4,517 research outputs found
Buneman instability in a magnetized current-carrying plasma with velocity shear
Buneman instability is often driven in magnetic reconnection. Understanding
how velocity shear in the beams driving the Buneman instability affects the
growth and saturation of waves is relevant to turbulence, heating, and
diffusion in magnetic reconnection. Using a Mathieu-equation analysis for weak
cosine velocity shear together with Vlasov simulations, the effects of shear on
the kinetic Buneman instability are studied in a plasma consisting of strongly
magnetized electrons and cold unmagnetized ions. In the linearly unstable
phase, shear enhances the coupling between oblique waves and the sheared
electron beam, resulting in a wider range of unstable eigenmodes with common
lower growth rates. The wave couplings generate new features of the electric
fields in space, which can persist into the nonlinear phase when electron holes
form. Lower hybrid instabilities simultaneously occur at
with a much lower growth
rate, and are not affected by the velocity shear.Comment: Accepted by Physics of Plasm
Persistent spin current in mesoscopic ferrimagnetic spin ring
Using a semiclassical approach, we study the persistent magnetization current
of a mesoscopic ferrimagnetic ring in a nonuniform magnetic field. At zero
temperature, there exists persistent spin current because of the quantum
fluctuation of magnons, similar to the case of an antiferromagnetic spin ring.
At low temperature, the current shows activation behavior because of the
field-induced gap. At higher temperature, the magnitude of the spin current is
proportional to temperature T, similar to the reported result of a
ferromagnetic spin ring.Comment: 6 pages, 3 figures, one more reference adde
Robotic flow shop scheduling with parallel machines and no-wait constraints in an aluminium anodising plant with the CMAES algorithm
Player valuation in thin markets: the case of European Association football
The amount of money in football is staggering and is a concern for people of all walks of life. While these concerns are valid, the money in football is justified and consumers of football as a form of entertainment, actively participate in the set-up of this labour market. Thanks to the availability of market value, wage, and transfer fee data for the most valued production workers (players) and bolstered by the emergence of data analytics firms to crunch large amounts of performance data in real time, it is possible to analyse and better understand the monetary worth of the most talented players, and the role of each stakeholder in the buildup of this value. This 3-essay series uses Mincer’s (1985) human capital formulation and multilevel regression analyses to provide a complete study of the different money centers that underlie player valuation.Essay 1 analyses player market values – values attributed by football fans via crowd-sourced open forums online. Market values (Transfermarkt values) that are used in actual transfer and salary negotiations are driven by both football and non-football related factors. From a sample of 500 offensive player observations in the big 5 European leagues for the 2017/18 and 2018/19 seasons, this essay analyses 12 data points per player observation, hence 6,000 data points in total, using a series of multilevel regression models to isolate the proportion of player market value based solely on talent (performance and demographic). Results show that the proportion of market value due to talent decreases as market value increases. For the players sampled, the mean impact of talent on overall market value is 77%. Essay 2 analyses the transfer fee premia. The difference between the amount paid for the transfer of a football player and his crowd-sourced market valuation at the time of transfer (transfer premium) is dependent on several factors some of which are not measurable. This essay analyses 30 top transfers per season over the decade 2011 – 2020 and shows that buying clubs exhibit risk tolerance in that they spend a sizeable premium on young promising players compared to mature players with proven talent. The breach of a player’s current contract and player’s overall performance rating during the previous season also play significant roles in the size of the transfer premium.Essay 3 looks at the top end of the football market valuation and shows that there are no diminishing returns on player wages as age increases. An analysis of the 90th percentile of football players in Europe’s ‘big 5’ leagues, ranked by Transfermarkt market value, shows that mature players earn 112% more than young players, while mid age players earn 64% more than young players. Transfers in this market segment come with a wage penalty, but compared to young players, mature players get an offset. Player performance and minutes played in the preceding season do not matter much in wage determination as players in this market segment already have reputation built over the years. Player popularity has a small positive effect on the basic wage of football players compared to the impact on their bonuses and image rights.The player labour markets shows that clubs exhibit risk tolerance in player transfers by their willingness to spend huge amounts on the transfer of young players with no proven talent in the hopes that this investment will pay-off in the future. On the other hand, when it comes to wages, clubs exhibit risk aversion as they pay much higher wages to mature players with proven talent
Optical Interferometry of early-type stars with PAVO@CHARA. I. Fundamental stellar properties
We present interferometric observations of 7 main-sequence and 3 giant stars
with spectral types from B2 to F6 using the PAVO beam combiner at the CHARA
array. We have directly determined the angular diameters for these objects with
an average precision of 2.3%. We have also computed bolometric fluxes using
available photometry in the visible and infrared wavelengths, as well as
space-based ultraviolet spectroscopy. Combined with precise \textit{Hipparcos}
parallaxes, we have derived a set of fundamental stellar properties including
linear radius, luminosity and effective temperature. Fitting the latter to
computed isochrone models, we have inferred masses and ages of the stars. The
effective temperatures obtained are in good agreement (at a 3% level) with
nearly-independent temperature estimations from spectroscopy. They validate
recent sixth-order polynomial (B-V)- empirical relations
\citep{Boyajian2012a}, but suggest that a more conservative third-order
solution \citep{vanBelle2009} could adequately describe the
(V-K)- relation for main-sequence stars of spectral type A0 and
later. Finally, we have compared mass values obtained combining surface gravity
with inferred stellar radius (\textit{gravity mass}) and as a result of the
comparison of computed luminosity and temperature values with stellar
evolutionary models (\textit{isochrone mass}). The strong discrepancy between
isochrone and gravity mass obtained for one of the observed stars,
\,Lyr, suggests that determination of the stellar atmosphere parameters
should be revised.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Effect of in-plane magnetic field on magnetic phase transitions in nu=2 bilayer quantum Hall systems
By using the effective bosonic spin theory, which is recently proposed by
Demler and Das Sarma [ Phys. Rev. Lett. 82, 3895 (1999) ], we analyze the
effect of an external in-plane magnetic field on the magnetic phase transitions
of the bilayer quantum Hall system at filling factor nu=2. It is found that the
quantum phase diagram is modified by the in-plane magnetic field. Therefore,
quantum phase transitions can be induced simply by tilting the magnetic field.
