356 research outputs found
Optical lattice quantum simulator for QED in strong external fields: spontaneous pair creation and the Sauter-Schwinger effect
Spontaneous creation of electron-positron pairs out of the vacuum due to a
strong electric field is a spectacular manifestation of the relativistic
energy-momentum relation for the Dirac fermions. This fundamental prediction of
Quantum Electrodynamics (QED) has not yet been confirmed experimentally as the
generation of a sufficiently strong electric field extending over a large
enough space-time volume still presents a challenge. Surprisingly, distant
areas of physics may help us to circumvent this difficulty. In condensed matter
and solid state physics (areas commonly considered as low energy physics), one
usually deals with quasi-particles instead of real electrons and positrons.
Since their mass gap can often be freely tuned, it is much easier to create
these light quasi-particles by an analogue of the Sauter-Schwinger effect. This
motivates our proposal of a quantum simulator in which excitations of
ultra-cold atoms moving in a bichromatic optical lattice represent particles
and antiparticles (holes) satisfying a discretized version of the Dirac
equation together with fermionic anti-commutation relations. Using the language
of second quantization, we are able to construct an analogue of the spontaneous
pair creation which can be realized in an (almost) table-top experiment.Comment: 21 pages, 10 figure
Tails for the Einstein-Yang-Mills system
We study numerically the late-time behaviour of the coupled Einstein
Yang-Mills system. We restrict ourselves to spherical symmetry and employ
Bondi-like coordinates with radial compactification. Numerical results exhibit
tails with exponents close to -4 at timelike infinity and -2 at future
null infinity \Scri.Comment: 12 pages, 5 figure
Saddle-point dynamics of a Yang-Mills field on the exterior Schwarzschild spacetime
We consider the Cauchy problem for a spherically symmetric SU(2) Yang-Mills
field propagating outside the Schwarzschild black hole. Although solutions
starting from smooth finite energy initial data remain smooth for all times,
not all of them scatter since there are non-generic solutions which
asymptotically tend to unstable static solutions. We show that a static
solution with one unstable mode appears as an intermediate attractor in the
evolution of initial data near a border between basins of attraction of two
different vacuum states. We study the saddle-point dynamics near this
attractor, in particular we identify the universal phases of evolution: the
ringdown approach, the exponential departure, and the eventual decay to one of
the vacuum states.Comment: 15 pages, 5 figure
Late-time tails of a self-gravitating massless scalar field, revisited
We discuss the nonlinear origin of the power-law tail in the long-time
evolution of a spherically symmetric self-gravitating massless scalar field in
even-dimensional spacetimes. Using third-order perturbation method, we derive
explicit expressions for the tail (the decay rate and the amplitude) for
solutions starting from small initial data and we verify this prediction via
numerical integration of the Einstein-scalar field equations in four and six
dimensions. Our results show that the coincidence of decay rates of linear and
nonlinear tails in four dimensions (which has misguided some tail hunters in
the past) is in a sense accidental and does not hold in higher dimensions.Comment: 10 pages, 6 figures, one reference added, updated to conform with
published versio
Late-time tails of a Yang-Mills field on Minkowski and Schwarzschild backgrounds
We study the late-time behavior of spherically symmetric solutions of the
Yang-Mills equations on Minkowski and Schwarzschild backgrounds. Using
nonlinear perturbation theory we show in both cases that solutions having
smooth compactly supported initial data posses tails which decay as at
timelike infinity. Moreover, for small initial data on Minkowski background we
derive the third-order formula for the amplitude of the tail and confirm
numerically its accuracy.Comment: 7 pages, 3 figure
Feasibility study of biogas upgrading coupled with nutrient removal from anaerobic effluents using microalgae-based processes
Producción CientÃficaThe present research was conducted to simultaneously optimize biogas upgrading and carbon and nutrient removal from centrates in a 180-L high-rate algal pond interconnected to an external CO2 absorption unit. Different biogas and centrate supply strategies were assessed to increase biomass lipid content. Results showed 99 % CO2 removal efficiencies from simulated biogas at liquid recirculation rates in the absorption column of 9.9 m3 m−2 h−1, concomitant with nitrogen and phosphorus removal efficiencies of 100 and 82 %, respectively, using a 1:70 diluted centrate at a hydraulic retention time of 7 days. The lipid content of the harvested algal–bacterial biomass remained low (2.9–11.2 %) regardless of the operational conditions, with no particular trend over time. The good settling characteristics of the algal–bacterial flocs resulted in harvesting efficiencies over 95 %, which represents a cost-effective alternative for algal biomass reutilization compared to conventional physical–chemical techniques. Finally, high microalgae biodiversity was found regardless of the operational conditions.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. (Project GR76, VA024U14, and RTA2013-00056-C03-02
Octupole transitions in the 208Pb region
The 208Pb region is characterised by the existence of collective octupole states.
Here we populated such states in 208Pb + 208Pb deep-inelastic reactions. γ-ray angular
distribution measurements were used to infer the octupole character of several E3 transitions.
The octupole character of the 2318 keV 17− → 14+ in 208Pb, 2485 keV 19/2
− → 13/2
+ in
207Pb, 2419 keV 15/2
− → 9/2
+ in 209Pb and 2465 keV 17/2
+ → 11/2
− in 207Tl transitions was
demonstrated for the first time. In addition, shell model calculations were performed using two
different sets of two-body matrix elements. Their predictions were compared with emphasis on
collective octupole states.This work is supported by the Science and Technology Facilities Council
(STFC), UK, US Department of Energy, Office of Nuclear Physics, under Contract No. DEAC02-06CH11357
and DE-FG02-94ER40834, NSF grant PHY-1404442
Thermal Analysis of Potted Litz Wire for High-Power-Density Aerospace Electric Machines
Increasing the power density and efficiency of electric machines (motors and generators) is integral to bringing Electrified Aircraft (EA) to commercial realization. To that end an effort to create a High Efficiency Megawatt Motor (HEMM) with a goal of exceeding 98% efficiency and 1.46 MW of power has been undertaken at the NASA Glenn Research Center. Of the motor components the resistive losses in the stator windings are by far the largest contributor (34%) to total motor loss. The challenge is the linear relationship between resistivity and temperature, making machine operation sensitive to temperature increases. In order to accurately predict the thermal behavior of the stator the thermal conductivity of the Litz wire-potting-electrical insulation system must be known. Unfortunately, this multi material system has a wide range of thermal conductivities (0.1 W/m-K 400 W/m-K) and a high anisotropy (axial vs transverse) making the prediction of the transverse thermal conductivity an in turn the hot spot temperatures in the windings is difficult. In order to do this a device that simulates the thermal environment found in the HEMM stator was designed. This device is not unlike the motorettes (little motors) that are described in IEEE standards for testing electrical insulation lifetimes or other electric motor testing. However, because the HEMM motor design includes significant rotor electrical and thermal considerations the term motorette was not deemed appropriate. Instead statorette (or little stator) was adopted as the term for this test device. This paper discussed the design, thermal heat conjugate analysis (thermal model), manufacturing and testing of HEMM's statorette. Analysis of the results is done by thermal resistance network model and micro thermal model and is compared to analytical predictions of thermal conductivity of the insulated and potted Litz wire system
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