Showing papers in "Journal of the Korean Physical Society in 2016"

A brief review on relaxor ferroelectrics and selected issues in lead-free relaxors

Abstract: Relaxor ferroelectricity is one of the most widely investigated but the least understood material classes in the condensed matter physics. This is largely due to the lack of experimental tools that decisively confirm the existing theoretical models. In spite of the diversity in the models, they share the core idea that the observed features in relaxors are closely related to localized chemical heterogeneity. Given this, this review attempts to overview the existing models of importance chronologically, from the diffuse phase transition model to the random-field model and to show how the core idea has been reflected in them to better shape our insight into the nature of relaxor-related phenomena. Then, the discussion will be directed to how the models of a common consensus, developed with the so-called canonical relaxors such as Pb(Mg1/3Nb2/3)O3 (PMN) and (Pb, La)(Zr, Ti)O3 (PLZT), are compatible with phenomenological explanations for the recently identified relaxors such as (Bi1/2Na1/2)TiO3 (BNT)-based lead-free ferroelectrics. This review will be finalized with a discussion on the theoretical aspects of recently introduced 0−3 and 2−2 ferroelectric/relaxor composites as a practical tool for strain engineering.

Fe2O3/Co3O4 composite nanoparticle ethanol sensor

Abstract: In this study Fe2O3/Co3O4 nanocomposites were synthesized by using a simple hydrothermal route. The X-ray diffraction analysis results showed that the synthesized powders were pure and nanocrystalline in nature. Moreover, scanning electron microscopy revealed that Fe2O3 nanoparticles had spherical shapes while Co3O4 particles had a rod-like morphology. The ethanol sensing properties of Fe2O3/Co3O4 nanocomposites were examined and compared with those of pristine Fe2O3 nanoparticles. The gas sensing properties of Fe2O3/Co3O4 nanocomposites were shown to be superior to those of pristine Fe2O3 nanoparticles and for all concentrations of ethanol, the response of the nanocomposite sensor was shown to be higher than that of the pristine Fe2O3 nanoparticle sensor. In detail, the response of the Fe2O3/Co3O4 nanocomposite sensor to 200 ppm of ethanol at 300 ◦C was about 3 times higher than that of pristine sensor. Also, in general, the response and recovery times of the Fe2O3/Co3O4 nanocomposite sensor were shorter than those of the pristine one. The improved sensing characteristics of the Fe2O3/Co3O4 sensor were attributed to a combination of several effects: the formation of a potential barrier at the Fe2O3-Co3O4 interface, the enhanced modulation of the conduction channel width accompanying the adsorption and desorption of ethanol gas, the catalytic activity of Co3O4 for the oxidation of ethanol, the stronger oxygen adsorption of p-type Co3O4, and the formation of preferential adsorption sites.

Towards Future Circular Colliders

Abstract: The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) presently provides proton-proton collisions at a center-of-mass (c.m.) energy of 13 TeV. The LHC design was started more than 30 years ago, and its physics program will extend through the second half of the 2030’s. The global Future Circular Collider (FCC) study is now preparing for a post-LHC project. The FCC study focuses on the design of a 100-TeV hadron collider (FCC-hh) in a new ∼100 km tunnel. It also includes the design of a high-luminosity electron-positron collider (FCCee) as a potential intermediate step, and a lepton-hadron collider option (FCC-he). The scope of the FCC study comprises accelerators, technology, infrastructure, detectors, physics, concepts for worldwide data services, international governance models, and implementation scenarios. Among the FCC core technologies figure 16-T dipole magnets, based on Nb3Sn superconductor, for the FCC-hh hadron collider, and a highly-efficient superconducting radiofrequency system for the FCC-ee lepton collider. Following the FCC concept, the Institute of High Energy Physics (IHEP) in Beijing has initiated a parallel design study for an e+e− Higgs factory in China (CEPC), which is to be succeeded by a high-energy hadron collider (SPPC). At present a tunnel circumference of 54 km and a hadron collider c.m. energy of about 70 TeV are being considered. After a brief look at the LHC, this article reports the motivation and the present status of the FCC study, some of the primary design challenges and R&D subjects, as well as the emerging global collaboration.

