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We carried out the synchrotron X-ray diffraction measurements and investigated the oxygen octahedral distortions in compressively strained SrRuO3 epitaxial thin films on NdGaO3 substrates. We observed half-order Bragg reflections originating from the RuO6 octahedral distortions accommodated in the compressively strained films. Our structural analysis shows that the RuO6 octahedra are elongated along the out-of-plane direction and are rotated in the out-of-phase manner only about the out-of-plane direction. The RuO6 rotation angle γrot is 10.9° ± 1.2°, which is larger than that in the bulk SrRuO3. The results indicate that the substrate-induced compressive strain enhances the out-of-plane rotations while strongly suppressing the in-plane rotations. In fact, the half-order Bragg reflections arising from the octahedral rotations about the in-plane direction are found to be weak and broad, implying that the in-plane rotations exist only in the interface region, not in the entire film. This indicates that while the in-plane octahedral rotation propagates through the interfacial octahedral connections into the film, its propagation decays within the interface region because of the compressive strain. Our results indicate that both the substrate-induced strain and the interfacial octahedral connection play important roles in determining the octahedral distortions accommodated in the strained SrRuO3 films. [ABSTRACT FROM AUTHOR]

This work is devoted to the investigation of the broad band dielectric spectra of 0.83PbMg1/3Nb2/3O3-0.17PbTiO3 (PMN-17PT) single crystals which have intermediate random fields. The necessity to understand the impact of random fields for the phase transition in heterogeneous perovskite oxides is of central importance. The thorough studies of dielectric properties revealed that the structural phase transition in PMN-17PT has a very complex dynamics. The temperature dependences of dielectric permittivity show that this material has features characteristic of polar nano regions and order-disorder phase transition dynamics. We speculate that this is the fingerprint of the phase transition in such heterogeneous perovskite oxides. [ABSTRACT FROM AUTHOR]

Tunnel junctions are interesting for both studying fundamental physical phenomena and providing new technological applications. Modeling of the tunneling current is important for understanding the tunneling processes and interpreting experimental data. In this work, the tunneling current is modeled using the Tsu-Esaki formulation with numerically calculated transmission. The feasibility of analytical formulae used for fitting experimental results is studied by comparing them with this model. The Tsu-Esaki method with numerically calculated transmission provides the possibility to calculate tunneling currents and fit experimental I-V curves for wide bias voltage and barrier width ranges as opposed to the more restricted analytical formulae. I-V curve features typical of tilted barrier structures are further analyzed to provide insight into the question, which of the phenomena can be explained with this simple barrier model. In particular, a small change in the effective barrier width is suggested as a possible explanation for experimental I-V curve features previously interpreted by a change in the tilt and height of the barrier. [ABSTRACT FROM AUTHOR]

Three-dimensional numerical simulation of equilibrium micromagnetic configurations existing in thin ferromagnetic films with surface anisotropy is carried out taking into account the strong demagnetization field acting on the film magnetization and the true micromagnetic boundary condition on the film surface. The numerical results are obtained in the simplest Néel approximation for surface anisotropy energy, a surface anisotropy constant Ks being a single phenomenological parameter. It is found that the spin canted state has the lowest total energy as compared to various multi-domain configurations in the intermediate range of thickness, Lz,minLz,max, nearly uniform in-plane magnetization, or the vortex has been obtained depending on the film in-plane aspect ratio. On the other hand, different labyrinth domain structures with large in-plane magnetization have been calculated in a thick enough film, Lz>Lz,max, with a sufficiently large surface anisotropy constant. [ABSTRACT FROM AUTHOR]

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in short-reach optical communication links and interconnects because of their high modulation speed and low energy consumption. In this paper, the effect of heat dissipation on the modulation bandwidth of highspeed 850-nm VCSELs has been investigated. With increasing bottom distributed Bragg reflector (DBR) diameter from 34 μm to 46 μm, optical output power increased as well as the -3 dB modulation bandwidth. The maximum small-signal modulation bandwidth of 16.2 GHz was achieved with an aperture of 5 μm. Thermal analysis showed that the heat dissipation of the bottom DBR diameter, which determined the differential gain of the quantum wells, has a strong effect on the modulation bandwidth. [ABSTRACT FROM AUTHOR]

We simulate the mixing (demixing) process of a quiescent binary liquid mixture with a composition-dependent viscosity which is instantaneously brought from the two-phase (one-phase) to the one-phase (two-phase) region of its phase diagram. Our theoretical approach follows a standard diffuse-interface model of partially miscible regular binary mixtures wherein convection and diffusion are coupled via a nonequilibrium capillary force, expressing the tendency of the phase-separating system to minimize its free energy. Based on 2D simulation results, we discuss the influence of viscosity ratio on basic statistics of the mixing (segregation) process triggered by a rapid heating (quench), assuming that the ratio of capillary to viscous forces (a.k.a. the fluidity coefficient) is large. We show that, for a phase-separating system, at a fixed value of the fluidity coefficient (with the continuous phase viscosity taken as a reference), the separation depth and the characteristic length of single-phase microdomains decrease monotonically for increasing values of the viscosity of the dispersed phase. This variation, however, is quite small, in agreement with experimental results. On the other hand, as one might expect, at a fixed viscosity of the dispersed phase both of the above statistics increase monotonically as the viscosity of the continuous phase decreases. Finally, we show that for a mixing system the attainment of a single-phase equilibrium state by coalescence and diffusion is retarded by an increase in the viscosity ratio at a fixed fluidity for the dispersed phase. In fact, for large enough values of the viscosity ratio, a thin film of the continuous phase becomes apparent when two drops of the minority phase approach each other, which further retards coalescence. [ABSTRACT FROM AUTHOR]

