Parameter identification in a semilinear hyperbolic system

We consider the identification of a nonlinear friction law in a one-dimensional damped wave equation from additional boundary measurements. Well-posedness of the governing semilinear hyperbolic system is established via semigroup theory and contraction arguments. We then investigte the inverse problem of recovering the unknown nonlinear damping law from additional boundary measurements of the pressure drop along the pipe. This coefficient inverse problem is shown to be ill-posed and a variational regularization method is considered for its stable solution. We prove existence of minimizers for the Tikhonov functional and discuss the convergence of the regularized solutions under an approximate source condition. The meaning of this condition and some arguments for its validity are discussed in detail and numerical results are presented for illustration of the theoretical findings

A search for soft X-ray emission associated with prominent high-velocity-cloud complexes

We correlate the ROSAT 1/4 keV all-sky survey with the Leiden/Dwingeloo HI survey, looking for soft X-ray signatures of prominent high-velocity-cloud (HVC) complexes. We study the transfer of 1/4 keV photons through the interstellar medium in order to distinguish variations in the soft X-ray background (SXRB) intensity caused by photoelectric absorption effects from those due to excess X-ray emission. The X-ray data are modelled as a combination of emission from the Local Hot Bubble (LHB) and emission from a distant plasma in the galactic halo and extragalactic sources. The X-ray radiation intensity of the galactic halo and extragalactic X-ray background is modulated by the photoelectric absorption of the intervening galactic interstellar matter. We show that large- and small-scale intensity variations of the 1/4 keV SXRB are caused by photoelectric absorption which is predominantly traced by the total N(HI) distribution. The extensive coverage of the two surveys supports evidence for a hot, X-ray emitting corona. We show that this leads to a good representation of the SXRB observations. For four large areas on the sky, we search for regions where the modelled and observed X-ray emission differ. We find that there is excess X-ray emission towards regions near HVC complexes C, D, and GCN. We suggest that the excess X-ray emission is positionally correlated with the high-velocity clouds. Some lines of sight towards HVCs also pass through significant amounts of intermediate-velocity gas, so we cannot constrain the possible role played by IVC gas in these directions of HVC and IVC overlap, in determining the X-ray excesses.Comment: 16 pages, 8 figures, accepted for publication in Astronomy & Astrophysics main journa

Transport in Double-Crossed Luttinger Liquids

We study transport through two Luttinger liquids (one-dimensional electrons interacting through a Coulomb repulsion in a metal) coupled together at {\it two} points. External voltage biases are incorporated through boundary conditions. We include density-density couplings as well as single-particle hops at the contacts. For weak repulsive interactions, transport through the wires remains undisturbed by the inter-wire couplings, which renormalise to zero. For strong repulsive interactions, the inter-wire couplings become strong. For symmetric barriers and no external voltage bias, a single gate voltage is sufficient to tune for resonance transmission in both wires. However, for asymmetric couplings or for finite external biases, the system is insulating.Comment: Latex file, 11 pages, one eps figur

Measurement of the Michel Parameter xi" in Polarized Muon Decay and Implications on Exotic Couplings of the Leptonic Weak Interaction

The Michel parameter xi" has been determined from a measurement of the longitudinal polarization of positrons emitted in the decay of polarized and depolarized muons. The result, xi" = 0.981 +- 0.045stat +- 0.003syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.Comment: 15 pages, 16 figures, 3 tables; submitted to Phys. Rev.

Exclusion Statistics in a two-dimensional trapped Bose gas

We briefly explain the notion of exclusion statistics and in particular discuss the concept of an ideal exclusion statistics gas. We then review a recent work where it is demonstrated that a {\em two-dimensional} Bose gas with repulsive delta function interactions obeys ideal exclusion statistics, with a fractional parameter related to the interaction strength.Comment: 10 pages, RevTeX. Proceedings of the Salerno workshop "Theory of Quantum Gases and Quantum Coherence", to appear in a special issue of J.Phys. B, Dec. 200

Vacancy cluster growth and thermal recovery in hydrogen-irradiated tungsten

The thermal evolution of vacancies and vacancy clusters in tungsten (W) has been studied. W (100) single crystals were irradiated with 200 keV hydrogen (H) ions to a low damage level (5.8 x 10(-3) dpa) at 290 K and then annealed at temperatures in the range of 500-1800 K. The resulting defects were characterized by positron annihilation lifetime spectroscopy (PALS) and positron annihilation Doppler broadening spectroscopy (DBS). Annealing at 700 K resulted in the formation of clusters containing 10-15 vacancies, while at 800 K and higher temperatures clusters containing about 20 vacancies or more were formed. Reduction of the defect concentration likely accompanied by further coarsening of the clusters started at 1300 K and ended at 1800 K with the complete defect recovery. The determined cluster sizes at 700 K and 800 K were larger than the estimated minimum cluster sizes that are thermally stable at these temperatures, indicating that the migration and ensuing coalescence of small clusters plays an important role in cluster growth. (C) 2020 Max-Planck-Institut fur Plasmaphysik. Published by Elsevier B.V. All rights reserved.Peer reviewe

Analysis of Spatial Structure of the SPica H II Region

Far ultraviolet (FUV) spectral images of the Spica H II region are first presented here for the Si II* 1533.4A and Al II 1670.8A lines and then compared with the optical Halpha image. The H alpha and Si II* images show enhanced emissions in the southern part of the H II region where H I density increases outwards. This high density region, which we identify as part of the "interaction ring" of the Loop I superbubble and the Local Bubble, seems to bound the southern H II region. On the other hand, the observed profile of Al II shows a broad central peak, without much difference between the northern and southern parts, which we suspect results from multiple resonant scattering. The extended tails seen in the radial profiles of the FUV intensities suggest that the nebula may be embedded in a warm ionized gas. Simulation with a spectral synthesis code yields the values of the Lyman continuum luminosity and the effective temperature of the central star similar to previous estimates with 10^46.2 photons s^-1 and 26,000 K, respectively, but the density of the northern H II region, 0.22 cm^-3, is much smaller than previous estimates for the H alpha brightest region.Comment: 15 pages, 5 figures, accepted for Ap

Universality of electron correlations in conducting carbon nanotubes

Effective low-energy Hamiltonian of interacting electrons in conducting single-wall carbon nanotubes with arbitrary chirality is derived from the microscopic lattice model. The parameters of the Hamiltonian show very weak dependence on the chiral angle, which makes the low energy properties of conducting chiral nanotubes universal. The strongest Mott-like electron instability at half filling is investigated within the self-consistent harmonic approximation. The energy gaps occur in all modes of elementary excitations and estimate at $0.01-0.1$ eV.Comment: 4 pages, 2 figure