Prediction of the band structures of Bi2Te3-related binary and Sb/Se-doped ternary thermoelectric materials

Abstract: Density functional calculations are performed to study the band structures of Bi2Te3-related binary (Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3) and Sb/Se-doped ternary compounds [(Bi1−x Sb x )2Te3 and Bi2(Te1−y Se y )3]. The band gap was found to be increased by Sb doping and to be monotonically increased by Se doping. In ternary compounds, the change in the conduction band structure is more significant as compared to the change in the valence band structure. The band degeneracy of the valence band maximum is maintained at 6 in binaries and ternaries. However, when going from Bi2Te3 to Sb2Te3 (Bi2Se3), the degeneracy of the conduction band minimum is reduced from 6 to 2(1). Based on the results for the band structures, we suggest suitable stoichiometries of ternary compounds for high thermoelectric performance.

Hybrid-density functional theory study on the band structures of tetradymite-Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3 thermoelectric materials

Abstract: The low-energy band structure near the band gap determines the electrical performance of thermoelectric materials. Here, by using the hybrid-density functional theory (hybrid-DFT) calculations, we calculate the low-energy band structure of Bi2Te3, Sb2Te3, Bi2Se3 and Sb2Se3 in the tetradymite phase. We find that the band structure characteristics are very sensitive the selection of the exchange energy functional. The predictability of the band gaps and the band degeneracies is not enhanced in hybrid-DFT calculations, as compared to DFT calculations. The poor prediction of low-energy band structures originates from the poor prediction of interlayer distances and the high structure sensitivity on the band gap. We conclude that the hybrid-DFT calculations are not superior to DFT calculations when predicting band structures of tetradymite Bi2Te3, Sb2Te3, Bi2Se3 and Sb2Se3 thermoelectric materials.

Unconventional entropy production in the presence of momentum-dependent forces

Abstract: We investigate the unconventional nature of entropy production (EP) in nonequilibrium systems with odd-parity variables that change signs under time reversal. We consider the Brownian motion of a particle in contact with a heat reservoir, where the particle’s momentum is an odd-parity variable. In the presence of an external momentum-dependent force, the EP transferred to the environment is found to be not equivalent to the usual reservoir entropy change due to heat transfer. An additional unconventional contribution to the EP, which is crucial for maintaining the non-negativity of the (average) total EP enforced by the second law of thermodynamics, appears. A few examples are considered to elucidate the novel nature of the EP. We also discuss detailed balance conditions with a momentum-dependent force.

Control of the saturation temperature in magnetic heating by using polyethylene-glycol-coated rod-shaped nickel-ferrite (NiFe2O4) nanoparticles

Abstract: Polyethylene-glycol (PEG)-coated nickel-ferrite nanoparticles were prepared for magnetic hyperthermia applications by using the co-precipitation method. The PEG coating occurred during the synthesis of the nanoparticles. The coated nanoparticles were rod-shaped with an average length of 16 nm and an average diameter of 4.5 nm, as observed using transmission electron microscopy. The PEG coating on the surfaces of the nanoparticles was confirmed from the Fourier-transform infrared spectra. The nanoparticles exhibited superparamagnetic characteristics with negligible coercive force. Further, magnetic heating effects were observed in aqueous solutions of the coated nanoparticles. The saturation temperature could be controlled at 42 ℃ by changing the concentration of the nanoparticles in the aqueous solution. Alternately, the saturation temperature could be controlled for a given concentration of nanoparticles by changing the intensity of the magnetic field. The Curie temperature of the nanoparticles was estimated to be 495 ℃. These results for the PEG-coated nickel-ferrite nanoparticles showed the possibility of utilizing them for controlled magnetic hyperthermia at 42 ℃.

