Femtosecond laser pulse energy tunneling in the range of forbidden frequencies of a one-dimensional photonic crystal

The results of computer simulations demonstrate the possibility of light energy transmission through band gaps of a one-dimensional photonic crystal with fluctuating lengths of individual layers. In the case of weak (±5%) fluctuations, the light pulse energy partly penetrates in depth a photonic crystal. If fluctuations of the layer length are strong (±20%), the light pulse energy can be partly transmitted through the crystal.     

Phonon-Mediated Distributed Transition-Edge-Sensor X-Ray Detector with Deep Trenches

We continue our development of a phonon-mediated distributed-TES X-ray detector. X-rays are absorbed in a large silicon or germanium crystal, and the energy is read out by four distributed TESs. This design takes advantage of existing TES technology while overcoming the difficulties of designing spatially large arrays. In this paper, we discuss three detector designs. First, a silicon detector with 220 μm deep trenches through a 350 μm crystal. Second, a germanium detector with 275 μm deep trenches through a 550 μm crystal. Finally another silicon detector with 330 μm deep trenches through a 350 μm crystal. We discuss energy loss mechanisms in the detector and propose a reason for the energy resolution that we observe.      

Molecular dynamics calculation of the J-integral fracture criterion for nano-sized crystals

The critical J-integral is recognized as a valuable criterion for evaluating elastic-plastic fracture behavior of materials. It is, however, computationally complicated at the atomic scale for a nano-sized crystal when using the traditional integral method to reckon the traction force, strain, strain energy, etc. atom by atom. This paper attempts to propose an effective J-integral calculation method based on molecular dynamics (MD) simulations. The proposed method takes advantage of the potential energy interpretation of the J-integral, the main task in which is to calculate the potential energy difference of two identical crystal models with the neighboring crack length. As the potential energy of crystals at the atomic scale can be obtained easily from molecular dynamics simulations, the proposed method is thus computationally simple and especially applicable to complex crystal structures. A nickel crystal having an opening model I crack is investigated using this method. The critical J-integral in the ductile fracture plane (1\bar10) is calculated to be 4.424 Jm^–2. It is also found in this study that the onset of crack propagation does not coincide with the occurrence of the maximum tension stress for nano-sized crystals, which is different from conventional fracture mechanics observation.     

Thermal-and field-treatment induced reconstruction of a carbon fiber crystal

A carbon (PAN) fiber crystal was studied in a field electron microscope equipped with a dispersive energy analyzer. Annealing of the emitter crystal at a temperature of 750° C lead to a tenfold decrease in the emission current and to the appearance of an additional low-energy peak (shifted by 0.5 eV from the main peak toward lower energies) in the field electron energy distribution. An increase in the anode voltage restored both the initial emission current and the electron energy distribution.     

Dielectric properties of YNi2B2C

The dielectric function of the quarternary superconductor YNi₂B₂C has been derived in an energy range from WmeV up to 50eV by linking together optical reflectance and electron energy loss measurements, obtained from polycrystalline samples at room temperature. The low energy part of the spectrum was additionally investigated in a temperature range from 10K up to 300K. In order to determine the anisotropy of this compound, the reflectance spectra of a single crystal were taken, using polarized light with polarization along the crystallographic axes. The single crystal data exhibit a normal Drude-like metallic behaviour, while the polycrystalline data can only be interpreted by means of a strong energy dependent scattering rate, which has been interpreted as resulting from strong electron-phonon coupling.     

YAG(Ce) crystal characterization with proton beams

A YAG(Ce) crystal has been characterized with a proton beam up to 100 MeV. Tests were performed to investigate the possibility of using this detector as a proton calorimeter. A crystal size has been chosen that is able to stop up to 200 MeV. Energy resolution and light response have been measured at Laboratori Nazionali del Sud with a proton beam up to 60 MeV and a spatial homogeneity study of the crystal has been performed at Loma Linda University Medical Center with a 100 MeV proton beam. The YAG(Ce) crystal showed a good energy resolution equal to 3.7% at 60 MeV and measurements, performed in the 30-60 MeV proton energy range, were fitted by Birks' equation. Using a silicon tracker to determine the particle entry point in the crystal, a spatial homogeneity value of 1.7% in the light response has been measured.     

