624 research outputs found
Markov Decision Processes with Average-Value-at-Risk criteria
We investigate the problem of minimizing the Average-Value-at-Risk (AV aRr) of the discounted cost over a finite and an infinite horizon which is generated by a Markov Decision Process (MDP). We show that this problem can be reduced to an ordinary MDP with extended state space and give conditions under which an optimal policy exists. We also give a time-consistent interpretation of the AV aRr . At the end we consider a numerical example which is a simple repeated casino game. It is used to discuss the influence of the risk aversion parameter r of the AV aRr-criterion
Enhanced self-field critical current density of nano-composite YBa(2)Cu(3)O(7) thin films grown by pulsed-laser deposition
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ EPLA, 2008.Enhanced self-field critical current density Jc of novel, high-temperature superconducting thin films is reported. Layers are deposited on (001) MgO substrates by laser ablation of YBa2Cu3O7âÎŽ(Y-123) ceramics containing Y2Ba4CuMOx (M-2411, M=Ag, Nb, Ru, Zr) nano-particles. The Jc of films depends on the secondary-phase content of the ceramic targets, which was varied between 0 and 15âmol%. Composite layers (2âmol% of Ag-2411 and Nb-2411) exhibit Jc values at 77âK of up to 5.1âMA/cm2, which is 3 to 4 times higher than those observed in films deposited from phase pure Y-123 ceramics. Nb-2411 grows epitaxially in the composite layers and the estimated crystallite size is ~10ânm.The Austrian Science Fund, the Austrian Federal Ministry of Economics and Labour, the European Science Foundation and the Higher Education Commission of Pakistan
Bolometric calibration of a superfluid He detector for Dark Matter search: direct measurement of the scintillated energy fraction for neutron, electron and muon events
We report on the calibration of a superfluid He bolometer developed for
the search of non-baryonic Dark Matter. Precise thermometry is achieved by the
direct measurement of thermal excitations using Vibrating Wire Resonators
(VWRs). The heating pulses for calibration were produced by the direct quantum
process of quasiparticle generation by other VWRs present. The bolometric
calibration factor is analyzed as a function of temperature and excitation
level of the sensing VWR. The calibration is compared to bolometric
measurements of the nuclear neutron capture reaction and heat depositions by
cosmic muons and low energy electrons. The comparison allows a quantitative
estimation of the ultra-violet scintillation rate of irradiated helium,
demonstrating the possibility of efficient electron recoil event rejection.Comment: 17 pages, submitted to Nuc. Instr. Meth.
Evidence for Thermally Activated Spontaneous Fluxoid Formation in Superconducting Thin-Film Rings
We have observed spontaneous fluxoid generation in thin-film rings of the
amorphous superconductor MoSi, cooled through the normal-superconducting
transition, as a function of quench rate and externally applied magnetic field,
using a variable sample temperature scanning SQUID microscope. Our results can
be explained using a model of freezout of thermally activated fluxoids,
mediated by the transport of bulk vortices across the ring walls. This
mechanism is complementary to a mechanism proposed by Kibble and Zurek, which
only relies on causality to produce a freezout of order parameter fluctuations.Comment: 4 pages, 3 figure
Experimental Test of the Numerical Renormalization Group Theory for Inelastic Scattering from Magnetic Impurities
We present measurements of the phase coherence time \tauphi in quasi
one-dimensional Au/Fe Kondo wires and compare the temperature dependence of
\tauphi with a recent theory of inelastic scattering from magnetic impurities
(Phys. Rev. Lett. 93, 107204 (2004)). A very good agreement is obtained for
temperatures down to 0.2 . Below the Kondo temperature , the inverse
of the phase coherence time varies linearly with temperature over almost one
decade in temperature.Comment: 5 pages, 3 figure
The effective mass of two--dimensional 3He
We use structural information from diffusion Monte Carlo calculations for
two--dimensional 3He to calculate the effective mass. Static effective
interactions are constructed from the density-- and spin structure functions
using sumrules. We find that both spin-- and density-- fluctuations contribute
about equally to the effective mass. Our results show, in agreement with recent
experiments, a flattening of the single--particle self--energy with increasing
density, which eventually leads to a divergent effective mass.Comment: 4 pages, accepted in PR
Angle-dependence of the Hall effect in HgBa2CaCu2O6 thin films
Superconducting compounds of the family Hg-Ba-Ca-Cu-O have been the subject
of intense study since the current record-holder for the highest critical
temperature of a superconductor belongs to this class of materials. Thin films
of the compound with two adjacent copper-oxide layers and a critical
temperature of about 120 K were prepared by a two-step process that consists of
the pulsed-laser deposition of precursor films and the subsequent annealing in
mercury-vapor atmosphere. Like some other high-temperature superconductors,
Hg-Ba-Ca-Cu-O exhibits a specific anomaly of the Hall effect, a double-sign
change of the Hall coefficient close to the superconducting transition. We have
investigated this phenomenon by measurements of the Hall effect at different
angles between the magnetic field direction and the crystallographic c-axis.
