3,707 research outputs found
Glass transition in Ultrathin Polymer Films : A Thermal Expansion Study
Glass transition process gets affected in ultrathin films having thickness
comparable to the size of the molecules. We observe systematic broadening of
glass transition temperature (Tg) as the thickness of the polymer film reduces
below the radius of gyration but the change in the average Tg was found to be
very small. Existence of reversible negative and positive thermal expansion
below and above Tg increased the sensitivity of our thickness measurements
performed using energy dispersive x-ray reflectivity. A simple model of Tg
variation as a function of depth expected from sliding motion could explain the
results. We observe clear glass transition even for 4 nm polystyrene film that
was predicted to be absent from ellipsometry measurements of thicker films.Comment: 11 pages, 5 figure
Charge-induced spin polarization in non-magnetic organic molecule Alq
Electrical injection in organic semiconductors is a key prerequisite for the
realization of organic spintronics. Using density-functional theory
calculations we report the effect of electron transfer into the organic
molecule Alq. Our first-principles simulations show that electron injection
spontaneously spin-polarizes non-magnetic Alq with a magnetic moment
linearly increasing with induced charge. An asymmetry of the Al--N bond lengths
leads to an asymmetric distribution of injected charge over the molecule. The
spin-polarization arises from a filling of dominantly the nitrogen
orbitals in the molecule's LUMO together with ferromagnetic coupling of the
spins on the quinoline rings.Comment: Accepted 4 pages 4 figure
Energy dependent wavelength of the ion induced nanoscale ripple
Wavelength variation of ion beam induced nanoscale ripple structure has
received much attention recently due to its possible application in
nanotechnology. We present here results of Ar bombarded Si in the energy
range 50 to 140 keV to demonstrate that with beam scanning the ripple
wavelength increases with ion energy and decreases with energy for irradiation
without ion beam scanning. An expression for the energy dependence of ripple
wavelength is proposed taking into simultaneous effect of thermally activated
surface diffusion and ion induced effective surface diffusion.Comment: REVTeX (4 pages), 3 EPS figure
Sustaining supercooled mixed phase via resonant oscillations of the order parameter
We investigate the dynamics of a first order transition when the order
parameter field undergoes resonant oscillations, driven by a periodically
varying parameter of the free energy. This parameter could be a background
oscillating field as in models of pre-heating after inflation. In the context
of condensed matter systems, it could be temperature , or pressure, external
electric/magnetic field etc. We show that with suitable driving frequency and
amplitude, the system remains in a type of mixed phase, without ever completing
transition to the stable phase, even when the oscillating parameter of the free
energy remains below the corresponding critical value (for example, with
oscillating temperature, always remains below the critical temperature
). This phenomenon may have important implications. In cosmology, it will
imply prolonged mixed phase in a first order transition due to coupling with
background oscillating fields. In condensed matter systems, it will imply that
using oscillating temperature (or, more appropriately, pressure waves) one may
be able to sustain liquids in a mixed phase indefinitely at low temperatures,
without making transition to the frozen phase.Comment: 17 pages, 7 figures, Expanded version with more detail
Evolution towards and beyond accretion-induced collapse of massive white dwarfs and formation of millisecond pulsars
Millisecond pulsars (MSPs) are generally believed to be old neutron stars
(NSs), formed via type Ib/c core-collapse supernovae (SNe), which have been
spun up to high rotation rates via accretion from a companion star in a
low-mass X-ray binary (LMXB). In an alternative formation channel, NSs are
produced via the accretion-induced collapse (AIC) of a massive white dwarf (WD)
in a close binary. Here we investigate binary evolution leading to AIC and
examine if NSs formed in this way can subsequently be recycled to form MSPs
and, if so, how they can observationally be distinguished from pulsars formed
via the standard core-collapse SN channel in terms of their masses, spins,
orbital periods and space velocities. Numerical calculations with a detailed
stellar evolution code were used for the first time to study the combined pre-
and post-AIC evolution of close binaries. We investigated the mass transfer
onto a massive WD in 240 systems with three different types of non-degenerate
donor stars: main-sequence stars, red giants, and helium stars. When the WD is
able to accrete sufficient mass (depending on the mass-transfer rate and the
duration of the accretion phase) we assumed it collapses to form a NS and we
studied the dynamical effects of this implosion on the binary orbit.