Al/Pt nanolaminates with a bilayer thickness (tb, width of an Al/Pt pair-layer) of 164 nm were irradiated with single laser pulses with durations of 10ms and 0.5 ms at 189 W/cm² and 1189 W/cm², respectively. The time to ignition was measured for each pulse, and shorter ignition times were observed for the higher power/shorter pulse width. Videographic images of the irradiated area shortly after ignition show a non-uniform radial brightness for the longer pulse, while the shorter pulse shows uniform brightness. A diffusion-limited single step reaction mechanism was implemented in a finite element package to model the progress from reactants to products at both pulse widths. The model captures well both the observed ignition delay and qualitative observations regarding the non-uniform radial temperature. [ABSTRACT FROM AUTHOR]

The structural properties and the spin states of vacancies in ion implanted silicon carbide samples are analyzed by experimental measurements along with first-principles calculations. Different types and dosages of ions (N+, O+, and B+) were implanted in the 4H-silicon carbide single crystal. The Raman spectra, positron annihilation spectroscopy, and magnetization-magnetic field curves of the implanted samples were measured. The fitting results of magnetization-magnetic field curves reveal that samples implanted with 1×1016cm-2 N+ and O+ ions generate paramagnetic centers with various spin states of J = 1 and J=0.7, respectively. While for other implanted specimens, the spin states of the paramagnetic centers remain unchanged compared with the pristine sample. According to the positron annihilation spectroscopy and first-principles calculations, the change in spin states originates from the silicon vacancy carrying a magnetic moment of 3.0 μB in the high dosage N-implanted system and 2.0 μB in the O-doped system. In addition, the ratio of the concentration of implanted N ions and silicon vacancies will affect the magnetic moment of VSi. The formation of carbon vacancy which does not carry a local magnetic moment in B-implanted SiC can explain the invariability in the spin states of the paramagnetic centers. These results will help to understand the magnetic moments of vacancies in ion implanted 4H-SiC and provide a possible routine to induce vacancies with high spin states in SiC for the application in quantum technologies and spintronics. [ABSTRACT FROM AUTHOR]

A glass with the mol. % compositions 51.7 B2O3/9.3 K2O/1 P2O5/27.6 Li2O/10.4 Fe2O3 was crystallized at temperatures in the range from 400 to 540 °C for different periods of time (2-12 h). X-ray diffraction showed cubic Li ferrite with a spinel structure, LiFe5O8, with small crystallites with sizes in the range from 3 to 31 nm. While the samples crystallized at 440, 480, and 500 °C reveal a disordered phase, crystallization at 540 °C indicates a phase transformation to the ordered state. Magnetization curves showed that the samples crystallized at 440 °C are superparamagnetic with very low maximum magnetization, while the samples crystallized at 480 and 500 °C show thin clear S-shaped magnetization curves with zero coercivity and hence are also superparamagnetic. By contrast, the sample crystallized at 540 °C shows a coercive field of 40 Oe and thus is ferrimagnetic. The magnetic properties can be tailored by the size of the ferrite crystals and hence by the crystallization conditions. [ABSTRACT FROM AUTHOR]

We recently demonstrated a magnetic parallel dipole line (PDL) system that serves as a unique diamagnetic trap with a fascinating one-dimensional camelback potential along its longitudinal axis. The system can be realized with a pair of transversely magnetized cylindrical magnets and a cylindrical graphite rod as the trapped object. We present more detailed experimental and theoretical studies of the finite size effect of the rod and its impact on the stability and oscillation dynamics of the trap. We show that the camelback potential effect only occurs when the length of the PDL system is beyond certain critical length (LC). The length of the trapped rod determines the "effective camelback potential" and is subject to maximum and minimum values for the trap to be stable. Both length and radius of the rod determine the damping dynamics or the quality factor of the oscillator. These characteristics are important for designing the PDL trap system for various sensing applications, for example, we demonstrated a PDL trap gas viscometer system through measurement of the oscillation damping time constant. [ABSTRACT FROM AUTHOR]

H+ ion-induced damage of multilayer graphene (MLG) is investigated using Molecular Dynamics simulations as H2 plasmas could provide a possible route to pattern graphene. Low-energy (5-25 eV) H+ cumulative bombardment of ABA-stacked MLG samples shows an increase of the hydrogenation rate with the ion dose and ion energy. At 5 eV, the H coverage grows with the ion fluence only on the upper-side of the top layer but saturates around 35%. Hydrogenation of multilayers and carbon etching are observed at higher energies. Layer-by-layer peeling/erosion of the MLG sample is observed at 10 eV and occurs in two phases: the MLG sample is first hydrogenated before carbon etching starts via the formation of CHx (∼60%) and C2Hx (∼30%) by-products. A steady state is reached after an ion dose of ∼5×1016 H+/cm², as evidenced by a constant C etch yield (∼0.02 C/ion) and the saturation of the hydrogenation rate. At 25 eV, an original etching mechanism—lifting-off the entire top layer—is observed at low fluences due to the accumulation of H2 gas in the interlayer space and the absence of holes/vacancies in the top layer. However, as the underneath layers contain more defects and holes, this Smartcut-like mechanism cannot be not repeated and regular ion-assisted chemical etching is observed at higher fluences, with a yield of ∼0.05 C/ion. [ABSTRACT FROM AUTHOR]