溅射Ar+能量对离子束溅射制备Ge薄膜结晶性的影响

采用离子束溅射技术在不同Ar 能量下溅射Ge单层膜.用Raman光谱和AFM图谱对薄膜进行表征,得到Ge薄膜结晶性和晶粒大小随Ar 能量变化的波动性关系.在0.6keV的Ar 能量下,得到晶粒较小的Ge晶化薄膜,0.8keV能量下,Ge薄膜为非晶结构,1.0keV能量下,得到晶粒较大的Ge晶化薄膜.通过对沉积原子数与沉积原子能量的分析,解释了这一波动性变化.     

Comparative crystal field calculations of the Cr<Superscript>3+</Superscript> energy level schemes in ZnAl<Subscript>2</Subscript>S<Subscript>4</Subscript> and ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript>

The exchange charge model of crystal field (which includes the covalent effects) is used to analyze the energy level schemes of Cr³⁺ ion in ZnAl₂S₄ and ZnGa₂O₄ crystals with spinel structure. Calculations of the overlap integrals and crystal field parameters were performed before getting the Cr³⁺ energy levels. The calculated energy level schemes are compared with available experimental data; a new interpretation of the absorption peaks is suggested.     

Electronic structure, charge density, and chemical bonding properties of C11H8N2O o-methoxydicyanovinylbenzene (DIVA) single crystal

A comprehensive theoretical density functional theory (DFT) study of the electronic crystal structure, bonding properties, electron charge density of C₁₁H₈N₂O o-methoxydicyanovinylbenzene (DIVA) single crystals were performed. The exchange and correlation potential was described within a framework of the local density approximation (LDA) by Ceperley-Alder and gradient approximation (GGA) based on exchange–correlation energy optimization to calculate the total energy. In addition, we have used Engel–Vosko generalized gradient approximation (EV-GGA) and the modified Becke–Johnson potential (mBJ) for the electronic crystal structure, bonding properties, electron charge density calculations. There is systematically increasing in the energy gap from 2.25 eV (LDA), 2.34 eV (GGA), 2.50 eV (EV-GGA), 2.96 eV (mBJ). Our calculations show that this crystal possess direct energy gap. Furthermore, the electronic charge density space distribution contours in the (1 1 0) crystallographic plane clarifies the nature of chemical bonding.     

Observations on dual-ended readout of 100 mm long LYSO crystals

We are investigating using dual-ended readout of axially oriented long thin scintillator crystals in detectors for a compact geometry, small ring diameter animal PET system. The axial position of interaction is determined from the light sharing between two photodetectors at opposite ends of the crystal. We examine the light output, energy resolution and axial spatial resolution of 1.5-5×2×100 mm³ polished LYSO crystals by irradiating with an electronically collimated beam of 511 keV photons oriented perpendicular to the long axis and read out at either end by position sensitive photomultiplier tubes (PSPMTs). Three reflector materials, namely Teflon, 3 M enhanced specular reflector (ESR) and black paint are examined for the 2×2×100 mm³ crystal size. The light output increases and energy resolution improves with the crystal cross-section. Generally, the spatial resolution worsens with increase in crystal cross-section. For the 2×2×100 mm³ crystal size, the mean energy resolutions of the photopeak over the nine irradiation positions were 14.4±0.4%, 16.0±1.2% and 28.3±2.1% with mean spatial resolutions of 7.0±1.0, 9.4±3.3 and 26.0±5.0 mm using ESR, Teflon and black paint, respectively. ESR reflector gave the best light output, energy and axial spatial resolutions. These characterization results of PSPMT-based dual-ended long LYSO crystals will be useful in the design of detector modules for a highly compact geometry preclinical PET system using this detector technology.     

