8,749 research outputs found
Molecular Dynamics in grafted layers of poly(dimethylsiloxane) (PDMS)
Dielectric relaxation spectroscopy 10^-1 Hz to 10^6 Hz) is employed to study
the molecular dynamics of poly(dimethylsiloxane) (PDMS, Mw=1.7 10^5 g/mol and
Mw=9.6 10^4 g/mol as grafted films with thicknesses d below and above the
radius of gyration Rg. For d smaller than Rg the molecular dynamics becomes
faster by up to three orders of magnitude with respect to the bulk resulting in
a pronounced decrease of the Vogel temperature T0 and hence the calorimetric
glass transition temperature Tg. For d larger than Rg the molecular dynamics is
comparable to that of the bulk melt. The results are interpreted in terms of a
chain confinement effect and compared with the findings for low molecular eight
glass forming liquids contained in nanoporous glasses and zeolites.
Crystallization effects - well known for PDMS - are observed for films of
thicknesses above and below Rg.Comment: 20 pages, 4 figure
Accelerated expansion in bosonic and fermionic 2D cosmologies with quantum effects
In this work we analyze the effects produced by bosonic and fermionic
constituents, including quantum corrections, in two-dimensional (2D)
cosmological models. We focus on a gravitational theory related to the
Callan-Giddings-Harvey-Strominger model, to simulate the dynamics of a young,
spatially-lineal, universe. The cosmic substratum is formed by an {\it
inflaton} field plus a matter component, sources of the 2D gravitational field;
the degrees of freedom also include the presence of a dilaton field. We show
that this combination permits, among other scenarios, the simulation of a
period of inflation, that would be followed by a (bosonic/fermionic) matter
dominated era. We also analyse how quantum effects contribute to the destiny of
the expansion, given the fact that in 2D we have a consistent (renormalizable)
quantum theory of gravity. The dynamical behavior of the system follows from
the solution of the gravitational field equations, the (Klein-Gordon and Dirac)
equations for the sources and the dilaton field equation. Consistent
(accelerated) regimes are present among the solutions of the 2D equations; the
results depend strongly on the initial conditions used for the dilaton field.
In the particular case where fermions are included as matter fields a
transition to a decelerated expansion is possible, something that does not
happen in the exclusively bosonic case.Comment: 6 pages, 5 figures, to appear in EP
Specific Heat of the Ca-Intercalated Graphite Superconductor CaC
The superconducting state of Ca-intercalated graphite CaC6 has been
investigated by specific heat measurements. The characteristic anomaly at the
superconducting transition (Tc = 11.4 K) indicates clearly the bulk nature of
the superconductivity. The temperature and magnetic field dependence of the
electronic specific heat are consistent with a fully-gapped superconducting
order parameter. The estimated electron-phonon coupling constant is lambda =
0.60 - 0.74 suggesting that the relatively high Tc of CaC6 can be explained
within the weak-coupling BCS approach.Comment: 4 pages, 4 figs, submitted to Phys. Rev. Let
Tidal Disruptions of Stars by Black Hole Remnants in Dense Star Clusters
In a dense stellar environment, such as the core of a globular cluster (GC),
dynamical interactions with black holes (BHs) are expected to lead to a variety
of astrophysical transients. Here we explore tidal disruption events (TDEs) of
stars by stellar-mass BHs through collisions and close encounters. Using
state-of-the-art -body simulations, we show that these TDEs occur at
significant rates throughout the evolution of typical GCs and we study how
their relative rates relate to cluster parameters such mass and size. By
incorporating a realistic cosmological model of GC formation, we predict a BH -
main-sequence-star TDE rate of approximately
in the local universe () and a cosmological rate that peaks at roughly
for redshift 3. Furthermore, we show that the
ejected mass associated with these TDEs could produce optical transients of
luminosity with timescales of about a
day to a month. These should be readily detectable by optical transient surveys
such as the Zwicky Transient Facility. Finally, we comment briefly on BH -
giant encounters and discuss how these events may contribute to the formation
of BH - white-dwarf binaries.Comment: Accepted for publication in ApJ. Comments welcom
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