8,784 research outputs found
Novel substrates for Helium adsorption: Graphane and Graphene-Fluoride
The discovery of fullerenes has stimulated extensive exploration of the
resulting behavior of adsorbed films. Our study addresses the planar substrates
graphene-fluoride (GF) and graphane (GH) in comparison to graphene. We present
initial results concerning the potential energy, energy bands and low density
behavior of 4He and 3He films on such different surfaces. For example, while
graphene presents an adsorption potential that is qualitatively similar to that
on graphite, GF and GH yield potentials with different symmetry, a number of
adsorption sites double that on graphene/graphite and a larger corrugation for
the adatom. In the case of GF, the lowest energy band width is similar to that
on graphite but the He atom has a significantly larger effective mass and the
adsorption energy is about three time that on graphite. Implications concerning
the monolayer phase diagram of 4He are explored with the exact path integral
ground state method. A commensurate ordered state similar to the sqrt{3} x
sqrt{3} R30^o state on graphite is found the be unstable both on GF and on GH.
The ground states of submonolayer 4He on both GF and GH are superfluids with a
Bose Einstein condensate fraction of about 10%.Comment: 6 pages, 3 figures, LT26 proceedings, accepted for publication in
Journal of Physics: Conference Serie
The Fine Structure Constant and the CMB Damping Scale
The recent measurements of the Cosmic Microwave Background anisotropies at
arcminute angular scales performed by the ACT and SPT experiments are probing
the damping regime of CMB fluctuations. The analysis of these datasets
unexpectedly suggests that the effective number of relativistic degrees of
freedom is larger than the standard value of Neff = 3.04, and inconsistent with
it at more than two standard deviations. In this paper we study the role of a
mechanism that could affect the shape of the CMB angular fluctuations at those
scales, namely a change in the recombination process through variations in the
fine structure constant. We show that the new CMB data significantly improve
the previous constraints on variations of {\alpha}, with {\alpha}/{\alpha}0 =
0.984 \pm 0.005, i.e. hinting also to a more than two standard deviation from
the current, local, value {\alpha}0. A significant degeneracy is present
between {\alpha} and Neff, and when variations in the latter are allowed the
constraints on {\alpha} are relaxed and again consistent with the standard
value. Deviations of either parameter from their standard values would imply
the presence of new, currently unknown physics.Comment: 4 pages, 1 figur
Triggering the Formation of Halo Globular Clusters with Galaxy Outflows
We investigate the interactions of high-redshift galaxy outflows with
low-mass virialized (Tvir < 10,000K) clouds of primordial composition. While
atomic cooling allows star formation in larger primordial objects, such
"minihalos" are generally unable to form stars by themselves. However, the
large population of high-redshift starburst galaxies may have induced
widespread star formation in these objects, via shocks that caused intense
cooling both through nonequilibrium H2 formation and metal-line emission. Using
a simple analytic model, we show that the resulting star clusters naturally
reproduce three key features of the observed population of halo globular
clusters (GCs). First, the 10,000 K maximum virial temperature corresponds to
the ~ 10^6 solar mass upper limit on the stellar mass of GCs. Secondly, the
momentum imparted in such interactions is sufficient to strip the gas from its
associated dark matter halo, explaining why GCs do not reside in dark matter
potential wells. Finally, the mixing of ejected metals into the primordial gas
is able to explain the ~ 0.1 dex homogeneity of stellar metallicities within a
given GC, while at the same time allowing for a large spread in metallicity
between different clusters. To study this possibility in detail, we use a
simple 1D numerical model of turbulence transport to simulate mixing in
cloud-outflow interactions. We find that as the shock shears across the side of
the cloud, Kelvin-Helmholtz instabilities arise, which cause mixing of enriched
material into > 20% of the cloud. Such estimates ignore the likely presence of
large-scale vortices, however, which would further enhance turbulence
generation. Thus quantitative mixing predictions must await more detailed
numerical studies.Comment: 21 pages, 11 figures, Apj in pres
Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for
The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the
paramagnetic phase of La_{2-x}Sr_xCuO_4 (for ) due to the
injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation
time T_1 measurements. The results are discussed in the framework of the
connection between T_1 and the in-plane magnetic correlation length
. It is found that at high temperatures the system remains in
the renormalized classical regime, with a spin stiffness constant
reduced by small doping to an extent larger than the one due to Zn doping. For
the effect of doping on appears to level off. The
values for derived from T_1 for K are much larger
than the ones estimated from the temperature behavior of sublattice
magnetization in the ordered phase (). It is argued that these
features are consistent with the hypothesis of formation of stripes of
microsegregated holes.Comment: 10 pages, 3 figure
Metal Cooling in Simulations of Cosmic Structure Formation
The addition of metals to any gas can significantly alter its evolution by
increasing the rate of radiative cooling. In star-forming environments,
enhanced cooling can potentially lead to fragmentation and the formation of
low-mass stars, where metal-free gas-clouds have been shown not to fragment.
