5,071 research outputs found
Protostellar Feedback Processes and the Mass of the First Stars
We review theoretical models of Population III.1 star formation, focusing on
the protostellar feedback processes that are expected to terminate accretion
and thus set the mass of these stars. We discuss how dark matter annihilation
may modify this standard feedback scenario. Then, under the assumption that
dark matter annihilation is unimportant, we predict the mass of stars forming
in 12 cosmological minihalos produced in independent numerical simulations.
This allows us to make a simple estimate of the Pop III.1 initial mass function
and how it may evolve with redshift.Comment: 6 pages, Proceedings of 'The First Stars and Galaxies: Challenges for
the Next Decade", Austin, TX, March 8-11, 201
Radiation Induced Damage in GaAs Particle Detectors
The motivation for investigating the use of GaAs as a material for detecting
particles in experiments for High Energy Physics (HEP) arose from its perceived
resistance to radiation damage. This is a vital requirement for detector
materials that are to be used in experiments at future accelerators where the
radiation environments would exclude all but the most radiation resistant of
detector types.Comment: 5 pages. PS file only - original in WORD Also available at
http://ppewww.ph.gla.ac.uk/preprints/97/06
Exploring transmission Kikuchi diffraction using a Timepix detector
Electron backscatter diffraction (EBSD) is a well-established scanning electron microscope (SEM)-based technique [1]. It allows the non-destructive mapping of the crystal structure, texture, crystal phase and strain with a spatial resolution of tens of nanometers. Conventionally this is performed by placing an electron sensitive screen, typically consisting of a phosphor screen combined with a charge coupled device (CCD) camera, in front of a specimen, usually tilted 70° to the normal of the exciting electron beam. Recently, a number of authors have shown that a significant increase in spatial resolution is achievable when Kikuchi diffraction patterns are acquired in transmission geometry; that is when diffraction patterns are generated by electrons transmitted through an electron-transparent, usually thinned, specimen. The resolution of this technique, called transmission Kikuchi diffraction (TKD), has been demonstrated to be better than 10 nm [2,3]. We have recently demonstrated the advantages of a direct electron detector, Timepix [4,5], for the acquisition of standard EBSD patterns [5]. In this article we will discuss the advantages of Timepix to perform TKD and for acquiring spot diffraction patterns and more generally for acquiring scanning transmission electron microscopy micrographs in the SEM. Particularly relevant for TKD, is its very compact size, which allows much more flexibility in the positioning of the detector in the SEM chamber. We will furthermore show recent results using Timepix as a virtual forward scatter detector, and will illustrate the information derivable on producing images through processing of data acquired from different areas of the detector. We will show results from samples ranging from gold nanoparticles to nitride semiconductor nanorods
Optical diode based on the chirality of guided photons
Photons are nonchiral particles: their handedness can be both left and right.
However, when light is transversely confined, it can locally exhibit a
transverse spin whose orientation is fixed by the propagation direction of the
photons. Confined photons thus have chiral character. Here, we employ this to
demonstrate nonreciprocal transmission of light at the single-photon level
through a silica nanofibre in two experimental schemes. We either use an
ensemble of spin-polarised atoms that is weakly coupled to the nanofibre-guided
mode or a single spin-polarised atom strongly coupled to the nanofibre via a
whispering-gallery-mode resonator. We simultaneously achieve high optical
isolation and high forward transmission. Both are controlled by the internal
atomic state. The resulting optical diode is the first example of a new class
of nonreciprocal nanophotonic devices which exploit the chirality of confined
photons and which are, in principle, suitable for quantum information
processing and future quantum optical networks
The Biermann Battery in Cosmological MHD Simulations of Population III Star Formation
We report the results of the first self-consistent three-dimensional adaptive
mesh refinement magnetohydrodynamical simulations of Population III star
formation including the Biermann Battery effect. We find that the Population
III stars formed including this effect are both qualitatively and
quantitatively similar to those from hydrodynamics-only (non-MHD) cosmological
simulations. We observe peak magnetic fields of ~10^-9 G in the center of our
star-forming halo at z ~ 17.55. The magnetic fields created by the Biermann
Battery effect are predominantly formed early in the evolution of the
primordial halo at low density and large spatial scales, and then grow through
compression and by shear flows. The fields seen in this calculation are never
large enough to be dynamically important (with beta >= 10^{15} at all times),
and should be considered the minimum possible fields in existence during
Population III star formation, and may be seed fields for the stellar dynamo or
the magnetorotational instability at higher densities and smaller spatial
scales.Comment: 4 pages, 3 figures; Submitted to the Astrophysical Journal Letters.
Comments welcome. Typo found (and fixed) in equation
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