Direct measurements of the adiabatic temperature change (DTAD) of Ni50Mn35In14.5B0.5 have been done using an adiabatic magnetocalorimeter in a temperature range of 250-350 K, and with magnetic field changes up to DH=1.8 T. The initial susceptibility in the low magnetic field region drastically increases with temperature starting at about 300 K. Magnetocaloric effects parameters, adiabatic temperature changes, and magnetic entropy changes were found to be a linear function of H2/3 in the vicinity of the second order transitions (SOT), whereas the first order transitions do not obey the H2/3 law due to the discontinuity of the transition. The relative cooling power based on the adiabatic temperature change for a magnetic field change of 1.8 T has been estimated. Maximum values of DTAD=-2.6K and 1.7K were observed at the magnetostructural transition (MST) and SOT for DH=1.8 T, respectively. The observed DTAD at the MST exceeds the DTAD for Ni50Mn35In14X with X=In, Al, and Ge by more than 20% and is larger than the Gd based Heusler alloys. [ABSTRACT FROM AUTHOR]

e compare the plasma emission and harmonic generation spectra in the cases of resonance enhancement of single harmonic using various laser-produced plasmas. The analysis of these spectra showed that the enhancement of harmonics depended on the oscillator strengths of the nearby ionic transitions rather than the plasma emission transitions. To prove it, we used the tunable source of mid-infrared radiation from an optical parametric amplifier (1200-1600 nm) and the fixed wavelength source (810 nm) of femtosecond pulses. The comparison of resonantly enhanced and featureless plateaulike harmonic spectra from different laser-produced plasmas, as well as their plasma emissions, confirmed this assumption. [ABSTRACT FROM AUTHOR]

High quality hybrid perovskite films are grown by the pulsed laser deposition (PLD) process through target stoichiometry and gas ambient control in the commonly used on-axis geometry which is compatible with PLD of other materials such as metal oxides. The use of an off-stoichiometric target and a momentum softening gas mixture of argon and hydrogen (90%:10%) is shown to yield dense films in the on-axis geometry with excellent stoichiometry and optical quality. The band gap can be easily tuned from 1.6 to 2.3 eV with either mixed halide composite target or successive depositions of interdiffusing MAPbBr3 and MAPbI3 layers. A photosensor based on PLD grown MAPbI3 is shown to render impressive performance over the broad range of wavelengths from 375 to 800 nm at a voltage of 5V bias. A planar heterojunction solar cell based on the dry-processed on-axis PLD grown film exhibits a champion conversion efficiency of 10.9%. A heterostructure comprising in situ pulsed laser deposited Cu2O (p-type) followed by the hybrid perovskite film shows a rectifying characteristic and photoresponse. [ABSTRACT FROM AUTHOR]

We present a novel terahertz (THz) quantum cascade laser (QCL) design where C-valley states are used for lasing transition and X-valley states—in particular, Xz-states—are used as injector subbands. Since the lasing states in our proposed structure are populated and depopulated mainly through the interface roughness assisted C-Xz electron scattering, we present a model to describe this intervalley carrier transport. In the injector region of the proposed THz QCL, we use a quaternary AlGaAsP material to introduce tensile strain, which plays a crucial role in increasing the gain. To compensate the strain per period, we propose to grow the periodic heterostructure on a GaAs0.94P0.06 virtual substrate. To simulate the carrier transport and hence calculate the gain and lasing performance of the proposed THz QCL, we use a simplified density matrix formalism that considers resonant tunneling, dephasing, and the important intersubband scattering mechanisms. Since electron temperature significantly varies from lattice temperature for QCLs, we take their difference into account using the kinetic energy balance method. We show that the proposed structure is capable of lasing up to a maximum lattice temperature of ∼119K at 4.8 THz. For future improvements of the device, we identify major performance-degrading factors of the proposed design. [ABSTRACT FROM AUTHOR]

We studied the optical properties of periodic InGaN/GaN multiple quantum well systems with different numbers of periods. A resonant increase in the optical reflection and simultaneous suppression of the optical absorption have been revealed experimentally at room temperature when the Bragg and exciton resonances were tuned to each other. Numerical modeling with a single set of parameters gave a quantitatively accurate fit of the experimental reflection and transmission spectra in a wide wavelength range and various angles of the light incidence. The model included both exciton resonance and non-resonant band-to-band transitions in the InGaN quantum wells, as well as Rayleigh light scattering in the GaN buffer layer. The analysis also involved x-ray diffraction and photoluminescence data. It allowed us to determine the key parameters of the structure. In particular, the radiative broadening of the InGaN QW excitons was evaluated as 0.20 ± 0.02 meV. [ABSTRACT FROM AUTHOR]