Current status of the CXI beamline at the PAL-XFEL

Abstract: The Pohang Accelerator Laboratory’s X-ray free electron laser (PAL-XFEL) is a research facility currently under construction. It will provide ultra-bright (1 × 1012 photons/pulse at 12.4 keV) and ultra-short (10 − 60 femtosecond) X-ray pulses. The CXI (coherent X-ray imaging) hard X-ray experimental station is designed to deliver brilliant hard X-rays (2 − 20.4 keV) and to measure diffraction signals with a forward scattering geometry. It will not only offer imaging studies of biological, chemical and physical samples with the “diffraction-before-destruction” scheme, but also be helpful in high-field hard X-ray physics and material science. The scientific programs are currently aimed at serial femtosecond crystallography (SFX) and coherent diffraction imaging (CDI) for bio specimens, nano materials, etc. In this paper, we describe the beamline layout, beam diagnostics, X-ray focusing optics, sample environments and detector system at the CXI experimental hutch.

Theoretical investigation of the structural, magnetic and band structure characteristics of Co2FeGe1−xSix (x = 0, 0.5, 1) full-Heusler alloys

Abstract: In this study, the structural, magnetic and electronic properties of the Co2FeGe1−x Si x (x = 0, 0.5, 1) Heusler compounds have been calculated using the full-potential linearized augmented plane-wave method based on the spin density functional theory within the generalized gradient approximation of Perdew-Burke-Ernzerhof. In order to take into account the correlation effects, we have also performed GGA + U calculations, where the Hubbard on-site Coulomb interaction correction U is calculated by using the constraint local density approximation for the Co and the Mn atoms. The Cu2MnAl-type structure is found to be energetically more favorable than the Hg2CuTitype structure for both the Co2FeSi and the Co2FeGe compounds. The calculated atomic resolved densities of states of Co2FeSi and Co2FeGe indicate nearly half-metallic behaviors with small spindown electronic densities of states at the Fermi level. This behavior is corrected by including the Hubbard Coulomb energy U term. The Coulomb exchange correlation U confirms the halfmetallic property in both the Co2FeSi and the Co2FeGe compounds. We also discuss the electronic structures, the total and the partial densities of states, and the local magnetic moments. The Co2FeGe0.5Si0.5 compound shows a nearly half-metallic behavior with a small spin-down electronic density of states at the Fermi level in both the GGA and GGA+U approximations.

Hyperbolic octonion formulation of the fluid Maxwell equations

Abstract: The equations of compressible ideal fluids analogous to those of electromagnetism are reformulated in terms of hyperbolic octonions. Furthermore, the wave equations with source terms are generalized in a compact and elegant form. The analogy between fluid mechanics and electromagnetism is also argued by considering the previous octonionic formulations in relevant literature.

First-principles study on the structural, electronic, and optical properties of Ca 1− x Sr x Se alloys

Abstract: The structural, electronic, and optical properties of binary CaSe and SrSe compounds and Ca1−xSrxSe alloys were studied by using the full potential linearized augmented plane wave (FPLAPW) method within density functional theory (DFT). The band structure calculations showed that the CaSe and the SrSe binary compounds in the rocksalt (RS), zinc-blende (ZB) and wurtzite (WZ) phases were semiconductors while they had a metallic characteristic in the CsCl phase. The lattice constant and bulk modulus values for the Ca1−xSrxSe alloys in the RS and the ZB phases at different concentrations were calculated and compared with those obtained by using Vegard’s law. The energy band gap values in the RS and the ZB phases were estimated for different x values by using both define acronyms the Perdew, Burke, and Ernzerhof (PBE-GGA) and the Engel and Vosko (EV-GGA) schemes, and the results were compared with those obtained by using the empirical electronegativity expression. The band gap bowing parameters were calculated by using quadratic functions and the procedure of Bernard and Zunger to fit the non-linear variation of the band gaps. The static dielectric constant e1(0) was calculated at different concentrations. The energy loss function L(ω) for the Ca1−xSrxSe alloys in the RS and the ZB phases has a main peak corresponding to the plasmon frequency. The values of the static refractive index (n(0)) for the Ca1−xSrxSe alloys were calculated and compared with the values predicted by using the Moss, Ravindra, and Vandamme models. Finally, the extinction indic incident photon energies. es (k(ω)) and the reflectivities (R(ω)) for the Ca1−xSrxSe alloys were calculated within a wide range of incident photon energies.

