498 research outputs found
Investigation of Single Boron Acceptors at the Cleaved Si:B (111) Surface
The cleaved and (2 x 1) reconstructed (111) surface of p-type Si is
investigated by scanning tunneling microscopy (STM). Single B acceptors are
identified due to their characteristic voltage-dependent contrast which is
explained by a local energetic shift of the electronic density of states caused
by the Coulomb potential of the negatively charged acceptor. In addition,
detailed analysis of the STM images shows that apparently one orbital is
missing at the B site at sample voltages of 0.4 - 0.6 V, corresponding to the
absence of a localized dangling-bond state. Scanning tunneling spectroscopy
confirms a strongly altered density of states at the B atom due to the
different electronic structure of B compared to Si.Comment: 6 pages, 7 figure
Moduli Stabilisation and de Sitter String Vacua from Magnetised D7 Branes
Anomalous U(1)'s are ubiquitous in 4D chiral string models. Their presence
crucially affects the process of moduli stabilisation and cannot be neglected
in realistic set-ups. Their net effect in the 4D effective action is to induce
a matter field dependence in the non-perturbative superpotential and a
Fayet-Iliopoulos D-term. We study flux compactifications of IIB string theory
in the presence of magnetised D7 branes. These give rise to anomalous U(1)'s
that modify the standard moduli stabilisation procedure. We consider simple
orientifold models to determine the matter field spectrum and the form of the
effective field theory. We apply our results to one-modulus KKLT and
multi-moduli large volume scenarios, in particular to the Calabi-Yau
P^4_{[1,1,1,6,9]}. After stabilising the matter fields, the effective action
for the Kahler moduli can acquire an extra positive term that can be used for
de Sitter lifting with non-vanishing F- and D-terms. This provides an explicit
realization of the D-term lifting proposal of hep-th/0309187.Comment: 35 pages, 1 figure. v2: Minor changes, references adde
Linear Paul trap design for an optical clock with Coulomb crystals
We report on the design of a segmented linear Paul trap for optical clock
applications using trapped ion Coulomb crystals. For an optical clock with an
improved short-term stability and a fractional frequency uncertainty of 10^-18,
we propose 115In+ ions sympathetically cooled by 172Yb+. We discuss the
systematic frequency shifts of such a frequency standard. In particular, we
elaborate on high precision calculations of the electric radiofrequency field
of the ion trap using the finite element method. These calculations are used to
find a scalable design with minimized excess micromotion of the ions at a level
at which the corresponding second- order Doppler shift contributes less than
10^-18 to the relative uncertainty of the frequency standard
Building MSSM Flux Vacua
We construct N=1 and N=0 chiral four-dimensional vacua of flux
compactification in Type IIB string theory. These vacua have the common
features that they are free of tadpole instabilities (both NSNS and RR) even
for models with N=0 supersymmetry. In addition, the dilaton/complex structure
moduli are stabilised and the supergravity background metric is warped. We
present an example in which the low energy spectrum contains the MSSM spectrum
with three generations of chiral matter. In the N=0 models, the background
fluxes which stabilise the moduli also induce soft supersymmetry breaking terms
in the gauge and chiral sectors of the theory, while satisfying the equation of
motion. We also discuss some phenomenological features of these three
generation MSSM flux vacua. Our techniques apply to other closed string
backgrounds as well and, in fact, also allow to find new N=1 D-brane models
which were believed not to exist. Finally, we discuss in detail the consistency
conditions of these flux compactifications. Cancellation of K-theory charges
puts additional constraints on the consistency of the models, which render some
chiral D-brane models in the literature inconsistent.Comment: 33 pages, 1 figure. Minor correction
Pre - Inflationary Clues from String Theory ?