Adding metal cooling to numerical simulations has traditionally required a
choice between speed and accuracy. We introduce a method that uses the
sophisticated chemical network of the photoionization software, Cloudy, to
include radiative cooling from a complete set of metals up to atomic number 30
(Zn) that can be used with large-scale three-dimensional hydrodynamic
simulations. Our method is valid over an extremely large temperature range (10
K < T < 10^8 K), up to hydrogen number densities of 10^12 cm^-3. At this
density, a sphere of 1 Msun has a radius of roughly 40 AU. We implement our
method in the adaptive mesh refinement (AMR) hydrodynamic/N-body code, Enzo.
Using cooling rates generated with this method, we study the physical
conditions that led to the transition from Population III to Population II star
formation. While C, O, Fe, and Si have been previously shown to make the
strongest contribution to the cooling in low-metallicity gas, we find that up
to 40% of the metal cooling comes from fine-structure emission by S, when solar
abundance patterns are present. At metallicities, Z > 10^-4 Zsun, regions of
density and temperature exist where gas is both thermally unstable and has a
cooling time less than its dynamical time. We identify these doubly unstable
regions as the most inducive to fragmentation. At high redshifts, the CMB
inhibits efficient cooling at low temperatures and, thus, reduces the size of
the doubly unstable regions, making fragmentation more difficult.Comment: 19 pages, 12 figures, significant revision, including new figure
Stability of Magnetized Disks and Implications for Planet Formation
This paper considers gravitational perturbations in geometrically thin disks
with rotation curves dominated by a central object, but with substantial
contributions from magnetic pressure and tension. The treatment is general, but
the application is to the circumstellar disks that arise during the
gravitational collapse phase of star formation. We find the dispersion relation
for spiral density waves in these generalized disks and derive the stability
criterion for axisymmetric disturbances (the analog of the Toomre
parameter ) for any radial distribution of the mass-to-flux ratio
. The magnetic effects work in two opposing directions: on one hand,
magnetic tension and pressure stabilize the disk against gravitational collapse
and fragmentation; on the other hand, they also lower the rotation rate making
the disk more unstable. For disks around young stars the first effect generally
dominates, so that magnetic fields allow disks to be stable for higher surface
densities and larger total masses. These results indicate that magnetic fields
act to suppress the formation of giant planets through gravitational
instability. Finally, even if gravitational instability can form a secondary
body, it must lose an enormous amount of magnetic flux in order to become a
planet; this latter requirement represents an additional constraint for planet
formation via gravitational instability and places a lower limit on the
electrical resistivity.Comment: accepted in Ap
Exciton polaritons in two-dimensional photonic crystals
Experimental evidence of strong coupling between excitons confined in a
quantum well and the photonic modes of a two-dimensional dielectric lattice is
reported. Both resonant scattering and photoluminescence spectra at low
temperature show the anticrossing of the polariton branches, fingerprint of
strong coupling regime. The experiments are successfully interpreted in terms
of a quantum theory of exciton-photon coupling in the investigated structure.
These results show that the polariton dispersion can be tailored by properly
varying the photonic crystal lattice parameter, which opens the possibility to
obtain the generation of entangled photon pairs through polariton stimulated
scattering.Comment: 5 pages, 4 figure
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