Synthesis of nano precipitated calcium carbonate by using a carbonation process through a closed loop reactor

Abstract: Nano calcium carbonate particles have a wide range of industrial applications due to their beneficial properties such as high porosity and high surface area to volume ratio and due to their strengthening the mechanical properties of plastics and paper. Consequently, significant research has been done to deliver a new approach for the synthesis of precipitated nano calcium carbonate by using a carbonation process through a closed loop reactor. Both the experimental and the instrumental parameters, i.e. the CO2 flow rate, the concentration of the starting materials (Ca(OH)2 and CaO), the pH, the orifice diameter, etc., were investigated. The carbonation efficiency was increased due to the diffusion process involved in the loop reactor. The particle size was affected by the CO2 flow rate, reaction time, and orifice diameter. Finally, precipitated nano calcite calcium carbonate (50 to 100 nm) was synthesized by optimizing all the experimental and the instrumental parameters. The synthesized precipitated nano calcium carbonate was characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. This study has proved that the carbonation efficiency can be enhanced for a short time by using a loop reactor and that the carbonation process was more energy efficient and cost effective than other conventional methods.

Shannon information entropies for the three-dimensional Klein-Gordon problem with the Poschl-Teller potential

Abstract: The Shannon information entropies for the Klein-Gordon equations are evaluated for the Poschl-Teller potential, and the position-space information entropies for the ground and the excited states are calculated.

Electrical and optical properties of VO2 thin films grown on various sapphire substrates by using RF sputtering deposition

Abstract: VO2 thin films were grown on a-, c-, m-, and r-plane sapphire and SiO2/Si substrates under identical conditions by using RF sputtering deposition from a VO2 target. The structural and the morphological properties of all VO2 films were investigated. The grain sizes of the VO2 films varied between 268 nm and 355 nm depending on the substrate’s orientation. The electrical and the optical properties of all VO2 thin films were examined in detail. The metal-insulator transition temperature (TMI) varied with the substrate’s orientation. The (200)/( $$\bar 211$$ )-oriented VO2 films on the a-plane sapphire showed the lowest TMI of about 329.3 K (56.3 °C) while the (020)/(002)-VO2 films on the c-plane sapphire displayed the highest TMI of about 339.6 K (66.6 °C). The VO2 films showed reversible changes in the resistivity as large as 1.19 × 105 and a hysteresis of ~2 K upon traversing the transition temperature. The variations observed in the TMI with respect to the substrate’s orientation were due to changes in the lattice strain and the grain size distribution. Raman spectroscopy showed that metal (rutile) - insulator (monoclinic) transitions occurred via the M2 phase for VO2 films on the c-plane substrate rather than the direct M1 to rutile transition. The shifts in the phonon frequencies of the VO2 film grown on various sapphire substrates were explained in terms of the strain along the V-V atomic bond direction (cR). Our work shows a possible correlation between the transition parameters (e.g., TMI, sharpness, and hysteresis width) and the width (σ) of the grain size distribution. It also shows a possible correlation between the TMI and the resistivities at the insulating and the metallic phases for VO2 films grown on various sapphire substrates.

Surface coverage enhancement of a mixed halide perovskite film by using an UV-ozone treatment

