28,244 research outputs found
Ultrahigh energy neutrino scattering: an update
We update our estimates of charged and neutral current neutrino total cross
sections on isoscalar nucleons at ultrahigh energies using a global (x, Q^2)
fit, motivated by the Froissart bound, to the F_2 (electron-proton) structure
function utilizing the most recent analysis of the complete ZEUS and H1 data
sets from HERA I. Using the large Q^2, small Bjorken-x limits of the "wee"
parton model, we connect the ultrahigh energy neutrino cross sections directly
to the large Q^2, small-x extrapolation of our new fit, which we assume
saturates the Froissart bound. We compare both to our previous work, which
utilized only the smaller ZEUS data set, as well as to recent results of a
calculation using the ZEUS-S based global perturbative QCD parton distributions
using the combined HERA I results as input. Our new results substantiate our
previous conclusions, again predicting significantly smaller cross sections
than those predicted by extrapolating pQCD calculations to neutrino energies
above 10^9 GeV.Comment: 8 pages, 1 figure, 3 table
Bloch-mode analysis for retrieving effective parameters of metamaterials
We introduce a new approach for retrieving effective parameters of
metamaterials based on the Bloch-mode analysis of quasi-periodic composite
structures. We demonstrate that, in the case of single-mode propagation, a
complex effective refractive index can be assigned to the structure, being
restored by our method with a high accuracy. We employ both surface and volume
averaging of the electromagnetic fields of the dominating (fundamental) Bloch
modes to determine the Bloch and wave impedances, respectively. We discuss how
this method works for several characteristic examples, and demonstrate that
this approach can be useful for retrieval of both material and wave effective
parameters of a broad range of metamaterials.Comment: 12 pages, 10 figure
Interplay of charge, spin and lattice degrees of freedom on the spectral properties of the one-dimensional Hubbard-Holstein model
We calculate the spectral function of the one dimensional Hubbard-Holstein
model using the time dependent Density Matrix Renormalization Group (tDMRG),
focusing on the regime of large local Coulomb repulsion, and away from
electronic half-filling. We argue that, from weak to intermediate
electron-phonon coupling, phonons interact only with the electronic charge, and
not with the spin degrees of freedom. For strong electron-phonon interaction,
spinon and holon bands are not discernible anymore and the system is well
described by a spinless polaronic liquid. In this regime, we observe multiple
peaks in the spectrum with an energy separation corresponding to the energy of
the lattice vibrations (i.e., phonons). We support the numerical results by
introducing a well controlled analytical approach based on Ogata-Shiba's
factorized wave-function, showing that the spectrum can be understood as a
convolution of three contributions, originating from charge, spin, and lattice
sectors. We recognize and interpret these signatures in the spectral properties
and discuss the experimental implications.Comment: 8 pages, 7 figure
Modeling and optimal control of an energy-efficient hybrid solar air conditioning system
© 2014 Elsevier B.V. All rights reserved. The paper addresses the modeling and optimal control problem of a new hybrid solar-assisted air conditioning system developed for performance enhancement and energy efficiency improvement. To regulate the mass flow rate of the refrigerant vapor passing through a water storage tank for increasing the refrigerant's sub-cooling process at partial loads, we propose a new discharge bypass line together with an inline solenoid valve, installed after the compressor. In addition, to control the air flow rate, a variable speed drive is coupled with the condenser fan. For the control purpose, a lumped parameter model is first developed to describe the system dynamics in an explicit input-output relationship; then, a linear optimal control scheme is applied for the system's multivariable control. The system has been fully-instrumented to examine its performance under different operation conditions. The system model is then validated by extensive experimental tests. Based on the obtained dynamic model, an optimal controller is designed to minimize a quadratic cost function. Numerical algorithms, implemented in a simulation tool, are then employed to predict the energy performance of the system under transient loads. The experimental results obtained from implementation with PLC demonstrate that the newly-developed system can deliver higher system efficiency owing to amelioration of the refrigeration effect in the direct expansion evaporator and adjustment of its air flow rate. The development is thus promising for improvement of energy efficiency, enhancement of the system performance while fulfilling the cooling demand
- …