6,401 research outputs found
On the Azimuthal Stability of Shock Waves around Black Holes
Analytical studies and numerical simulations of time dependent axially
symmetric flows onto black holes have shown that it is possible to produce
stationary shock waves with a stable position both for ideal inviscid and for
moderately viscous accretion disks.
We perform several two dimensional numerical simulations of accretion flows
in the equatorial plane to study shock stability against non-axisymmetric
azimuthal perturbations. We find a peculiar new result. A very small
perturbation seems to produce an instability as it crosses the shock, but after
some small oscillations, the shock wave suddenly transforms into an asymmetric
closed pattern, and it stabilizes with a finite radial extent, despite the
inflow and outflow boundary conditions are perfectly symmetric. The main
characteristics of the final flow are: 1) The deformed shock rotates steadily
without any damping. It is a permanent feature and the thermal energy content
and the emitted energy vary periodically with time. 2) This behavior is also
stable against further perturbations. 3) The average shock is still very strong
and well defined, and its average radial distance is somewhat larger than that
of the original axially symmetric circular shock. 4) Shocks obtained with
larger angular momentum exhibit more frequencies and beating phenomena. 5) The
oscillations occur in a wide range of parameters, so this new effect may have
relevant observational consequences, like (quasi) periodic oscillations, for
the accretion of matter onto black holes. Typical time scales for the periods
are 0.01 and 1000 seconds for black holes with 10 and 1 million solar mass,
respectively.Comment: 15 pages, 7 figures, accepted by the Astrophysical Journa
Magnetic soft modes in the locally distorted triangular antiferromagnet alpha-CaCr2O4
In this paper we explore the phase diagram and excitations of a distorted
triangular lattice antiferromagnet. The unique two-dimensional distortion
considered here is very different from the 'isosceles'-type distortion that has
been extensively investigated. We show that it is able to stabilize a 120{\deg}
spin structure for a large range of exchange interaction values, while new
structures are found for extreme distortions. A physical realization of this
model is \alpha-CaCr2O4 which has 120{\deg} structure but lies very close to
the phase boundary. This is verified by inelastic neutron scattering which
reveals unusual roton-like minima at reciprocal space points different from
those corresponding to the magnetic order.Comment: 5 pages, 3 figures and lots of spin-wave
Mammals of Kenya’s protected areas from 1888 to 2013
Kenya is a world leader in conservation and host to one of the most diverse array of mammals on the planet. As a focus of scientific attention, it is important to be able to assess not only the current state of Kenya’s mammal communities, but also how they have changed over anthropogenic timescales. Comprehensive lists of mammal species from known areas are essential for this goal, and these also provide comparative baselines for assessing changes in mammalian diversity in the future and in the fossil record. Though there is considerable literature available for mammals inhabiting Kenyan protected areas (National Parks and Reserves), species compilation projects vary greatly in scope, completeness, agreement, and accuracy. We combine the information in these databases for Kenya and supplement them with the most up-to-date knowledge available up to November 2013. Comprehensive historical species lists were compiled from specimen lists collected during 1888–1950 in ecosystems that today correspond to 13 different protected areas. We also provide analogous modern species lists based on data collected during 1950–2012. The data sets include both large and small mammals. A master list of a total of 413 species provides ecological information including body mass, diet, feeding and shelter habitat, and activity time. Historical data are based on museum specimens and sighting records, and modern data are based on museum data as well as literature, books, field guides, written accounts, photos, and videos. We used this compilation for an analysis comparing the two data sets (excluding volant and domestic species) for six protected areas with the most complete historical records and have shown in a separate publication that species richness is preserved, but beta diversity, based on pairwise comparisons of sites in this database, is being lost over the entire study area
Hadronic vacuum polarization contribution to the anomalous magnetic moments of leptons from first principles
We compute the leading, strong-interaction contribution to the anomalous
magnetic moment of the electron, muon and tau using lattice quantum
chromodynamics (QCD) simulations. Calculations include the effects of , ,
and quarks and are performed directly at the physical values of the
quark masses and in volumes of linear extent larger than . All
connected and disconnected Wick contractions are calculated. Continuum limits
are carried out using six lattice spacings. We obtain
,
and
, where the first error
is statistical and the second is systematic.Comment: 17 pages, 8 figures (in 13 PDF files), RevTeX 4.1. Minor changes to
results and to text. References updated. Matches version published in
Physical Review Letter
QCD thermodynamics with continuum extrapolated dynamical overlap fermions
We study the finite temperature transition in QCD with two flavors of dynamical fermions at a pseudoscalar pion mass of about 350 MeV. We use lattices with temporal extent of =8, 10 and 12. For the first time in the literature a continuum limit is carried out for several observables with dynamical overlap fermions. These findings are compared with results obtained within the staggered fermion formalism at the same pion masses and extrapolated to the continuum limit. The presented results correspond to fixed topology and its effect is studied in the staggered case. Nice agreement is found between the overlap and staggered results
Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays
We study the role of dissipation and structural defects on the time evolution
of quantum dot arrays with mobile charges under external driving fields. These
structures, proposed as quantum dot cellular automata, exhibit interesting
quantum dynamics which we describe in terms of equations of motion for the
density matrix. Using an open system approach, we study the role of asymmetries
and the microscopic electron-phonon interaction on the general dynamical
behavior of the charge distribution (polarization) of such systems. We find
that the system response to the driving field is improved at low temperatures
(and/or weak phonon coupling), before deteriorating as temperature and
asymmetry increase. In addition to the study of the time evolution of
polarization, we explore the linear entropy of the system in order to gain
further insights into the competition between coherent evolution and
dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR
Ab initio calculation of the neutron-proton mass difference
The existence and stability of atoms rely on the fact that neutrons are more massive than protons. The measured mass difference is only 0.14% of the average of the two masses. A slightly smaller or larger value would have led to a dramatically different universe. Here, we show that this difference results from the competition between electromagnetic and mass isospin breaking effects. We performed lattice quantum-chromodynamics and quantum-electrodynamics computations with four nondegenerate Wilson fermion flavors and computed the neutron-proton mass-splitting with an accuracy of 300 kilo-electron volts, which is greater than 0 by 5 standard deviations. We also determine the splittings in the Sigma, Xi, D, and Xi(cc) isospin multiplets, exceeding in some cases the precision of experimental measurements
Secular trends in the initiation of therapy in secondary fracture prevention in Europe : a multi-national cohort study including data from Denmark, Catalonia, and the United Kingdom
Altres ajuts: UCB funded this study. All analyses were conducted independently by the academic researchers involved. MKJ is supported by the National Institute for Health Research (NIHR) Oxford Biomedical Research Centre (BRC).This paper demonstrates a large post-fracture anti-osteoporosis treatment gap in the period 2005 to 2015. The gap was stable in Denmark at around 88-90%, increased in Catalonia from 80 to 88%, and started to increase in the UK towards the end of our study. Improved post-fracture care is needed. Patients experiencing a fragility fracture are at high risk of subsequent fractures, particularly within the first 2 years after the fracture. Previous studies have demonstrated that only a small proportion of fracture patients initiate therapy with an anti-osteoporotic medication (AOM), despite the proven fracture risk reduction of such therapies. The aim of this paper is to evaluate the changes in this post-fracture treatment gap across three different countries from 2005 to 2015. This analysis, which is part of a multinational cohort study, included men and women, aged 50 years or older, sustaining a first incident fragility fracture. Using routinely collected patient data from three administrative health databases covering Catalonia, Denmark, and the United Kingdom, we estimated the treatment gap as the proportion of patients not treated with AOM within 1 year of their first incident fracture. A total of 648,369 fracture patients were included. Mean age 70.2-78.9 years; 22.2-31.7% were men. In Denmark, the treatment gap was stable at approximately 88-90% throughout the 2005 to 2015 time period. In Catalonia, the treatment gap increased from 80 to 88%. In the UK, an initially decreasing treatment gap-though never smaller than 63%-was replaced by an increasing gap towards the end of our study. The gap was more pronounced in men than in women. Despite repeated calls for improved secondary fracture prevention, an unacceptably large treatment gap remains, with time trends indicating that the problem may be getting worse in recent years. The online version of this article (10.1007/s00198-020-05358-4) contains supplementary material, which is available to authorized users
Proton drip-line nuclei in relativistic mean-field theory
The position of the two-proton drip line has been calculated for even-even
nuclei with in the framework of the relativistic mean-field
(RMF) theory. The current model uses the NL3 effective interaction in the
mean-field Lagrangian and describes pairing correlations in the
Bardeen-Cooper-Schrieffer (BCS) formalism. The predictions of the RMF theory
are compared with those of the Hartree-Fock+BCS approach (with effective force
Skyrme SIII) and the finite-range droplet model (FRDM) and with the available
experimental information.Comment: 18 pages, RevTeX, 2 p.s figures, to appear in Phys. Rev.
Jahn-Teller versus quantum effects in the spin-orbital material LuVO3
We report on combined neutron and resonant x-ray scattering results,
identifying the nature of the spin-orbital ground state and magnetic
excitations in LuVO3 as driven by the orbital parameter. In particular, we
distinguish between models based on orbital Peierls dimerization, taken as a
signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor
of the latter. In order to solve this long-standing puzzle, polarized neutron
beams were employed as a prerequisite in order to solve details of the magnetic
structure, which allowed quantitative intensity-analysis of extended magnetic
excitation data sets. The results of this detailed study enabled us to draw
definite conclusions about classical vs quantum behavior of orbitals in this
system and to discard the previous claims about quantum effects dominating the
orbital physics of LuVO3 and similar systems.Comment: Phys. Rev. B 91, 161104(R) (2015
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