"Brane supersymmetry breaking" occurs in String Theory when the only
available combinations of D-branes and orientifolds are not mutually BPS and
yet do not introduce tree-level tachyon instabilities. It is characterized by
the emergence of a steep exponential potential, and thus by the absence of
maximally symmetric vacua. The corresponding low-energy supergravity admits
intriguing spatially-flat cosmological solutions where a scalar field is forced
to climb up toward the steep potential after an initial singularity, and
additional milder terms can inject an inflationary phase during the ensuing
descent. We show that, in the resulting power spectra of scalar perturbations,
an infrared suppression is typically followed by a pre-inflationary peak that
reflects the end of the climbing phase and can lie well apart from the
approximately scale invariant profile. A first look at WMAP9 raw data shows
that, while the chi^2 fits for the low-l CMB angular power spectrum are clearly
compatible with an almost scale invariant behavior, they display nonetheless an
eye-catching preference for this type of setting within a perturbative string
regime.Comment: 34 pages, LaTeX, 16 eps figures. Relative displacement in fig. 14 and
some typos corrected, references and acknowledgments updated. To appear in
JCA
Preembryo Personhood: An Assessment of the Presidentâs Council Arguments
The Presidentâs Council on Bioethics has addressed the moral status of human preembryos in its reports on stem cell research and human therapeutic cloning. Although the Council has been criticized for being hand-picked to favor the right-to-life viewpoint concerning human preembryos, it has embraced the idea that the right-to-life position should be defended in secular terms. This is an important feature of the Councilâs work, and it demonstrates a recognition of the need for genuine engagement between opposing sides in the debate over stem cell research. To promote this engagement, the Council has stated in secular terms several arguments for the personhood of human preembryos. This essay presents and critiques those arguments, and it concludes that they are unsuccessful. If the best arguments in support of the personhood of human preembryos have been presented by the Council, then there are no reasonable secular arguments in support of that view
High-time Resolution Astrophysics and Pulsars
The discovery of pulsars in 1968 heralded an era where the temporal
characteristics of detectors had to be reassessed. Up to this point detector
integration times would normally be measured in minutes rather seconds and
definitely not on sub-second time scales. At the start of the 21st century
pulsar observations are still pushing the limits of detector telescope
capabilities. Flux variations on times scales less than 1 nsec have been
observed during giant radio pulses. Pulsar studies over the next 10 to 20 years
will require instruments with time resolutions down to microseconds and below,
high-quantum quantum efficiency, reasonable energy resolution and sensitive to
circular and linear polarisation of stochastic signals. This chapter is review
of temporally resolved optical observations of pulsars. It concludes with
estimates of the observability of pulsars with both existing telescopes and
into the ELT era.Comment: Review; 21 pages, 5 figures, 86 references. Book chapter to appear
in: D.Phelan, O.Ryan & A.Shearer, eds.: High Time Resolution Astrophysics
(Astrophysics and Space Science Library, Springer, 2007). The original
publication will be available at http://www.springerlink.co
Evidence for a mixed mass composition at the `ankle' in the cosmic-ray spectrum
We report a first measurement for ultra-high energy cosmic rays of the
correlation between the depth of shower maximum and the signal in the water
Cherenkov stations of air-showers registered simultaneously by the fluorescence
and the surface detectors of the Pierre Auger Observatory. Such a correlation
measurement is a unique feature of a hybrid air-shower observatory with
sensitivity to both the electromagnetic and muonic components. It allows an
accurate determination of the spread of primary masses in the cosmic-ray flux.
Up till now, constraints on the spread of primary masses have been dominated by
systematic uncertainties. The present correlation measurement is not affected
by systematics in the measurement of the depth of shower maximum or the signal
in the water Cherenkov stations. The analysis relies on general characteristics
of air showers and is thus robust also with respect to uncertainties in
hadronic event generators. The observed correlation in the energy range around
the `ankle' at differs significantly from
expectations for pure primary cosmic-ray compositions. A light composition made
up of proton and helium only is equally inconsistent with observations. The
data are explained well by a mixed composition including nuclei with mass . Scenarios such as the proton dip model, with almost pure compositions, are
thus disfavoured as the sole explanation of the ultrahigh-energy cosmic-ray
flux at Earth.Comment: Published version. Added journal reference and DOI. Added Report
Numbe
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
- âŠ