The results concerning the upper part of the transition, where the first sign
change occurs, are discussed in terms of the renormalized fluctuation model for
the Hall conductivity, adapted through the field rescaling procedure in order
to take into account the arbitrary orientation of the magnetic field.Comment: to be published in Phys. Rev.
Contrasting the beam interaction characteristics of selected lasers with a partially stabilised zirconia (PSZ) bio-ceramic
Differences in the beam interaction characteristics of a CO2 laser, a Nd:YAG laser, a high power diode laser (HPDL) and an excimer laser with a partially stabilised zirconia (PSZ) bio-ceramic have been studied. A derivative of Beer-Lambertâs law was applied and the laser beam absorption lengths of the four lasers were calculated as 33.55 x 10-3 cm for the CO2 laser, 18.22 x 10-3 cm for the Nd:YAG laser, 17.17 x 10-3 cm for the HPDL and 8.41 x 10-6 cm for the excimer laser. It was determined graphically that the fluence threshold values at which significant material removal was effected by the CO2 laser, the Nd:YAG laser, the HPDL and the excimer laser were 52 J/cm2, 97 J/cm2, 115 J/cm2 and 0.48 J/cm2 respectively. The thermal loading value for the CO2 laser, the Nd:YAG laser, the HPDL and the excimer laser were calculated as being 1.55 kJ/cm3, 5.32 kJ/cm3, 6.69 kJ/cm3 and 57.04 kJ/cm3 respectively
Transmission Phase in the Kondo Regime Revealed in a Two-Path Interferometer
We report on the direct observation of the transmission phase shift through a
Kondo correlated quantum dot by employing a new type of two-path
interferometer. We observed a clear -phase shift, which persists up to
the Kondo temperature . Above this temperature, the phase shifts by
more than at each Coulomb peak, approaching the behavior observed for
the standard Coulomb blockade regime. These observations are in remarkable
agreement with 2-level numerical renormalization group calculations. The unique
combination of experimental and theoretical results presented here fully
elucidates the phase evolution in the Kondo regime.Comment: 4 pages, 3 figure
Are there static texture?
We consider harmonic maps from Minkowski space into the three sphere. We are
especially interested in solutions which are asymptotically constant, i.e.
converge to the same value in all directions of spatial infinity. Physical
3-space can then be compactified and can be identified topologically (but not
metrically!) with a three sphere. Therefore, at fixed time, the winding of the
map is defined. We investigate whether static solutions with non-trivial
winding number exist. The answer which we can proof here is only partial: We
show that within a certain family of maps no static solutions with non-zero
winding number exist. We discuss the existing static solutions in our family of
maps. An extension to other maps or a proof that our family of maps is
sufficiently general remains an open problem.Comment: 12 page Latex file, 1 postscript figure, submitted to PR
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