We explore in numerical simulation the degree to which the thermal emission in the mid-infrared (IR) range can be controlled using a lattice of plasmonic spheres. While it has been shown that a great deal of spectral control can be achieved using a single sphere, obviously no directional control exists. We propose gaining this additional directional control over the thermal emission by designing a 2D lattice of resonant spheres. In this paper, we demonstrate that directional control as well as additionally spectral control over the thermal emission is achieved with this method. We do this by simulating a 2D layer of spheres composed of SiO2 cores and a SiC shells with a polaritonic resonance in the mid-IR. We then analyze how changing the lattice geometry affects the thermal emission characteristics of the ensemble of spheres. We show that for closely spaced spheres, modest directional control is possible with the additional benefit of some spectral control. When the spheres are spaced farther apart, Bragg scattering allows for more directional control, however, the spectrum of the emission is mostly governed by the properties of a single sphere. [ABSTRACT FROM AUTHOR]

Annealed Czochralski Silicon wafers containing SiOx precipitates have been studied by high energy X-ray diffraction in a defocused Laue setup using a laboratory tungsten tube. The energy dispersive evaluation of the diffracted Bragg intensity of the 220 reflection within the framework of the statistical dynamical theory yields the static Debye-Waller factor E of the crystal, which gives access to the strain induced by the SiOx precipitates. The results are correlated with precipitate densities and sizes determined from transmission electron microscopy measurements of equivalent wafers. This allows for the determination of the constrained linear misfit ϵ between precipitate and crystal lattice. For samples with octahedral precipitates the values ranging from ϵ=0.39 (+0.28/–0.12) to ϵ=0.48 (+0.34/–0.16) indicate that self-interstitials emitted into the matrix during precipitate growth contribute to the lattice strain. In this case, the expected value calculated from literature values is ϵ=0.26±0.05. Further, the precise evaluation of Pendellösung oscillations in the diffracted Bragg intensity of as-grown wafers reveals a thermal Debye-Waller parameter for the 220 reflection B220(293 K) of 0.5582±0.0039 Å2 for a structure factor based on spherically symmetric scattering contributions. [ABSTRACT FROM AUTHOR]

We present an analytical method for evaluating the first and second moments of the effective exciton spatial distribution in organic light-emitting diodes (OLED) from measured emission patterns. Specifically, the suggested algorithm estimates the emission zone mean position and width, respectively, from two distinct features of the pattern produced by interference between the emission sources and their images (induced by the reflective cathode): the angles in which interference extrema are observed, and the prominence of interference fringes. The relations between these parameters are derived rigorously for a general OLED structure, indicating that extrema angles are related to the mean position of the radiating excitons via Bragg's condition, and the spatial broadening is related to the attenuation of the image-source interference prominence due to an averaging effect. The method is applied successfully both on simulated emission patterns and on experimental data, exhibiting a very good agreement with the results obtained by numerical techniques. We investigate the method performance in detail, showing that it is capable of producing accurate estimations for a wide range of source cathode separation distances, provided that the measured spectral interval is large enough; guidelines for achieving reliable evaluations are deduced from these results as well. As opposed to numerical fitting tools employed to perform similar tasks to date, our approximate method explicitly utilizes physical intuition and requires far less computational effort (no fitting is involved). Hence, applications that do not require highly resolved estimations, e.g., preliminary design and production-line verification, can benefit substantially from the analytical algorithm, when applicable. This introduces a novel set of efficient tools for OLED engineering, highly important in the view of the crucial role the exciton distribution plays in determining the device performance. [ABSTRACT FROM AUTHOR]

After α + β-zirconium has fully transformed into β-phase upon heating, the intensities of all β-Zr Bragg reflections decrease simultaneously as a function of time. It is shown that this effect represents a transition from the kinematic to the dynamic theory of diffraction due to the ever increasing crystal perfection driven by thermal recovery of the system. The best fitting coherent crystallite size of 30 μm and other microstructural features are verified by in situ laser scanning confocal microscopy. This effect of primary extinction in neutron diffraction has been employed to further investigate the crystal perfection kinetics. Upon further heating, crystal recovery is identified as a process of dislocation annihilation, suffering from lattice friction. Upon cooling, precipitating α-Zr induces strain into the perfect β-crystallites, re-establishing the kinematic diffraction intensities. An Avrami analysis leads to the estimations of nucleation time, consumption of nucleation sites and lower-dimensional growth. Such technique bears great value for further investigation on all metal systems annealed close to the melting temperature. [ABSTRACT FROM AUTHOR]

To know the residual stress state within a component is essential for predicting its service life time. Neutron diffraction is a reliable tool for non-destructive determination of macroscopic residual stresses, even deep inside a component. One standard procedure in neutron residual stress analysis is to cut out small macrostress free coupons from the strained component, which serve as reference samples. Redistribution of intergranular stresses due to relaxation when the coupon is cut out, however, may lead to spurious macroscopic stresses. A quantitative study of the change of intergranular stresses shows that, contrary to conventional thought, even the {311} Bragg reflection is affected strongly by this phenomenon. In this case, the use of reference values calculated using boundary conditions of mechanical equilibrium enables to extract reliable macroscopic residual stress values. [ABSTRACT FROM AUTHOR]

