9,869 research outputs found
Invariance of fluid limits for the Shortest Remaining Processing Time and Shortest Job First policies
We consider a single-server queue with renewal arrivals and i.i.d. service
times, in which the server employs either the preemptive Shortest Remaining
Processing Time (SRPT) policy, or its non-preemptive variant, Shortest Job
First (SJF). We show that for given stochastic primitives (initial condition,
arrival and service processes), the model has the same fluid limit under either
policy. In particular, we conclude that the well-known queue length optimality
of preemptive SRPT is also achieved, asymptotically on fluid scale, by the
simpler-to-implement SJF policy. We also conclude that on fluid scale, SJF and
SRPT achieve the same performance with respect to response times of the
longest-waiting jobs in the system.Comment: 24 page
Spider (Arachnida: Araneae) distribution across the timberline in the Swiss Central Alps (Alp Flix, Grisons) and three morphologically remarkable species
We collected 6251 adult epigeic spiders from the dwarf-shrub heath to subalpine coniferous forest on Alp Flix (CH, canton Grisons, 1950 m) between May 2005 and May 2006 using pitfall traps. Total species richness and activity density of all species decreased from the open land to the forest, although this pattern varied according to family. The distribution of the 102 species found indicates that the small area around a single tree at the timberline provides habitats for both open land and forest spider species as well as some possible timberline specialists. Five species were new to the canton Grisons: Centromerita bicolor, Centromerita concinna, Hilaira excisa, Meioneta alpica and Tallusia experta. Three species showed remarkable morphological characteristics and were analysed in more detail. We found males of Pelecopsis radicicola without the characteristic longitudinal depression on the raised carapace. It is shown that the males of Meioneta alpica have a considerably variable lamella characteristica, which is nevertheless distinct from the sister species Meioneta ressli. Because we found intermediate forms of the head region described for Metopobactrus prominulus and M. schenkeli, respectively, M. schenkeli is considered a syn. nov. of M. prominulus. This study shows that the known distribution and taxonomic status of various spider taxa in the Central Alps are still incomplete and further work on arthropods in remote areas should be strongly encouraged
Analytical and numerical results for the elasticity and adhesion of elastic films with arbitrary Poisson’s ratio and confinement
We present an approximate, analytical treatment for the linearly elastic response of a film with arbitrary Poisson's ratio ν, which is indented by a flat cylindrical punch while resting on a rigid foundation. Our approach is based on a simple scaling argument allowing the vast changes of the elastomer’s effective modulus E¯¯¯ with the ratio of film height h and indenter radius a to be described with a compact, analytical expression. This yields exact asymptotics for large and small reduced film heights h/a, whereby it also reproduces the observation that E¯¯¯(h/a) has a pronounced minimum for ν>0.49 at h/a≈1.6. Using Green’s function molecular dynamics (GFMD), we demonstrate that the predictions for E¯¯¯(h/a) are reasonably correct and generate accurate reference data for effective modulus and pull-off force. GFMD also reveals that the nature of surface instabilities occurring during stable crack growth as well as the crack initiation itself depend sensitively on the way how continuum mechanics is terminated at small scales, that is, on parameters beyond the two dimensionless numbers h/a and ν defining the continuum problem
Collisional effects in the formation of cold guided beams of polar molecules
High fluxes of cold polar molecules are efficiently produced by electric
guiding and velocity filtering. Here, we investigate different aspects of the
beam formation. Variations of the source parameters such as density and
temperature result in characteristic changes in the guided beam. These are
observed in the velocity distribution of the guided molecules as well as in the
dependence of the signal of guided molecules on the trapping electric field. A
model taking into account velocity-dependent collisional losses of cold
molecules in the region close to the nozzle accurately reproduces this
behavior. This clarifies an open question on the parameter dependence of the
detected signal and gives a more detailed understanding of the velocity
filtering and guiding process
Lorentz meets Fano spectral line shapes: A universal phase and its laser control
Symmetric Lorentzian and asymmetric Fano line shapes are fundamental
spectroscopic signatures that quantify the structural and dynamical properties
of nuclei, atoms, molecules, and solids. This study introduces a universal
temporal-phase formalism, mapping the Fano asymmetry parameter q to a phase
{\phi} of the time-dependent dipole-response function. The formalism is
confirmed experimentally by laser-transforming Fano absorption lines of
autoionizing helium into Lorentzian lines after attosecond-pulsed excitation.
We also prove the inverse, the transformation of a naturally Lorentzian line
into a Fano profile. A further application of this formalism amplifies
resonantly interacting extreme-ultraviolet light by quantum-phase control. The
quantum phase of excited states and its response to interactions can thus be
extracted from line-shape analysis, with scientific applications in many
branches of spectroscopy.Comment: 11 pages, 4 figure
Algebraic order and the Berezinskii-Kosterlitz-Thouless transition in an exciton-polariton gas
We observe quasi-long range coherence in a two-dimensional condensate of
exciton-polaritons. Our measurements are the first to confirm that the spatial
correlation algebraically decays with a slow power-law, whose exponent
quantitatively behaves as predicted by the Berezinskii-Kosterlitz-Thouless
theory. The exciton-polaritons are created by non-resonant optical pumping of a
micro-cavity sample with embedded GaAs quantum-wells at liquid helium
temperature. Michelson interference is used to measure the coherence of the
photons emitted by decaying exciton-polaritons
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