The MRO-accompanied modes of Re-implantation into SiO2-host matrix: XPS and DFT based scenarios

The following scenarios of Re-embedding into SiO2-host by pulsed Re-implantation were derived and discussed after XPS-and-DFT electronic structure qualification: (i) low Re-impurity concentration mode -> the formation of combined substitutional and interstitial impurities with Re2O7-like atomic and electronic structures in the vicinity of oxygen vacancies; (ii) high Re-impurity concentration mode -> the fabrication of interstitial Re-metal clusters with the accompanied formation of ReO2-like atomic structures and (iii) an intermediate transient mode with Re-impurity concentration increase, when the precursors of interstitial defect clusters are appeared and growing in the host-matrix structure occur. An amplification regime of Re-metal contribution majority to the final Valence Band structure was found as one of the sequences of intermediate transient mode. It was shown that most of the qualified and discussed modes were accompanied by the MRO (middle range ordering) distortions in the initial oxygen subnetwork of the a-SiO2 host-matrix because of the appeared mixed defect configurations.Comment: 19 pages, 7 figures, accepted to J. Alloys and Compound

Identifying Nearby UHECR Accelerators using UHE (and VHE) Photons

Ultra-high energy photons (UHE, E > 10^19 eV) are inevitably produced during the propagation of 10^20 eV protons in extragalactic space. Their short interaction lengths (<20 Mpc) at these energies, combined with the impressive sensitivity of the Pierre Auger Observatory detector to these particles, makes them an ideal probe of nearby ultra-high-energy cosmic ray (UHECR) sources. We here discuss the particular case of photons from a single nearby (within 30 Mpc) source in light of the possibility that such an object might be responsible for several of the UHECR events published by the Auger collaboration. We demonstrate that the photon signal accompanying a cluster of a few > 6x10^19 eV UHECRs from such a source should be detectable by Auger in the near future. The detection of these photons would also be a signature of a light composition of the UHECRs from the nearby source.Comment: 4 pages, 2 figures, accepted for publication in PR

Electronic band gap reduction and intense luminescence in Co and Mn ion-implanted SiO2_2

Cobalt and manganese ions are implanted into SiO$_2$ over a wide range of concentrations. For low concentrations, the Co atoms occupy interstitial locations, coordinated with oxygen, while metallic Co clusters form at higher implantation concentrations. For all concentrations studied here, Mn ions remain in interstitial locations and do not cluster. Using resonant x-ray emission spectroscopy and Anderson impurity model calculations, we determine the strength of the covalent interaction between the interstitial ions and the SiO$_2$ valence band, finding it comparable to Mn and Co monoxides. Further, we find an increasing reduction in the SiO$_2$ electronic band gap for increasing implantation concentration, due primarily to the introduction of Mn- and Co-derived conduction band states. We also observe a strong increase in a band of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to oxygen deficient centers formed during implantation.Comment: 8 pages, 6 figure

Constraints on the origin of the ultra-high energy cosmic-rays using cosmic diffuse neutrino flux limits: An analytical approach

Astrophysical neutrinos are expected to be produced in the interactions of ultra-high energy cosmic-rays with surrounding photons. The fluxes of the astrophysical neutrinos are highly dependent on the characteristics of the cosmic-ray sources, such as their cosmological distributions. We study possible constraints on the properties of cosmic-ray sources in a model-independent way using experimentally obtained diffuse neutrino flux above 100 PeV. The semi-analytic formula is derived to estimate the cosmogenic neutrino fluxes as functions of source evolution parameter and source extension in redshift. The obtained formula converts the upper-limits on the neutrino fluxes into the constraints on the cosmic-ray sources. It is found that the recently obtained upper-limit on the cosmogenic neutrinos by IceCube constrains the scenarios with strongly evolving ultra-high energy cosmic-ray sources, and the future limits from an 1 km^3 scale detector are able to further constrain the ultra-high energy cosmic-rays sources with evolutions comparable to the cosmic star formation rate.Comment: 9 pages, 3 figures and 1 table. Accepted by Phys. Rev.

The Spectral Shape and Photon Fraction as Signatures of the GZK-Cutoff

With the prospect of measuring the fraction of arriving secondary photons, produced through photo-pion energy loss interactions of ultra high energy cosmic ray (UHECR) protons with the microwave background during propagation, we investigate how information about the local UHECR source distribution can be inferred from the primary (proton) to secondary (photon) ratio. As an aid to achieve this, we develop an analytic description for both particle populations as a function of propagation time. Through a consideration of the shape of the GZK cut-off and the corresponding photon fraction curve, we investigate the different results expected for both different maximum proton energies injected by the sources, as well as a change in the local source distribution following a perturbative deformation away from a homogeneous description. At the end of the paper, consideration is made as to how these results are modified through extra-galactic magnetic field effects on proton propagation. The paper aims to demonstrate how the shape of the cosmic ray flux in the cut-off region, along with the photon fraction, are useful indicators of the cutoff origin as well as the local UHECR source distribution.Comment: Accepted for publication in PRD, 12 pages, 9 figure

On the dual equivalence between self-dual and Maxwell-Chern-Simons models with Lorentz symmetry breaking

In this paper, we use gauge embedding procedure and master action approach to establish the equivalence between the self-dual and Maxwell-Chern-Simons models with Lorentz symmetry breaking. As a result, new kinds of Lorentz-breaking terms arise.Comment: 14 pages, minor corrections, version accepted to Physical Review

Comparative Analysis of the Electronic Energy Structure of Nanocrystalline Polymorphs of Y2O3 Thin Layers: Theory and Experiments

The results of fabrication and characterization of atomic structure of nanocrystalline thin layers of Y2O3 in cubic and monoclinic phases is reported. Experimental data demonstrate crystalline ordering in nanocrystalline films with average grain size of ~10-14 nm both for cubic and monoclinic studied structures. Density Functional Theory (DFT) based simulations demonstrate insignificant differences of electronic structure of these phases in the bulk and on the surfaces. Theoretical modeling also pointed out the significant broadening of valence and conductive bands caused by means of energy levels splitting in agreement with experimental data (X-ray photoelectron and photoluminescence spectra). The presence of various intrinsic and extrinsic defects (including surface adsorption of carbon mono- and dioxide) does not promote visible changes in electronic structure of Y2O3 surface for both studied phases. Optical absorption and luminescence measurements indicate insignificant bandgap reduction of Y2O3 nanocrystalline layers and the very little contribution from defect states. Simulation of extrinsic compression and expanding demonstrate stability of the electronic structure of nanocrystalline Y2O3 even under significant strain. Results of comprehensive studies demonstrate that yttrium oxide based nanocrystalline layers are prospective for various optical applications as a stable material.Comment: 24 pages, 13 figures, accepted to Applied Surface Scienc