Direct spectroscopic observation of the atomic-scale mechanisms of clustering and homogenization of rare-earth dopant ions in vitreous silica

Structural aspects of clustering of rare earth ions in oxide glasses have been studied for the last several years in relation to their applications in photonics. However, the mechanism of homogenization of the spatial distribution of rare earth ions by codoping, typically with Al or P, is still not well understood. In this work we report direct experimental determination of the homogenization mechanism of Yb3+ ion clusters in silica glasses doped with 0.1 wt. % Yb2 O3 and codoped with Al or P, using two-dimensional HYSCORE-EPR spectroscopy. The results lead us to conclude that Yb creates its coordination environment via formation of Yb-O-Si and Yb-O-Yb bonds in a Yb-doped silica glass and even the light codoping with Al starts replacing these bonds with Yb-O-Al linkages. Heavy codoping with P replaces all Yb-O-Si Yb linkages with Yb-O-P linkages. The formation of a next-nearest neighbor shell of Al or P creates suitable structural pockets, which ultimately leads to homogenization. © 2006 The American Physical Society

The Jahn-Teller effect in Cr5+-doped PbTiO3: A multi-frequency electron paramagnetic resonance study

Electron paramagnetic resonance (EPR) spectra of Cr5+ defects incorporated on Ti4+ sites in powdered ceramics of PbTiO3 were investigated in the temperature range 50-400K at 9GHz (X), 34GHz (Q) and 94GHz (W band). The Jahn-Teller effect stabilizes the vibronic ground state of the 3d1 electron of the Cr5+ ion and leads to a tetragonally distorted defect- O6 octahedron with the point symmetry D4h. The spontaneous electrical polarization present in the ferroelectric phase of PbTiO3 appears as a further perturbation producing an additional g-tensor contribution by the quadratic field effect. Its symmetry is dependent on the orientation of the electrical polarization with respect to the Jahn-Teller distortion axis, the tetragonal axis of the defect- O6 octahedron. If the polarization of a domain is anti-or parallel to this axis, the local tetragonal symmetry of the Cr5+ ion persists whereas it is reduced by a perpendicular orientation. Anisotropic EPR spectra of tetragonally and orthorhombic distorted Cr5+O6 12- are detected at low temperatures. Increasing the temperature, the peaks of the two spectra are broadened and a motionally averaged isotropic spectrum appears at 200K. © 2010 IOP Publishing Ltd

A pulsed EPR study of clustering of Yb3+ ions incorporated in GeO2 glass

The structural aspects of clustering of Yb3+ ions and the paramagnetic behavior of these clusters have been investigated in GeO 2 glasses doped with 140-1100 ppm by weight of Yb2O 3 using time-domain electron paramagnetic resonance (EPR) spectroscopic techniques. The echo-detected EPR (EDEPR) spectra of Yb 3+ ions and their unusual dependencies on microwave power and magnetic field have been found to be indicative of the formation of clusters of these rare earth ions in GeO2 glass that behave as non-Kramers type spin systems. The magnetic field and concentration dependence of phase relaxation rates of Yb3+ in these glasses further substantiate such a scenario and indicate the formation of clusters of Yb3+ ions. A comparison of the EDEPR spectra with calculated cw-EPR line shapes yields a semi-quantitative measure of the typical cluster size of ≥3 Yb atoms and intra-cluster Yb-Yb distances of 3.5-4.0 Å in these glasses at doping levels of ≥350 ppm of Yb2O3. © 2003 Elsevier B.V. All rights reserved

Microwave resonant technique in studies of photodielectric properties of bulk, thin film and nanoparticle materials

© 2016 IOP Publishing Ltd Printed in the UK.An enhanced contactless microwave technique allows us to study the photoconductivity of materials. The transient response of the complex permittivity of matter (ϵ = ϵ1 - jϵ 2) under optical irradiation is measured with nanosecond time resolution. The main advantage of the novel methodology is the elimination of the polarization effect in evaluating photoconductivity. The potential of the methodology was demonstrated by photoconductivity measurements in Si [1 0 0] crystal, CeO2 nanocrystalline powder and Ce-doped LiYF4 single crystal. The variations of complex permittivity (δϵ1 and δϵ 2) of Si [1 0 0] crystal, CeO2 nanocrystalline powder and Ce-doped LiYF4 single crystal under optical irradiation was measured and accurate values for crystalline band gaps were extracted. Finally, quantum confinement effects were observed in nano-size crystalline powders

EPR Evidence of Unusual Dopant Valency States in Nanocrystalline Er-doped CeO<inf>2</inf>