Abstract: Recently, a significant breakthrough in emerging photovoltaics occurred. Now, perovskite solar cells, hybrid types of organic and inorganic solar cells, are considered as reliable next-generation solar cells due to their outstanding photovoltaic performance. Records of the National Renewable Energy Laboratory (NREL) on cell efficiency research indicates a prominent growth in the power conversion efficiency (PCE) of a perovskite solar cells which is now approaching 20.1%. Perovskite solar cells are, in general, classified into three types based on their structures; the mesoporous type with TiO2 nanoparticles, the meso-superstructure type with Al2O3 and the planar hetero-junction type. Among them, planar-structured perovskite solar cells have strong advantages due to their easy processibility and flexibility. We can replace the materials in the electron transport layer (ETL) and the hole transport layer (HTL) with common materials that are available in organic solar cells. However, a great challenge is to fabricate a high-quality perovskite film because the perovskite morphology is highly sensitive to its fabrication conditions. For control of the film’s morphology, some experiments, such as changing the annealing temperature or time and adding some additives, have been done to increase the surface coverage of perovskite films. In this work, we introduce normal, planar, perovskite solar cells with a hetero-junction structure based on compact TiO2 and a mixed halide perovskite (CH3NH3PbI3−x Cl x ). To enlarge the surface coverage of perovskite film, we used an UV-ozone treatment on top of the compact TiO2, which made the surface of TiO2 hydrophilic. Because a perovskite precursor is hydrophilic, an UV-ozone treatment is expected to improve the wettability between the compact TiO2 and the perovskite film. Here, we present the photovoltaic performance, along with the surface coverage difference, for various UV-ozone treatment time. In addition, the effect of the UV-ozone treatment was examined by using an opto-electronic analysis.

First-Principles Calculation the Electronic Structure and the Optical Properties of Mn-Decorated g-C3N4 for Photocatalytic Applications

Abstract: The electronic structure and the optical properties of Mn-decorated graphitic carbon nitride (g-C3N4) were investigated using the density functional method. The large absorption energy of the Mn atoms on the g-C3N4 surface was found to suppress the clustering of the Mn atoms, which led to a conservation of the photocatalytic activity. The electronic structures of the Mn-decorated g-C3N4 showed that impurity energy levels emerged in the forbidden band of g-C3N4 and that the band edge of g-C3N4 shifted upward to 0.40 eV. In addition, the calculated optical constants showed that the novel photon absorption in the range of visible light originated from electronic transitions from the N 2p states in the upper valence band to impurity Mn 3d states. Moreover, the photon absorption reached a maximum when all sites of triangular N holes were decorated with Mn atoms. Our results provide evidence that the Mn-decorated C3N4 system could be a highly-efficient photocatalyst for solar light due to the extension of the range of photon absorption to include almost all visible light.

Numerical analysis on the effect of angle of attack on evaluating radio-frequency blackout in atmospheric reentry

Abstract: A three-dimensional numerical simulation model that considers the effect of the angle of attack was developed to evaluate plasma flows around reentry vehicles. In this simulation model, thermochemical nonequilibrium of flowfields is considered by using a four-temperature model for high-accuracy simulations. Numerical simulations were performed for the orbital reentry experiment of the Japan Aerospace Exploration Agency, and the results were compared with experimental data to validate the simulation model. A comparison of measured and predicted results showed good agreement. Moreover, to evaluate the effect of the angle of attack, we performed numerical simulations around the Atmospheric Reentry Demonstrator of the European Space Agency by using an axisymmetric model and a three-dimensional model. Although there were no differences in the flowfields in the shock layer between the results of the axisymmetric and the three-dimensional models, the formation of the electron number density, which is an important parameter in evaluating radio-frequency blackout, was greatly changed in the wake region when a non-zero angle of attack was considered. Additionally, the number of altitudes at which radio-frequency blackout was predicted in the numerical simulations declined when using the three-dimensional model for considering the angle of attack.

Optimization and performance improvement of an electromagnetic-type energy harvester with consideration of human walking vibration

Abstract: In this study, we derived an equation of motion for an electromechanical system in view of the components and working mechanism of an electromagnetic-type energy harvester (ETEH). An electromechanical transduction factor (ETF) was calculated using a finite-element analysis (FEA) based on Maxwell’s theory. The experimental ETF of the ETEH measured by means of sine wave excitation was compared with and FEA data. Design parameters for the stationary part of the energy harvester were optimized in terms of the power performance by using a response surface method (RSM). With optimized design parameters, the ETEH showed an improvement in performance. We experimented with the optimized ETEH (OETEH) with respect to changes in the external excitation frequency and the load resistance by taking human body vibration in to account. The OETEH achieved a performance improvement of about 30% compared to the initial model.

