intramolecular and
charge-transfer character, favouring the transition towards charge separation.
These findings support the recently proposed scenario for charge separation at
donor-acceptor interfaces through delocalized hot charge-transfer states.Comment: 9 pages, 4 figure
Atomistic calculation of the thermal conductance of large scale bulk-nanowire junctions
We have developed an efficient scalable kernel method for thermal transport
in open systems, with which we have computed the thermal conductance of a
junction between bulk silicon and silicon nanowires with diameter up to 10 nm.
We have devised scaling laws for transmission and reflection spectra, which
allow us to predict the thermal resistance of bulk-nanowire interfaces with
larger cross sections than those achievable with atomistic simulations. Our
results indicate the characteristic size beyond which atomistic systems can be
treated accurately by mesoscopic theories.Comment: 6 pages, 4 figure
Impact on floating membranes
When impacted by a rigid object, a thin elastic membrane with negligible
bending rigidity floating on a liquid pool deforms. Two axisymmetric waves
radiating from the impact point propagate. In the first place, a longitudinal
wave front -- associated with in-plane deformation of the membrane and
traveling at constant speed -- separates an outward stress free domain with a
stretched but flat domain. Then, in the stretched domain a dispersive
transverse wave travels at a wave speed that depends on the local stretching
rate. We study the dynamics of this fluid-body system and we show that the wave
dynamics is similar to the capillary waves that propagate at a liquid-gas
interface but with a surface tension coefficient that depends on impact speed.
We emphasize the role of the stretching in the membrane in the wave dynamics
but also in the development of a buckling instability that give rise to radial
wrinkles
Atomistic simulations of heat transport in real-scale silicon nanowire devices
Utilizing atomistic lattice dynamics and scattering theory, we study thermal
transport in nanodevices made of 10 nm thick silicon nanowires, from 10 to 100
nm long, sandwiched between two bulk reservoirs. We find that thermal transport
in devices differs significantly from that of suspended extended nanowires, due
to phonon scattering at the contact interfaces. We show that thermal
conductance and the phonon transport regime can be tuned from ballistic to
diffusive by varying the surface roughness of the nanowires and their length.
In devices containing short crystalline wires phonon tunneling occurs and
enhances the conductance beyond that of single contacts.Comment: 5 pages, 5 figure
Inviscid coalescence of drops
We study the coalescence of two drops of an ideal fluid driven by surface
tension. The velocity of approach is taken to be zero and the dynamical effect
of the outer fluid (usually air) is neglected. Our approximation is expected to
be valid on scales larger than , which is for water. Using a high-precision boundary integral method, we show that
the walls of the thin retracting sheet of air between the drops reconnect in
finite time to form a toroidal enclosure. After the initial reconnection,
retraction starts again, leading to a rapid sequence of enclosures. Averaging
over the discrete events, we find the minimum radius of the liquid bridge
connecting the two drops to scale like
Rayleigh-B\'enard convection with a melting boundary
We study the evolution of a melting front between the solid and liquid phases
of a pure incompressible material where fluid motions are driven by unstable
temperature gradients. In a plane layer geometry, this can be seen as classical
Rayleigh-B\'enard convection where the upper solid boundary is allowed to melt
due to the heat flux brought by the fluid underneath. This free-boundary
problem is studied numerically in two dimensions using a phase-field approach,
classically used to study the melting and solidification of alloys, which we
dynamically couple with the Navier-Stokes equations in the Boussinesq
approximation. The advantage of this approach is that it requires only moderate
modifications of classical numerical methods. We focus on the case where the
solid is initially nearly isothermal, so that the evolution of the topography
is related to the inhomogeneous heat flux from thermal convection, and does not
depend on the conduction problem in the solid. From a very thin stable layer of
fluid, convection cells appears as the depth -- and therefore the effective
Rayleigh number of the layer increases. The continuous melting of the solid
leads to dynamical transitions between different convection cell sizes and
topography amplitudes. The Nusselt number can be larger than its value for a
planar upper boundary, due to the feedback of the topography on the flow, which
can stabilize large-scale laminar convection cells.Comment: 36 pages, 16 figure
Curvature singularity and film-skating during drop impact
We study the influence of the surrounding gas in the dynamics of drop impact
on a smooth surface. We use an axisymmetric 3D model for which both the gas and
the liquid are incompressible; lubrication regime applies for the gas film
dynamics and the liquid viscosity is neglected. In the absence of surface
tension a finite time singularity whose properties are analysed is formed and
the liquid touches the solid on a circle. When surface tension is taken into
account, a thin jet emerges from the zone of impact, skating above a thin gas
layer. The thickness of the air film underneath this jet is always smaller than
the mean free path in the gas suggesting that the liquid film eventually wets
the surface. We finally suggest an aerodynamical instability mechanism for the
splash.Comment: 5 figure
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