The general behavior of the critical tilted angle for different layer
separations and interlayer tunneling amplitudes is shown. We find that the
critical tilted angles being calculated agree very well with the reported
values. Moreover, a universal critical exponent for the transition from the
canted antiferromagnetic phase to the ferromagnetic phase is found to be equal
to 1/2 within the present effective theory.Comment: RevTeX, 4 pages with 3 EPS figures include
Finite-temperature phase transitions in bilayer quantum Hall systems
In this paper, the influence of an in-plane magnetic field B_\parallel on the
finite-temperature phase transitions in nu=2 bilayer quantum Hall systems are
examined. It is found that there can exist two types of finite-temperature
phase transitions. The first is the Kosterlitz-Thouless (KT) transitions, which
can have an unusual non-monotonic dependence on B_\parallel; the second type
originates from the crossing of energy levels and always increases with
B_\parallel. Based on these results, we point out that the threshold
temperature observed in the inelastic light scattering experiments cannot be
the KT transition temperature, because the latter shows a totally different
B_\parallel-dependence as compared with the experimental observation. Instead,
it should be the level-crossing temperature, which we found agrees with the
B_\parallel-dependence observed. Moreover, combining the knowledge of these two
transition temperatures, a complete finite-temperature phase diagram is
presented.Comment: RevTeX, 5 pages with 3 EPS figures include
Hysteresis effect in \nu=1 quantum Hall system under periodic electrostatic modulation
The effect of a one-dimensional periodic electrostatic modulation on quantum
Hall systems with filling factor \nu=1 is studied. We propose that, either when
the amplitude of the modulation potential or the tilt angle of the magnetic
field is varied, the system can undergo a first-order phase transition from a
fully spin-polarized homogeneous state to a partially spin-polarized
charge-density-wave state, and show hysteresis behavior of the spin
polarization. This is confirmed by our self-consistent numerical calculations
within the Hartree-Fock approximation. Finally we suggest that the \nu=1/3
fractional quantum Hall state may also show similar hysteresis behavior in the
presence of a periodic potential modulation.Comment: RevTeX, 4 page, 3 EPS figure
A comparison of weak-turbulence and PIC simulations of weak electron-beam plasma interaction
Quasilinear theory has long been used to treat the problem of a weak electron
beam interacting with plasma and generating Langmuir waves. Its extension to
weak-turbulence theory treats resonant interactions of these Langmuir waves
with other plasma wave modes, in particular ion-sound waves. These are strongly
damped in plasma of equal ion and electron temperatures, as sometimes seen in,
for example, the solar corona and wind. Weak turbulence theory is derived in
the weak damping limit, with a term describing ion-sound wave damping then
added. In this paper we use the EPOCH particle-in-cell code to numerically test
weak turbulence theory for a range of electron-ion temperature ratios. We find
that in the cold ion limit the results agree well, but increasing ion
temperature the three-wave resonance becomes broadened in proportion to the
ion-sound wave damping rate. This may be important in, for example, the theory
of solar radio bursts, where the spectrum of Langmuir waves is critical.
Additionally we establish lower limits on the number of simulation particles
needed to accurately reproduce the electron and wave distributions in their
saturated states, and to reproduce their intermediate states and time
evolution.Comment: Accepted by PO
Horn-Coupled, Commercially-Fabricated Aluminum Lumped-Element Kinetic Inductance Detectors for Millimeter Wavelengths
We discuss the design, fabrication, and testing of prototype horn-coupled,
lumped-element kinetic inductance detectors (LEKIDs) designed for cosmic
microwave background (CMB) studies. The LEKIDs are made from a thin aluminum
film deposited on a silicon wafer and patterned using standard
photolithographic techniques at STAR Cryoelectronics, a commercial device
foundry. We fabricated twenty-element arrays, optimized for a spectral band
centered on 150 GHz, to test the sensitivity and yield of the devices as well
as the multiplexing scheme. We characterized the detectors in two
configurations. First, the detectors were tested in a dark environment with the
horn apertures covered, and second, the horn apertures were pointed towards a
beam-filling cryogenic blackbody load. These tests show that the multiplexing
scheme is robust and scalable, the yield across multiple LEKID arrays is 91%,
and the noise-equivalent temperatures (NET) for a 4 K optical load are in the
range 26\thinspace\pm6 \thinspace \mu \mbox{K} \sqrt{\mbox{s}}
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