Abstract In order to achieve monochromatic X-rays of higher energy, and so as to calibrate the hard X-ray satellite-borne detectors and to test the energy response at high energy values, a hard X-ray test facility which is based on a single crystal monochromator was designed and built. The facility consists of four parts, namely the X-ray source, the collimating structure, the single crystal monochromator and the detectors. In this work, silicon crystals Si111, Si220, Si551 were utilized in the monochromator to achieve monoenergetic X-ray photons, which is based on crystal Bragg diffraction. By means of adjusting Bragg angle, different energy values can be obtained. The 21–301 keV monochromatic X-rays were realized and studied in this work. The experiment results indicate that there is a good linear relationship between monochromatic light flux and X-ray tube current. The broadening of energy spectra gets wider with increasing energy value, thus worsening the energy resolution. Stability of the hard X-ray test facility was studied, and results suggest that the energy stability is better than 0.4% within 50 h. Besides, the stability of monochromatic light flux is better than 1.4% within 50 h. Therefore, the hard X-ray test facility has a wide energy range and shows very good performance about stability. Thus, it can be used to carry out some experiments, especially to calibrate the hard X-ray satellite-borne detectors. This work will be helpful to the development of the X-ray astronomy. [ABSTRACT FROM AUTHOR]

Abstract The purpose of this paper is to identify the fragmentation patterns of molecules and ions that give the best fulfilment of a Bragg's rule estimation of the mean excitation energy of the molecules and ions. We investigate the effect of chemical binding on the validity of Bragg's rule for the calculation of stopping cross sections and mean excitation energies. As test cases we use a series of small molecules and molecular ions, primarily carbon and nitrogen hydrides. We are using several nuclear fragments of the same molecule or ion to test the dependence of Bragg's rule on the binding energy of the molecules/ions. The mean excitation energies of the molecules/ions and their fragments are computed with the same method. We find that neglect of chemical binding nearly always results in an underestimation of the mean excitation energies. We also find that the fully atomic decomposition of any molecule, but a diatomic molecule never gives the best fulfilment of Bragg's rule. The best fulfilment of Bragg's rule is obtained when using a fragmentation pattern with as few bonds as possible broken and when the choice of fragments is guided by chemical knowledge and intuition. By investigating several alternative fragmentation patterns for a molecule/ion, guidelines for choice of optimal Bragg rule fragmentation are suggested. Using the best fragmentation pattern for the tested molecules and ions, the error in the mean excitation energy is of the order of 5% or less, implying an error of not more than 1% in the pure Bethe stopping power because of applying Bragg's rule. [ABSTRACT FROM AUTHOR]

A technique to synthesize a new category of radar cross-section (RCS)-reduction metasurfaces is presented. The appealing feature of the proposed technique is that by representing the generalized Snell–Descartes’ law of reflection in the form of array factor, it unifies the two most widely studied and well-established modern RCS-reduction methods: checkerboard metasurfaces and gradient index metasurfaces. More importantly, it refines the concepts of both of these metasurfaces and overcomes numerous limitations associated with them. It can synthesize the RCS-reduction metasurfaces which can be placed along with almost any existing metasurface, without replacing the original metasurface that may be designed to serve a specific purpose, to reduce its RCS. The proposed technique is fundamentally based on scattering cancellation, and it is validated using artificial magnetic conductor technology for several configurations. Moreover, a phasor representation is developed and implemented for complex configurations to make the synthesis simpler and more insightful. Finally, to display the versatility of the proposed technique, an example of RCS-reduction metasurface has been synthesized and introduced to an existing high-gain metasurface ground plane available in the literature. This paper is confined to the conventional specular direction. [ABSTRACT FROM AUTHOR]

Abstract A new mechanism of neutron acceleration in the accelerated perfect crystal is proposed and studied experimentally. The effect arises due to the resonance energy dependence of the neutron refraction index in vicinity of the Bragg's energy. During the neutron time-of-flight through the accelerated crystal the relative neutron-crystal velocity will be changing (with the deviation from the exact Bragg condition) and so the refraction index and the velocity of outgoing neutron will change as well. [ABSTRACT FROM AUTHOR]

The ability to control wave propagation is of fundamental interest in many areas of physics. Photonic and phononic crystals have proved very useful for this purpose but, because they are based on Bragg interferences, these artificial media require structures with large dimensions. In [Ammari et al., J. Differential Equations, 263 (2017), pp. 5610--5629], it has been proved that a subwavelength bandgap opening occurs in bubble phononic crystals. To demonstrate the opening of a subwavelength phononic bandgap, a periodic arrangement of bubbles is considered and their subwavelength Minnaert resonance is exploited. In this paper, this subwavelength bandgap is used to demonstrate cavities, very similar to those obtained in photonic and phononic crystals, albeit of deeply subwavelength dimensions. The key idea is to increase the size of a single bubble inside the crystal, thus creating a defect. The goal is then to analytically and numerically show that this crystal has a localized eigenmode close to the defect bubble. [ABSTRACT FROM AUTHOR]