Subsequently, we followed the mass-transfer epoch which resumes once the donor
star refills its Roche lobe and calculated the continued LMXB evolution until
the end. We demonstrate that the final properties of these MSPs are, in
general, remarkably similar to those of MSPs formed via the standard
core-collapse SN channel. However, the resultant MSPs created via the AIC
channel preferentially form in certain orbital period intervals. Finally, we
discuss the link between AIC and young NSs in globular clusters. Our
calculations are also applicable to progenitor binaries of SNe Ia under certain
conditions. [Abridged]Comment: 26 pages, 20 figures, 2 tables. A few references added. A&A in pres
Two-dimensional Induced Ferromagnetism
Magnetic properties of materials confined to nanometer length scales are
providing important information regarding low dimensional physics. Using
gadolinium based Langmuir-Blodgett films, we demonstrate that two-dimensional
ferromagnetic order can be induced by applying magnetic field along the
in-plane (perpendicular to growth) direction. Field dependent exchange coupling
is evident in the in-plane magnetization data that exhibit absence of
hysteresis loop and show reduction in field required to obtain saturation in
measured moment with decreasing temperature.Comment: 4 pages, 3 postscript figures, corrected paper forma
Formation of rectifier with gold nanoclusters
Gold nanoclusters encapsulated with organic molecules are of great interest
for its possible applications in the fields of molecular electronics, catalysis
and medical science. Here we demonstrate that monolayer and bilayer films of
thiol-capped gold nanoclusters can exhibit diode-like properties provided
controlled spatial asymmetry exist between two tunnel junctions used to connect
a thiol capped gold nanoclusters. Current-voltage characteristics of this
rectifier were obtained from conducting probe atomic force microscopy
measurements and also from conventional two probe resistance measurements.
Systematic x-ray reflectivity and atomic force microscopy measurements were
carried out to characterize the spatial asymmetry introduced by a monolayer of
fatty acid salt gadolinium stearate used to deposit thiol-capped gold
nanocluster molecules on hydrophilic SiO2-Si(001) substrate by Langmuir
Blodgett technique. This information was used to explain prominent
rectification observed in these nano-structured films.Comment: 13 pages, 3 figure
Construction and Analysis of Projected Deformed Products
We introduce a deformed product construction for simple polytopes in terms of
lower-triangular block matrix representations. We further show how Gale duality
can be employed for the construction and for the analysis of deformed products
such that specified faces (e.g. all the k-faces) are ``strictly preserved''
under projection. Thus, starting from an arbitrary neighborly simplicial
(d-2)-polytope Q on n-1 vertices we construct a deformed n-cube, whose
projection to the last dcoordinates yields a neighborly cubical d-polytope. As
an extension of thecubical case, we construct matrix representations of
deformed products of(even) polygons (DPPs), which have a projection to d-space
that retains the complete (\lfloor \tfrac{d}{2} \rfloor - 1)-skeleton. In both
cases the combinatorial structure of the images under projection is completely
determined by the neighborly polytope Q: Our analysis provides explicit
combinatorial descriptions. This yields a multitude of combinatorially
different neighborly cubical polytopes and DPPs. As a special case, we obtain
simplified descriptions of the neighborly cubical polytopes of Joswig & Ziegler
(2000) as well as of the ``projected deformed products of polygons'' that were
announced by Ziegler (2004), a family of 4-polytopes whose ``fatness'' gets
arbitrarily close to 9.Comment: 20 pages, 5 figure
- …