Ta2O5-memristor synaptic array with winner-take-all method for neuromorphic pattern matching

Abstract: Pattern matching or pattern recognition is one of the elemental components that constitute the very complicated recalling and remembering process in human’s brain. To realize this neuromorphic pattern matching, we fabricated and tested a 3 × 3 memristor synaptic array with the winner-take-all method in this research. In the measurement, first, the 3 × 3 Ta2O5 memristor array is programmed to store [LLL], [LHH], and [HLH], where L is a low-resistance state and H is a high-resistance state, at the 1st, 2nd, and 3rd columns, respectively. After the programming, three input patterns, [111], [100], and [010], are applied to the memristor synaptic array. From the measurement results, we confirm that all three input patterns can be recognized well by using a twin memristor crossbar with synaptic arrays. This measurement can be thought of as the first real verification of the twin memristor crossbar with memristive synaptic arrays for neuromorphic pattern recognition.

Ar plasma treated polytetrafluoroethylene films for a highly efficient triboelectric generator

Abstract: We report an Ar plasma treated polytetrafluoroethylene (PTFE) film based triboelectric device for a highly enhanced electric power generation. The plasma treatment of the PTFE in flowing Ar gas results in a sharp increase in surface roughness (~46 nm), as compared with the as-received film (~25 nm). In addition, the F ion content decreases whereas the O ion increases with increasing plasma reaction time. Because of the increased number of polar O ions, the surface becomes hydrophilic, as confirmed by water contact angle measurements. After the Ar plasma treatment, the PTFE based triboelectric device, which is periodically contacted with and separated from the ITO electrode, generates a 715 V open-circuit voltage and a 16 μA closed-circuit current, which are almost 79 and 32 times larger than those for as-received PTFE based device. Using the Ar plasma treated PTFE based triboelectric generator, we can turn on the 120 light emitting diodes (LEDs) without any batteries.

Composition-dependent structural, dielectric and ferroelectric responses of lead-free Bi 0.5 Na 0.5 TiO 3 -SrZrO 3 ceramics

Abstract: The influence of SrZrO3 (SZ) addition on the crystal structure, piezoelectric and the dielectric properties of lead-free Bi0.5Na0.5TiO3 (BNT−SZ100x, with x = 0 − 0.10) ceramics was systematically investigated. A significant reduction in the grain size was observed with SZ substitution. The X-ray diffraction analysis of the sintered BNT−SZ ceramics revealed a single perovskite phase with a pseudocubic symmetry; however, electric poling indicated a non-cubic distortion in the poled BNT−SZ ceramics. With increase in the SZ content, the temperature of maximum dielectric constant (T m ) shifted towards lower temperatures, and the curves became more diffuse. Enhanced piezoelectric constant (d 33 = 102 pC/N) and polarization response were observed for the BNT−SZ5 ceramics. The results indicated that SZ substitution induced a transition from a ferroelectric to relaxor state with a field-induced strain of 0.24% for BNT−SZ9 corresponding to a normalized strain of 340 pm/V.

Electronic structure and optical properties of CsI, CsI(Ag), and CsI(Tl)

Abstract: The band structure, electronic density of states and optical properties of CsI and of CsI doped with silver or thallium are studied by using a first-principles calculation based on density functional theory (DFT). The exchange and the correlation potentials among the electrons are described by using the generalized gradient approximation (GGA). The results of our study show that the electronic structure changes somewhat when CsI is doped with silver or thallium. The band gaps of CsI(Ag) and CsI(Tl) are smaller than that of CsI, and the width of the conduction band of CsI is increased when CsI is doped with thallium or silver. Two peaks located in the conduction band of CsI(Ag) and CsI(Tl) are observed from their electronic densities of states. The absorption coefficients of CsI, CsI(Ag), and CsI(Tl) are zero when their photon energies are below 3.5 eV, 1.5 eV, and 3.1 eV, respectively. The results show that doping can improve the detection performance of CsI scintillators. Our study can explain why doping can improve the detection performance from a theoretical point of view. The results of our research provide both theoretical support for the luminescent mechanisms at play in scintillator materials when they are exposed to radiation and a reference for CsI doping from the point of view of the electronic structure.

