1,282 research outputs found
Observation of Buried Phosphorus Dopants near Clean Si(100)-(2x1) with Scanning Tunneling Microscopy
We have used scanning tunneling microscopy to identify individual phosphorus
dopant atoms near the clean silicon (100)-(2x1) reconstructed surface. The
charge-induced band bending signature associated with the dopants shows up as
an enhancement in both filled and empty states and is consistent with the
appearance of n-type dopants on compound semiconductor surfaces and passivated
Si(100)-(2x1). We observe dopants at different depths and see a strong
dependence of the signature on the magnitude of the sample voltage. Our results
suggest that, on this clean surface, the antibonding surface state band acts as
an extension of the bulk conduction band into the gap. The positively charged
dimer vacancies that have been observed previously appear as depressions in the
filled states, as opposed to enhancements, because they disrupt these surface
bands.Comment: 4 pages, 3 figures. TeX for OSX from Wierde
Importance of In-Plane Anisotropy in the Quasi Two-Dimensional Antiferromagnet BaNiVO
The phase diagram of the quasi two-dimensional antiferromagnet
BaNiVO is studied by specific heat, thermal expansion,
magnetostriction, and magnetization for magnetic fields applied perpendicular
to . At T, a crossover to a high-field state,
where increases linearly, arises from a competition of intrinsic and
field-induced in-plane anisotropies. The pressure dependences of and
are interpreted using the picture of a pressure-induced in-plane
anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot
be neglected, which implies deviations from pure
Berezinskii-Kosterlitz-Thouless behavior.Comment: 4 pages, 4 figure
Theory of Umklapp-assisted recombination of bound excitons in Si:P
We present the calculations for the oscillator strength of the recombination
of excitons bound to phosphorous donors in silicon. We show that the direct
recombination of the bound exciton cannot account for the experimentally
measured oscillator strength of the no-phonon line. Instead, the recombination
process is assisted by an umklapp process of the donor electron state. We make
use of the empirical pseudopotential method to evaluate the Umklapp-assisted
recombination matrix element in second-order perturbation theory. Our result is
in excellent agreement with the experiment. We also present two methods to
improve the optical resolution of the optical detection of the spin state of a
single nucleus in silicon.Comment: 9 pages, 6 EPS figures, Revtex
Pressure versus concentration tuning of the superconductivity in Ba(Fe(1-x)Cox)2As2
In the iron arsenide compound BaFe2As2, superconductivity can be induced
either by a variation of its chemical composition, e.g., by replacing Fe with
Co, or by a reduction of the unit-cell volume through the application of
hydrostatic pressure p. In contrast to chemical substitutions, pressure is
expected to introduce no additional disorder into the lattice. We compare the
two routes to superconductivity by measuring the p dependence of the
superconducting transition temperature Tc of Ba(Fe(1-x)Cox)2As2 single crystals
with different Co content x. We find that Tc(p) of underdoped and overdoped
samples increases and decreases, respectively, tracking quantitatively the
Tc(x) dependence. To clarify to which extent the superconductivity relies on
distinct structural features we analyze the crystal structure as a function of
x and compare the results with that of BaFe2As2 under pressure.Comment: 14 pages, 4 figures, to be published in JPSJ Vol. 79 No. 12. The
copyright is held by The Physical Society of Japa
A Search for Dark Matter Annihilation with the Whipple 10m Telescope
We present observations of the dwarf galaxies Draco and Ursa Minor, the local
group galaxies M32 and M33, and the globular cluster M15 conducted with the
Whipple 10m gamma-ray telescope to search for the gamma-ray signature of
self-annihilating weakly interacting massive particles (WIMPs) which may
constitute astrophysical dark matter (DM). We review the motivations for
selecting these sources based on their unique astrophysical environments and
report the results of the data analysis which produced upper limits on excess
rate of gamma rays for each source. We consider models for the DM distribution
in each source based on the available observational constraints and discuss
possible scenarios for the enhancement of the gamma-ray luminosity. Limits on
the thermally averaged product of the total self-annihilation cross section and
velocity of the WIMP, , are derived using conservative estimates for
the magnitude of the astrophysical contribution to the gamma-ray flux. Although
these limits do not constrain predictions from the currently favored
theoretical models of supersymmetry (SUSY), future observations with VERITAS
will probe a larger region of the WIMP parameter phase space, and
WIMP particle mass (m_\chi).Comment: 33 pages, 12 figures, accepted for publication in the Astrophysical
Journa
VERITAS Observations of the BL Lac Object 1ES 1218+304
The VERITAS collaboration reports the detection of very-high-energy (VHE)
gamma-ray emission from the high-frequency-peaked BL Lac object 1ES 1218+304
located at a redshift of z=0.182. A gamma-ray signal was detected with high
statistical significance for the observations taken during several months in
the 2006-2007 observing season. The photon spectrum between ~160 GeV and ~1.8
TeV is well described by a power law with an index of Gamma = 3.08 +/-
0.34(stat) +/- 0.2(sys). The integral flux above 200 GeV corresponds to ~6% of
that of the Crab Nebula. The light curve does not show any evidence for VHE
flux variability. Using lower limits on the density of the extragalactic
background light (EBL) in the near-IR to mid-IR we are able to limit the range
of intrinsic energy spectra for 1ES 1218+304. We show that the intrinsic photon
spectrum is harder than a power law with an index of Gamma = 2.32 +/- 0.37.
When including constraints from the spectra of 1ES 1101-232 and 1ES 0229+200,
the spectrum of 1ES 1218+304 is likely to be harder than Gamma = 1.86 +/- 0.37.Comment: Submitted to Proceedings of "4th Heidelberg International Symposium
on High Energy Gamma-Ray Astronomy 2008
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