159 research outputs found
Room temperature coherent spin alignment of silicon vacancies in 4H- and 6H-SiC
We report the realization of the optically induced inverse population of the
ground-state spin sublevels of the silicon vacancies () in
silicon carbide (SiC) at room temperature. The data show that the probed
silicon vacancy spin ensemble can be prepared in a coherent superposition of
the spin states. Rabi nutations persist for more than 80 s. Two opposite
schemes of the optical alignment of the populations between the ground-state
spin sublevels of the silicon vacancy upon illumination with unpolarized light
are realized in 4H- and 6H-SiC at room temperature. These altogether make the
silicon vacancy in SiC a very favorable defect for spintronics, quantum
information processing, and magnetometry.Comment: 4 pages, 3 picture
Clustering of Primordial Black Holes. II. Evolution of Bound Systems
Primordial Black Holes (PBHs) that form from the collapse of density
perturbations are more clustered than the underlying density field. In a
previous paper, we showed the constraints that this has on the prospects of PBH
dark matter. In this paper we examine another consequence of this clustering:
the formation of bound systems of PBHs in the early universe. These would
hypothetically be the earliest gravitationally collapsed structures, forming
when the universe is still radiation dominated. Depending upon the size and
occupation of the clusters, PBH merging occurs before they would have otherwise
evaporated due to Hawking evaporation.Comment: 23 pages, 1 figure. Submitted to PR
Generalised constraints on the curvature perturbation from primordial black holes
Primordial black holes (PBHs) can form in the early Universe via the collapse
of large density perturbations. There are tight constraints on the abundance of
PBHs formed due to their gravitational effects and the consequences of their
evaporation. These abundance constraints can be used to constrain the
primordial power spectrum, and hence models of inflation, on scales far smaller
than those probed by cosmological observations. We compile, and where relevant
update, the constraints on the abundance of PBHs before calculating the
constraints on the curvature perturbation, taking into account the growth of
density perturbations prior to horizon entry. We consider two simple
parameterizations of the curvature perturbation spectrum on the scale of
interest: constant and power-law. The constraints from PBHs on the amplitude of
the power spectrum are typically in the range 10^{-2}-10^{-1} with some scale
dependence.Comment: 10 pages, 2 figures, version to appear in Phys. Rev. D with minor
change to calculation of constraints for spectral index not equal to on
Cosmological constraints on primordial black holes produced in the near-critical gravitational collapse
The mass function of primordial black holes created through the near-critical
gravitational collapse is calculated in a manner fairly independent of the
statistical distribution of underlying density fluctuation, assuming that it
has a sharp peak on a specific scale. Comparing it with various cosmological
constraints on their mass spectrum, some newly excluded range is found in the
volume fraction of the region collapsing into black holes as a function of the
horizon mass.Comment: 9 pages. Typos corrected. To appear in Physical Review
Blue spectra and induced formation of primordial black holes
We investigate the statistical properties of primordial black hole (PBH)
formation in the very early Universe. We show that the high level of
inhomogeneity of the early Universe leads to the formation of the first
generation PBHs. %The existence of these PBHs This causes later the appearance
of a dust-like phase of the cosmological expansion. We discuss here a new
mechanism for the second generation of PBH formation during the dust-like
phase. This mechanism is based on the coagulation process. We demonstrate that
the blue power spectrum of initial adiabatic perturbations after inflation
leads to overproduction of primordial black holes with gg if the power index is .Comment: 16 pages, 2 figure
Primordial black hole constraints in cosmologies with early matter domination
Moduli fields, a natural prediction of any supergravity and
superstring-inspired supersymmetry theory, may lead to a prolonged period of
matter domination in the early Universe. This can be observationally viable
provided the moduli decay early enough to avoid harming nucleosynthesis. If
primordial black holes form, they would be expected to do so before or during
this matter dominated era. We examine the extent to which the standard
primordial black hole constraints are weakened in such a cosmology. Permitted
mass fractions of black holes at formation are of order , rather than
the usual or so. If the black holes form from density perturbations
with a power-law spectrum, its spectral index is limited to ,
rather than the obtained in the standard cosmology.Comment: 7 pages RevTeX file with four figures incorporated (uses RevTeX and
epsf). Also available by e-mailing ARL, or by WWW at
http://star-www.maps.susx.ac.uk/papers/infcos_papers.htm
Primordial black holes in braneworld cosmologies: astrophysical constraints
In two recent papers we explored the modifications to primordial black hole
physics when one moves to the simplest braneworld model, Randall--Sundrum type
II. Both the evaporation law and the cosmological evolution of the population
can be modified, and additionally accretion of energy from the background can
be dominant over evaporation at high energies. In this paper we present a
detailed study of how this impacts upon various astrophysical constraints,
analyzing constraints from the present density, from the present high-energy
photon background radiation, from distortion of the microwave background
spectrum, and from processes affecting light element abundances both during and
after nucleosynthesis. Typically, the constraints on the formation rate of
primordial black holes weaken as compared to the standard cosmology if black
hole accretion is unimportant at high energies, but can be strengthened in the
case of efficient accretion.Comment: 17 pages RevTeX4 file with three figures incorporated; final paper in
series astro-ph/0205149 and astro-ph/0208299. Minor changes to match version
accepted by Physical Review
Supersymmetry and primordial black hole abundance constraints
We study the consequences of supersymmetry for primordial black hole (PBH)
abundance constraints. PBHs with mass less than about 10^{11}g will emit
supersymmetric particles when they evaporate. In most models of supersymmetry
the lightest of these particles, the lightest supersymmetric particle (LSP), is
stable and will hence survive to the present day. We calculate the limit on the
initial abundance of PBHs from the requirement that the present day LSP density
is less than the critical density. We apply this limit, along with those
previously obtained from the effects of PBH evaporation on nucleosynthesis and
the present day density of PBHs, to PBHs formed from the collpase of
inflationary density perturbations, in the context of supersymmetric inflation
models. If the reheat temperature after inflation is low, so as to avoid the
overproduction of gravitinos and moduli, then the lightest PBHs which are
produced in significant numbers will be evaporating around the present day and
there are therefore no constraints from the effects of the evaporation products
on nucleosynthesis or from the production of LSPs. We then examine models with
a high reheat temperature and a subsequent period of thermal inflation. In
these models avoiding the overproduction of LSPs limits the abundance of low
mass PBHs which were previously unconstrained. Throughout we incorporate the
production, at fixed time, of PBHs with a range of masses, which occurs when
critical collapse is taken into account.Comment: 8 pages RevTeX file with 3 figures incorporated (uses RevTeX and
epsf). Version to appear in Phys. Rev. D: minor change to calculation and
added discussio
Structure of the silicon vacancy in 6H-SiC after annealing identified as the carbon vacancy–carbon antisite pair
We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S=1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite–carbon vacancy pair (CSi-VC) in the doubly positive charge state can explain all experimental findings. We propose that the (CSi-VC) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy
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