1,841 research outputs found
Stabilizing an atom laser using spatially selective pumping and feedback
We perform a comprehensive study of stability of a pumped atom laser in the
presence of pumping, damping and outcoupling. We also introduce a realistic
feedback scheme to improve stability by extracting energy from the condensate
and determine its effectiveness. We find that while the feedback scheme is
highly efficient in reducing condensate fluctuations, it usually does not alter
the stability class of a particular set of pumping, damping and outcoupling
parameters.Comment: 7 figure
Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2
Low energy Raman scattering of the s=1/2 spin tetrahedra system
Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an
energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated
temperatures this mode shows a soft mode-like decrease in energy pointing to an
instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2
with a presumably larger inter-tetrahedra coupling does not show such a low
energy mode. Instead its excitation spectrum and thermodynamic properties are
compatible with long range Neel-ordering. We discuss the observed effects in
the context of quantum fluctuations and competing ground states.Comment: 5 pages, 2 figures, ISSP-Kashiwa 2001, Conference on Correlated
Electron
Risk analysis for forecasting cyberattacks against connected and autonomous vehicles
Meyer, S.F., Elvik, R. & Johnsson, E. Risk analysis for forecasting cyberattacks against connected and autonomous vehicles. J Transp Secur 14, 227–247 (2021). https://doi.org/10.1007/s12198-021-00236-4A security risk analysis was conducted to identify possible cyberattacks against a future transport system consisting of autonomous and connected vehicles. Six scenarios were developed: joyriding, kidnapping, domestic abuse, autopilot manipulation, a large transport accident, and paralysis of the transport system. Even if it were possible to increase the difficulty of conducting such cyberattacks, it might be impossible to eliminate such attacks entirely. Measures that limit the consequences will therefore be necessary. Such measures include safety measures in vehicles to protect their occupants in traffic accidents and measures that make vehicles easier to remove in case they do not function.acceptedVersio
QCD on the connection machine: beyond *LISP
We report on the status of code development for a simulation of quantum chromodynamics (QCD) with dynamical Wilson fermions on the Connection Machine model CM-2. Our original code, written in Lisp, gave performance in the near-GFLOPS range. We have rewritten the most time-consuming parts of the code in the low-level programming systems CMIS, including the matrix multiply and the communication. Current versions of the code run at approximately 3.6 GFLOPS for the fermion matrix inversion, and we expect the next version to reach or exceed 5 GFLOPS
Paired atom laser beams created via four-wave mixing
A method to create paired atom laser beams from a metastable helium atom
laser via four-wave mixing is demonstrated. Radio frequency outcoupling is used
to extract atoms from a Bose Einstein condensate near the center of the
condensate and initiate scattering between trapped and untrapped atoms. The
unequal strengths of the interactions for different internal states allows an
energy-momentum resonance which leads to the creation of pairs of atoms
scattered from the zero-velocity condensate. The resulting scattered beams are
well separated from the main atom laser in the 2-dimensional transverse atom
laser profile. Numerical simulations of the system are in good agreement with
the observed atom laser spatial profiles, and indicate that the scattered beams
are generated by a four-wave mixing process, suggesting that the beams are
correlated.Comment: 5 pages, 3 figure
Cnidaria, Scleractinia, Siderastreidae, Siderastrea siderea (Ellis and Solander, 1786): Hartt Expedition and the first record of a Caribbean siderastreid in tropical Southwestern Atlantic
Samples of Siderastrea collected by the geologist C. F. Hartt during expedition to Brazil (19th century), anddeposited at the National Museum of the Natural History, Smithsonian Institution, have been re-examined. Taxonomicalanalyses resulted in the identification of a colony of S. siderea from offshore northern Bahia state. Following recentstudies, the occurrence of Caribbean siderastreids to western South Atlantic provides new criteria to assess intra- andinterpopulational morphological variation of the endemic S. stellata, refuting historical trends of synonymizations possiblybiased by long-term taxonomical misunderstandings
Synthesis of La\u3csub\u3ex\u3c/sub\u3eCe\u3csub\u3e1\u3c/sub\u3e-\u3csub\u3ex\u3c/sub\u3eB\u3csub\u3e6\u3c/sub\u3e Whiskers
The aim of this study was to investigate if LaxCe1-xB6 (0 ≤ x ≤ 1) whiskers can be produced by the carbothermal vapour–liquid–solid (VLS) growth mechanism
Evidence for an unconventional magnetic instability in the spin-tetrahedra system Cu_2Te_2O_5Br_2
Thermodynamic experiments as well as Raman scattering have been used to study
the magnetic instabilities in the spin-tetrahedra systems Cu_2Te_2O_5X_2, X=Cl
and Br. While the phase transition observed in the Cl system at T_o=18.2 K is
consistent with 3D AF ordering, the phase transition at T_o=11.3 K in the Br
system has several unusual features. We propose an explanation in terms of
weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets.Comment: 4 pages, 4 figure
Generation of directional, coherent matter beams through dynamical instabilities in Bose-Einstein condensates
We present a theoretical analysis of a coupled, two-state Bose-Einstein
condensate with non-equal scattering lengths, and show that dynamical
instabilities can be excited. We demonstrate that these instabilities are
exponentially amplified resulting in highly-directional,
oppositely-propagating, coherent matter beams at specific momenta. To
accomplish this we prove that the mean field of our system is periodic, and
extend the standard Bogoliubov approach to consider a time-dependent, but
cyclic, background. This allows us to use Floquet's theorem to gain analytic
insight into such systems, rather than employing the usual Bogoliubov-de Gennes
approach, which is usually limited to numerical solutions. We apply our theory
to the metastable Helium atom laser experiment of Dall et al. [Phys. Rev. A 79,
011601(R) (2009)] and show it explains the anomalous beam profiles they
observed. Finally we demonstrate the paired particle beams will be
EPR-entangled on formation.Comment: Corrected reference
Voidage distribution around bubbles in a fluidized bed: influence on throughflow
In this work, a new method for measuring void fraction distribution around endogenous bubbles in a 2D
fluidized bed is presented. The technique is based on illuminating a transparent-wall 2-dimensional bed with
diffuse light from the rear and recording the distribution of light that penetrates the bed. The recording is
made with a high speed video-camera, which gives frames with grey level corresponding to the light
penetration and from which the voidage distribution around the bubbles can be determined. In this way,
voidage distribution in the region very close to the bubble contour (r/Rb≲1.2) is obtained, which was not
possible in previous studies due to limitations in spatial resolution. A correlation is proposed for the voidage
at the contour of the bubble, with the voidage depending on the radial position and the polar angle ε(r, θ).
In addition, the effect of the voidage distribution on the throughflow crossing the bubbles was studied and an
increase of 20% was determined for the average bubble geometry of the more than 100 bubbles analysedPublicad
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