Chaos in Lifshitz Spacetimes

Abstract: We investigate the chaotic behavior of a circular test string in the Lifshitz spacetimes by considering the critical exponent z as an external control parameter. We demonstrate that two primary tools to observe chaos in this system are the Poincare section and the Lyapunov exponent. Finally, the numerical result shows that if z = 1, the string dynamics is regular while in a case slightly larger than z = 1, the dynamics can be irregular and chaotic, which implies that the space time anisotropy, which breaks Lorentz symmetry, may cause the system to be chaotic.

Radiation imaging with a rotational modulation collimator (RMC) coupled to a Cs2LiYCl6:Ce (CLYC) detector

Abstract: As an attempt to develop a gamma-ray/neutron dual-particle imager, we employed a rotating modulation collimator (RMC) coupled to a pulse shape discrimination-capable scintillator. We performed fundamental simulations on the proposed RMC system utilizing a CLYC detector to verify the basic properties of the RMC system and to optimize the computing methods for the Monte Carlo simulation. We obtained batches of modulation patterns for various source locations by using Monte Carlo N-Particle simulations, and we studied the characteristics of the modulation patterns, such as the rotational effect, the shift effect, and the symmetric effect. We compared simulated modulation patterns with those obtained from the mathematical model of the RMC system to investigate the feasibility of identifying the source location correctly based on the simulated patterns. When a source was located in the far-field region, the modulation patterns showed good agreement between the Monte Carlo simulation and the mathematical model. The results hold promises for reconstructing gamma and neutron images of radioactivity by using a RMC system on a CLYC detector.

Construction of exact Ermakov-Pinney solutions and time-dependent quantum oscillators

Abstract: The harmonic oscillator with a time-dependent frequency has a family of linear quantum invariants for the time-dependent Schrodinger equation, which are determined by any two independent solutions to the classical equation of motion. Ermakov and Pinney have shown that a general solution to the time-dependent oscillator with an inverse cubic term can be expressed in terms of two independent solutions to the time-dependent oscillator. We explore the connection between linear quantum invariants and the Ermakov-Pinney solution for the time-dependent harmonic oscillator. We advance a novel method to construct Ermakov-Pinney solutions to a class of time-dependent oscillators and the wave functions for the time-dependent Schrodinger equation. We further show that the first and the second Poschl-Teller potentials belong to a special class of exact time-dependent oscillators. A perturbation method is proposed for any slowly-varying time-dependent frequency.

Light dark matter and dark radiation

Abstract: Light (M ≤ 20 MeV) dark-matter particles freeze out after neutrino decoupling. If the dark-matter particle couples to a neutrino or an electromagnetic plasma, the late time entropy production from dark-matter annihilation can change the neutrino-to-photon temperature ratio, and equally the effective number of neutrinos Neff. We study the non-equilibrium effects of dark-matter annihilation on the Neff and the effects by using a thermal equilibrium approximation. Both results are constrained with Planck observations. We demonstrate that the lower bounds of the dark-matter mass and the possibilities of the existence of additional radiation particles are more strongly constrained for dark-matter annihilation process in non-equilibrium.

Fabrication of a high-density nano-porous structure on polyimide by using ultraviolet laser irradiation

Abstract: A new approach for fabricating a high-density nano-porous structure on polyimide (PI) by using a 355-nm UV laser is presented here. When PI was irradiated by using a laser, debris that had electrical conductivity was generated. Accordingly, that debris caused electrical defects in the field of electronics. Thus, many researchers have tried to focus on a clean processing without debris. However, this study focused on forming a high density of debris so as to fabricate a nano-porous structure consisting of nanofibers on the PI film. A PI film with closed pores and open pores was successfully formed by using a chemical blowing agent (azodicarbonamide, CBA) in an oven. Samples were precured at 130 °C and cured at 205 °C in sequence so that the closed pores might not coalesce in the film. When the laser irradiated the PI film with closed pores, nanofibers were generated because polyimide was not completely decomposed by photochemical ablation. Our results indicated that a film with micro-closed pores, in conjunction with a 355-nm pulsed laser, can facilitate the fabrication of a high-density nano-porous structure.