To guide the experimental design of a linear plasma device for studying the interaction between energetic ions and Alfvénic gap eigenmode (AGE), this work computes AGE referring to fusion conditions in an ultra-long large plasma cylinder ended with strong magnetic throats for axial confinement of charged particles. It is shown that (i) for uniform equilibrium field between the ending throats, the dispersion relation of the computed wave field agrees well with a simple analytical model for the shear Alfvénic mode and (ii) for periodic equilibrium field with local defect, clear AGE is formed inside the spectral gap for both low and high depths of magnetic throats, although lower depth yields easier observation. The strongest AGE can be on the order of 3.1 × 10−4 to equilibrium field, making it conveniently measurable in experiment. The AGE is a standing wave localized around the defect which is introduced to break the system's periodicity, and its wavelength is twice the system's period, consistent with Bragg's law. The parameter scan reveals that the AGE remains nearly the same when the number of magnetic ripples is reduced from 18 to 8; however, there occurs an upward frequency shift when the depth of magnetic ripples drops from 0.5 to 0.1, possibly due to a flute-like effect: shrinking resonant cavity of the spectral gap. [ABSTRACT FROM AUTHOR]

Damping Bragg scattering from the ocean surface is the basic underlying principle of synthetic aperture radar (SAR) oil slick detection, and they produce dark spots on SAR images. Dark spot detection is the first step in oil spill detection, which affects the accuracy of oil spill detection. However, some natural phenomena (such as waves, ocean currents, and low wind belts, as well as human factors) may change the backscatter intensity on the surface of the sea, resulting in uneven intensity, high noise, and blurred boundaries of oil slicks or lookalikes. In this paper, Segnet is used as a semantic segmentation model to detect dark spots in oil spill areas. The proposed method is applied to a data set of 4200 from five original SAR images of an oil spill. The effectiveness of the method is demonstrated through the comparison with fully convolutional networks (FCN), an initiator of semantic segmentation models, and some other segmentation methods. It is here observed that the proposed method can not only accurately identify the dark spots in SAR images, but also show a higher robustness under high noise and fuzzy boundary conditions. [ABSTRACT FROM AUTHOR]

The influence of statistics on calculated lattice strains has been studied by comparing crystal plasticity finite element (CPFE) calculations with strains measured experimentally. Experimentally, when Bragg's law is obeyed, a plane normal must lie within a narrow orientation range (∼ 0.02° for synchrotron diffraction), or Bragg tolerance. However, CPFE models consider only a small number of grains compared to experiments, necessitating a justification of the statistically representative volume. It also becomes necessary to assess the threshold of Bragg tolerance allowable for the determined statistically representative volume. In this study, an 8 × 8 × 8 model was deemed as statistically representative such that only small benefits are obtained in terms of lattice strain calculations by adopting larger models such as 10 × 10 × 10. Based on the selected model, an allowable Bragg tolerance of approximately 5° was calculated. Also highlighted was the coupling between lattice strain, texture, hardening and applied boundary condition which are discriminators that will affect the choice of model size and Bragg tolerance threshold. [ABSTRACT FROM AUTHOR]

We construct an electron optical system to investigate Bragg diffraction (the crystal lattice plane, 10−2 to 10−3 rad) with the objective lens turned off by adjusting the current in the intermediate lenses. A crossover was located on the selected-area aperture plane. Thus, the dark-field imaging can be performed by using a selected-area aperture to select Bragg diffraction spots. The camera length can be controlled in the range of 0.8–4 m without exciting the objective lens. Furthermore, we can observe the magnetic-field dependence of electron diffraction using the objective lens under weak excitation conditions. The diffraction mode for Bragg diffraction can be easily switched to a small-angle electron diffraction mode having a camera length of more than 100 m. We propose this experimental method to acquire electron diffraction patterns that depict an extensive angular range from 10−2 to 10−7 rad. This method is applied to analyze the magnetic microstructures in three distinct magnetic materials, i.e. a uniaxial magnetic structure of BaFe10.35Sc1.6Mg0.05O19, a martensite of a Ni–Mn–Ga alloy, and a helical magnetic structure of Ba0.5Sr1.5Zn2Fe12O22. [ABSTRACT FROM AUTHOR]

Simultaneous rotations of sample and X-ray detected counter are needed to evaluate orientation distribution of crystallites and amorphous chains oriented predominantly parallel to the film surface in addition to exact diffraction peak profiles obtained without the complicated intensity corrections. The rotation mode is known as “θ–2θ scanning” system (θ: film, 2θ: counter). The system has been mainly used in research and development institutes. However, such instruments are not produced at present. Recently, small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) intensities have been measured by using X-ray beam generated along one direction. The brand name of the instrument is “a simultaneous SAXS and WAXD measuring instrument”. The X-ray beam generated by the instrument has surely high luminance providing high degree resolution of peak profiles by diffraction and/or scattering. The sample stage and detector, however, are fixed, since the intensities for SAXS and WAXD are obtained by the digital display of the number of X-ray photons detected on the imaging plate. Such optical system contains controversial defect on evaluating orientation of crystal planes parallel to the surface of films prepared by T-die and inflation methods as well as the exact profile. The imaging plate cannot detect the diffraction intensity from the crystal planes existing in the angle range between incident beam and Bragg angle associated with the diffraction peak position of the individual crystal planes. [ABSTRACT FROM AUTHOR]