© 2015, Springer-Verlag Wien. The structure and oxidation state of the Er dopant cation in CeO2 single crystal and nanocrystals with size ranging from 22 to 300 nm are studied using electron paramagnetic resonance (EPR) spectroscopy at X- and at Q-band near liquid-He temperatures. Besides the expected EPR line due to Er3+ in cubic sites in the lattice, unusual EPR lines with g values around 14 and 20 are observed in nanocrystalline CeO2. The appearance of these lines suggests the formation of non-Kramers Er2+, Er4+ ions, which becomes increasing favorable with decreasing nanoparticle size. Formation of rare earth ions with such unusual oxidation states in nanoparticles can be exploited in tuning their catalytic activity and optical properties

Dark magnetic resonance in an electron-nuclear spin system

Dark magnetic resonance in the solid state is observed and is shown to be analogous to electromagnetically induced transparency in coherent optics. The basis of the two effects is coherent population trapping, which can conveniently be described by the product operator formalism. It is demonstrated that pulse electron paramagnetic resonance experiments on electron-nuclear spin systems provide a simple means for studying the physics of these types of coherence phenomena

EPR and spin-lattice relaxation of rare-earth ions in LiLuF4 monocrystals

The EPR and spin-lattice relaxation are studied of impurity rare-earth ions in LiLuF4 crystals at liquid-helium temperatures. It is detected that paramagnetic relaxation of Er3+ ions is retarded by the effect of the phonon throat. The effect of resonance attenuation of the phonon throat is clarified in two-phonon resonance relaxation processes of Er3+ ions. The Debye temperature of the crystal is determined from an analysis of experimental results. © 1988 Plenum Publishing Corporation

Predictors of In-Hospital Mortality Among Patients with COVID-19 Related Stroke

Introduction. COVID-19-related stroke is associated with a significantly higher mortality than COVID-19 or stroke alone. Mechanisms underlying the increased mortality of patients with stroke and COVID-19 should be thoroughly studied. Objective: to analyze predictors of hospital mortality associated with COVID-19-related stroke. Materials and methods. We retrospectively analyzed 1,386 cases of COVID-19-related stroke reported by an infectious diseases inpatient clinic in 2020. We studied clinical, laboratory, and instrumental parameters in patients with different outcomes. Logistic regression was used to identify independent predictors of mortality. Results. 539 (38.9%) patients died during their hospital stay, with 437 (38.0%) deaths from ischemic stroke and 102 (42.7%) deaths from hemorrhagic stroke (p = 0.0001). Independent predictors of mortality associated with COVID-19-related stroke included age, neurological deficit severity measured by NIHSS, COVID-19 severity, and laboratory parameters including white blood cell count, creatinine, glucose, and D-dimer blood levels. Discussion. The results of logistic regression analysis were able to explain only 41% of the variability in hospital deaths among patients with stroke associated with COVID-19. Conclusion. Hospital mortality in patients with COVID-19-related stroke is associated with severity of inflammatory response, impaired coagulation, age, neurological deficit severity, and somatic comorbidities

An improved coupling design for high-frequency TE011 electron paramagnetic resonance cavities

In high-frequency electron paramagnetic resonance (EPR) spectroscopy the sample is usually accommodated in a single-mode cylindrical TE011 microwave cavity. This cavity stands out in terms of flexibility for various types of EPR experiments due to convenient control of its resonance frequency and easy waveguide-to-cavity microwave coupling. In continuous wave and in pulsed EPR it is, however, essential to be able to vary the coupling efficiency over a large range. We present a new mechanical design to vary the microwave coupling to the cavity using a movable metal sphere. This coupling sphere is shifted in the plane of the iris wall inside the coupling waveguide. The design allows for a compact and robust construction of the EPR probehead that can be easily accommodated inside a limited space of helium flow cryostat. The construction details and characterization of the coupling element for 95 GHz (W-band) EPR as well as for 34 GHz (Q-band) are presented. © 2013 American Institute of Physics

Technique of the analysis and assessment of innovative industrial risks at different stages of innovative activity

© 2016, Econjournals. All rights reserved.High rates of innovation in enterprises require setting a number of scientific and technical problems related to the evaluation and analysis of innovative industrial risks and their control systems. The article proposes an original technique for evaluation of innovative industrial risks at different stages of innovation. Its feature is that the rate determined by the survivability of the economic system. The basis of the study developed the method for determining the survivability is the definition of the integral index of vitality based on a comprehensive assessment of the production system. The closer the result of this indicator to one, the better the control system will determine the problem areas in the implementation of innovation and more efficient use of a set of measures to address the problems identified. Methods of assessing and analyzing the vitality of enterprises is available for the purposes of risk management as a tool of analysis to identify reasons for the decline performance of innovation. In this regard, we determined preliminary values of specific factors and the limit values for these indicators