Color change of tourmaline by heat treatment and electron beam irradiation: UV-Visible, EPR, and Mid-IR spectroscopic analyses

Abstract: The color of pink tourmaline gemstone changed to colorless when heating at temperature of 600 °C in air. This colorless tourmaline recovered its pink color when irradiated with an electron beam (e-beam) of 800 kGy. The origin of the color change was investigated in three types of tourmaline gemstones, two pink are from Afghanistan and one green are from Nigeria, by using Ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FTIR), Electron paramagnetic resonance (EPR), and Energy Dispersive X-ray Fluorescence (EDXRF). The UV-Vis absorption spectrum of the pink tourmaline with higher Mn concentration (T2, 0.24 wt%) showed characteristic absorption peaks originating from the Mn3+ color center: two absorption bands centered at wavelength of 396 and 520 nm, respectively. Both absorption bands disappeared when heated in air at 600 °C and then reappeared when irradiated with an e-beam at 800 kGy. EPR T2 spectra showed that the color change was related to the valence change of Mn3+ to Mn2+ and vice versa. The pink tourmaline of lower MnO content (T1, 0.08 wt%) also became colorless when heated, but the color was not recovered when the gemstone underwent e-beam irradiation. Instead, a yellow color was obtained. UV-Vis and FTIR spectra indicated that this yellow color originated from a decomposition of the hydroxyl group (−OH) into O− and Ho by the e-beam irradiation. Green tourmaline did not show any color change with either heat treatment or e-beam irradiation.

Thermoelectric properties of the ceramic oxide Sr 1− x La x TiO 3

Abstract: The effect of lanthanum on the electric and the thermoelectric properties of the ceramic oxide Sr1−x La x TiO3 (where x = 0.0, 0.04, 0.06, 0.08 and 0.12 mole) have been studied. La-doped SrTiO3 was prepared by using the conventional mixed-oxide reaction method. XRD patterns indicated that almost all the La atoms incorporated into the SrTiO3 crystal provided charge carriers. The lattice parameter increases with increasing La doping content. The relative densities of all the samples varied from 89.6% to 94.8%. The electrical conductivity increased with La doping up to 0.08 moles and then decreased as the content of La was increased above 0.08 moles. The thermal conductivity decreased with increasing La content. The largest absolute value of the Seebeck coefficient, 394 μVK−1 at 973 K, was observed at x = 0.04. The Sr0.92La0.08TiO3 sample showed its maximum electrical conductivity at 773 K and its largest ZT value of 0.20 at 973 K.

Formation of a plano-convex micro-lens array in fused silica glass by using a CO 2 laser-assisted reshaping technique

Abstract: We report on fabricating high-fill-factor plano-convex spherical and square micro-lens arrays on fused silica glass surface by using a CO2 laser-assisted reshaping technique. Initially, periodic micro-pillars are encoded on glass surfaces by means of a femtosecond laser beam, afterwards, the micro-pillars are polished several times by irradiating a CO2 laser beam on top of the micro-pillars. Consequently, a spherical micro-lens array with micro-lens size of 50 μm × 50 μm and a square micro-lens array with micro-lens size of 100 μm × 100 μm are formed on the surface of the fused silica glass. We also study the intensity distribution of light passing through the glass sample engraved with a spherical micro-lens array. The simulation result shows that the focal length of the spherical micro-lens array is 35 μm. Furthermore, we investigate the optical properties of glass samples with engraved micro-lens arrays. The proposed CO2-laser-based reshaping technique is simple and fast and shows promises for fabricating arrays of smooth micro-lenses in various transparent materials.