A near-unity unselective absorption spectrum is desirable for high-performance photovoltaics. Nanowire (NW) arrays are promising candidates for efficient solar cells due to nanophotonic absorption resonances in the solar spectrum. The absorption spectra, however, display undesired dips between the resonance peaks. To achieve improved unselective broadband absorption, we propose to enclose distributed Bragg reflectors (DBRs) in the bottom and top parts of indium phosphide (InP) NWs, respectively. We theoretically show that by enclosing only two periods of In0.56Ga0.44As/InP DBRs, an unselective 78% absorption efficiency (72% for NWs without DBRs) is obtained at normal incidence in the spectral range from 300 nm to 920 nm. Under oblique light incidence, the absorption efficiency is enhanced up to about 85% at an incidence angle of 50°. By increasing the number of DBR periods from two to five, the absorption efficiency is further enhanced up to 95% at normal incidence. In this work, we calculated optical spectra for InP NWs, but the results are expected to be valid for other direct band gap III–V semiconductor materials. We believe that our proposed idea of integrating DBRs in NWs offers great potential for high-performance photovoltaic applications. [ABSTRACT FROM AUTHOR]

The geometric, electronic structure and thermodynamic stability of Sc 2 O@C 92 has been characterized by using hybrid density functional theory calculations combined with statistical thermodynamic analyses. Results indicate that the isolated pentagon rule (IPR) isomers Sc 2 O@ C s (126339)-C 92 , Sc 2 O@ C 1 (126367)-C 92 and Sc 2 O@ C 1 (126390)-C 92 are favorable. Noteworthy, it is the first time to declare that fullerene isomer C s (126339)-C 92 could be considered as the suitable cage to encapsulate metallic cluster. The electronic properties of these three isomers were performed with frontier molecular orbital (HOMO and LUMO) analyses and bond order calculations. Finally, 13 C NMR and UV-vis-NIR spectra were simulated to provide valuable information for future experiments. [ABSTRACT FROM AUTHOR]

Cadmium telluride (CdTe) has been successfully prepared by a simple wet chemical process at different reaction temperatures. Temperature is one parameter that thermodynamically plays an important role in controlling the growth rate, morphology, size and size distribution of the as-prepared nanoparticles (NPs). Effect of this parameter was investigated on the growth, structural and optical properties of CdTe NPs. It was observed that the Powder X-ray diffraction (XRD) pattern for samples prepared at 50 °C had many impurities from unreacted precursors while those prepared at > 100 °C displayed polycrystalline NPs. The XRD results revealed that the structure of the CdTe NPs was cubic with the planes (111), (220), (311) being the main observed peaks. The crystallite sizes obtained from Scherrer formula increased with the increase in growth temperature (2.86-3.62 nm grown at 50-200 °C respectively). The scanning electron microscopy micrographs showed that the morphology of the nanoparticles possessed spherical-shaped particles over the entire surface. This was further confirmed by high resolution transmission electron microscopy micrographs which also displayed increase in the particle size with an increase in the growth temperature. In the optic study, the photoluminescence (PL) spectra displayed a red shift (540-560 nm) in emission as growth temperature increased from 50 to 200 °C. The highest PL peak intensity was realized at a growth temperature of 150 °C. Absorption band maxima were observed to shift towards longer wavelength for higher growth temperatures. The optical band gap decreased with increase in the growth temperature from 2.67 to 2.08 eV for 50-200 °C respectively. [ABSTRACT FROM AUTHOR]

We present theoretical considerations for diffracted diffraction radiation and also propose an application of this process to diagnosing ultra-relativistic electron (positron) beams for the first time. Diffraction radiation is produced when relativistic particles move near a target. If the target is a crystal or X-ray mirror, diffraction radiation in the X-ray region is expected to be diffracted at the Bragg angle and therefore be detectable. We present a scheme for applying this process to measurements of the beam angular spread, and consider how to conduct a proof-of-principle experiment for the proposed method. [ABSTRACT FROM AUTHOR]

Due to the simplicity and speed of the analysis, calorimetric (DSC measurements and Gibbs-Thomson method) or x-ray methods (SAXS measurements and Bragg's law or correlation function method) are the most frequently used in determining the thickness of crystals. This paper revealed that the direct application of the above mentioned methods in order to analyze the lamellar structure of semicrystalline polymers, the amorphous phase of which was swollen, may lead to incorrect results. Swelling of the amorphous phase (without changing the thickness of crystals) leads to a shift of the observed melting temperature towards lower values, what in turn will result in underestimated values of thickness of crystals, in the case of calorimetric method. At the same time, an increase of the value of long period is observed, what in turn will result in overestimated values of thickness of crystals, in the case of classical x-ray method (with the use of value of long period estimated according to the Bragg's law and crystalline volume fraction). We have observed that only the analysis of x-ray data with the use of the correlation function method enables to obtain actual values of the thickness of lamellar crystals of the polymeric materials with swollen amorphous phase. The means for correct analysis are pointed out. The above effects have been presented on the example of model, miscible, non-cocrystallizing semicrystalline polymer/low molecular weight modifier systems. [ABSTRACT FROM AUTHOR]

In present investigation, cetyltrimethyl ammonium bromide stabilized La 3+ doped SrSnO 3 nanoparticles have been synthesized via wet chemical route with lanthanum content of 0–6 at.%. X-ray diffraction and transmission electron microscopic analysis has indicated that doping of lanthanum has no effect on morphology and crystal structure of SrSnO 3 . A slight shift of Bragg diffraction peaks towards higher angle has been noticed, indicating the lattice contraction on substitution of Sr 2+ lattice sites by La 3+ ions. The optimum efficiency of 4.7% was obtained for 6 at.% La doped SrSnO 3 photoanode based dye sensitized solar cell, which is more than 100% increase in efficiency as compared to pristine SrSnO 3 (2.3%) photoanode based cell. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) locally resonant phononic crystals (LRPCs) are studied with the aim of optimising the sub-wavelength band gaps of such composites. By analysing their effective acoustic properties, it has been found that the effective acoustic speed of the composite will drop to zero when local resonance arise, and will increase monotonically when Bragg scattering effects occur. Moreover, if the matrix is a low-shear-speed medium, local resonators can significantly reduce the effective acoustic speed of the composite and, therefore, lower the frequency where Bragg scattering effects occur. Hence, a specific LRPC with alternating elastic and fluid matrices is proposed, whose resonance and Bragg gaps are already close in frequency. The fluid matrix behaves as a wave filter, which prevents the shear waves from propagating in the composite. By using the layer-multiple-scattering theory, the coupling behaviour of local resonance and Bragg scattering band gaps has been investigated. Both gaps are enhanced when they move closer to each other. Finally, a gap-coupled case is obtained that displays a broad sub-wavelength band gap. Such proposal excels at the application of underwater acoustic materials since the arrangement of structure can be handily adjusted for tuning the frequency of coupled gap. [ABSTRACT FROM AUTHOR]

CuZnSnS (CZTS) thin films on Mo substrate were obtained by a simple sol-gel process and subsequent sulfurization. To decrease the impact of the back contact to CZTS thin film solar cells, the pre-annealing and sulfurization process was controlled. Based on the sol-gel method, CZTS thin film solar cells with a structure of ZnO:Al/i-ZnO/CdS/CZTS/Mo/glass have been fabricated. The best power conversion efficiency (PCE) of CZTS solar cells was up to 4.28% under the optimized condition: the sulfurization temperature of 570 °C and CdS thickness of 50 nm. [ABSTRACT FROM AUTHOR]

A coaxial line with periodically tapered inner rode corrugated ripples is introduced to act as a distributed Bragg reflector to increase output microwave power of relativistic diffraction generator (RDG). The RDG device consists of an overmoded cross-section, two-sectional disk-loaded cylindrical slow-wave structure (SWS), and coaxial reflector is placed upstream at the first SWS section. The RDG device with tapered Bragg reflector is investigated by full three-dimensional Particle-in-Cell (PIC) simulations and experiments in this paper. The structural parameters of the reflector are optimized for favorable characteristic of mode selection and frequency response. It is found that oscillation modes varied at different magnetic field magnitude. The measured radiation power at 29~31 GHz is over 300 MW for TM01 mode and over 700 MW for TM11 mode. Results show that the reflector has a great impact on performance of the device. Operating at proper conditions, the radiation power is remarkably enhanced. [ABSTRACT FROM AUTHOR]

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Propagation of wide optical beams in transverse periodic lattices have been reported to induce power oscillations between Fourier modes related by the Bragg resonance condition, resulting from the coupling between the beam and the periodic structure. These oscillations have been referred to as Rabi optical oscillations due to the analogy with matter Rabi oscillations. In this work, we investigate the behavior of Bragg-induced Rabi-type oscillations of a multimode Gaussian beam in the presence of optical nonlinearity. We find a combination of oscillation and spectrum broadening under both self-focusing and self-defocusing nonlinearities, in the sense that the oscillations are maintained while the spectrum is broadened and therefore partially transferred to the twin frequency. For intense self-focusing nonlinearities a complete leak of the initial mode profile to other modes is rapidly attained so that no oscillation is observed. In contrast, for intense self-defocusing nonlinearities the redistribution rate is so dramatic that oscillations cease and power only fades away. [ABSTRACT FROM AUTHOR]

A series of novel SrLu2O4: x Ho3+, y Yb3+ phosphors ( x=0.005-0.05, y=0.1-0.6) were synthesized by a simple solid-state reaction method. The phase purity, morphology, and upconversion luminescence were measured by X-ray diffraction ( XRD), scanning electron microscopy ( SEM), and photoluminescence ( PL) spectroscopy. The doping concentrations and sintering temperature were optimized to be x=0.01, y=0.5 and T=1400°C to obtain the strongest emission intensity. Under 980 nm laser diode excitation, the SrLu2O4:Ho3+, Yb3+ phosphors exhibit intense green upconversion ( UC) emission band centered at 541 nm (5F4,5S2→5I8) and weak red emission peaked at 673 nm (5F5→5I8). Under different pump-power excitation, the UC luminescence can be finely tuned from yellow-green to green light region to some extent. Based on energy level diagram, the energy-transfer mechanisms are investigated in detail according to the analysis of pump-power dependence and luminescence decay curves. The energy-transfer mechanisms for green and red UC emissions can be determined to be two-photon absorption processes. Compared with commercial NaYF4:Er3+, Yb3+ and common Y2O3:Ho3+, Yb3+ phosphors, the SrLu1.49Ho0.01Yb0.5O4 sample shows good color monochromaticity and relatively high UC luminescence intensity. The results imply that SrLu2O4:Ho3+, Yb3+ can be a good candidate for green UC material in display fields. [ABSTRACT FROM AUTHOR]