Investigation of the liquid crystal alignment layer: effect on electrical properties

Abstract

We investigate the electrical behavior of a symmetric liquid crystal (LC) cell: elecrode-silane-LC-silane-electrode. The silane (chlorodimethyloctadecyl-silane) layer induces a homeotropic orientation of the nematic liquid crystal (NLC) molecules. The wettability technique is used to detect the change of the surface energy of the electrode upon cleaning and silane layer deposition. We report on the dynamic impedance measurements of the nematic liquid crystal cell. It is found that the silane alignment layer has a blocking effect on the liquid crystal (LC) cell. We also study the relaxation behavior of the cell which is later assimilated as an electrical equivalent circuit.     

Determination of the activation energy for crystal growth by differential thermal analysis

A method for determining the activation energy for crystal growth was calculated on the basis of the heat balance in the differential thermal analysis (DTA) measurements and the mechanism of nucleation and growth. The theoretical analysis showed that the term ln[C _pd(δT)/dt+KδT] should be a linear function of l/T, whereC _p is the heat capacity of sample and sample holder,K is the heat transfer coefficient,δT is the temperature difference between the sample and reference substance andt is the time. The energy term,E _D, obtained by multiplying the slope of the resulting straight line byR is indicative of the activation energy for crystal growth. It was shown thatE _D should be three times the activation energy for crystal growth when bulk nucleation is dominant, and equal to that for crystal growth only when surface nucleation predominates. The result of the analysis was tested by comparing the experimentally determinedE _D's with the activation energy for viscous flow, which was known to represent that for crystal growth. TheE _D for Li₂O·2SiO₂ glass with dominant bulk nucleation, approached three times the activation energy for viscous flow, as the heating rate in DTA decreased. TheE _D for 33.3Li₂O·66.7SiO₂·3TiO₂ glass with dominant surface nucleation approached the activation energy for viscous flow as the heating rate increased, suggesting the validity of the analysis.     

Evolution of two-dimensional crystal morphologies by surface diffusion with anisotropic surface free energies

The Wulff shape of a crystal surface in equilibrium under anisotropic surface free energy has been widely reported and a typical differentiable anisotropic free energy functional form has been used in illustrations. Here we study the evolution of an arbitrary initial two-dimensional crystal shape to equilibrium and we classify the anisotropy in three cases. We find that when the typical surface free energy is critically anisotropic, multiple equilibrium states exist and the evolved equilibrium shape depends on the initial crystal configuration. Numerical simulations show that corners and planar facets develop on the surface in evolution. In cases of severe anisotropy, small surface facets and coarsening can occur. In contrast, when the typical surface free energy is mildly anisotropic, the evolving surface is smooth and the equilibrium shape is unique for a variety of the initial crystal shapes.     

A comparative study of the electrical properties of reduced and unreduced LiTaO<Subscript>3</Subscript> crystals

We have carried out a comparative study of the electrical properties of lithium tantalate (LiTaO₃) crystals in a wide temperature range (300–900 K) before and after reductive treatment in H₂O vapor and subsequent oxidative annealing. The results demonstrate that, in the temperature range of Li⁺ ion conduction (550–900 K), the activation enthalpy for ionic conduction in the reduced lithium tantalate crystal is H _a = 1.37 eV, which slightly exceeds that in the initial state of the crystal (1.34 eV). In the temperature range 390–450 K, the σ(T) data for the unannealed crystal are well represented by the Arrhenius law in the presence of two carrier types, with activation energies E ₁ = 1.03 eV and E ₂ = 0.29 eV, characteristic of proton and electron hopping conduction, respectively. After reductive annealing, the activation energy for conduction is ~0.65 eV, characteristic of the activation energy for bipolaron conduction. After subsequent oxidative annealing of the reduced crystals in dry air, the activation energy is ~1.2 eV. It seems likely that the presence of oxygen vacancies in the reduced LiTaO₃ crystal stimulates hydrogen release from the crystal during oxidative annealing.     

(Cr,Tm,Ho):YAG晶体中Tm~(3+)离子的中介能量转移作用

测定了晶体的吸收谱，荧光谱和荧光寿命，用Taylor和Dexter理论计算了Cr3+－Ho3+,Cr3+－Tm3+和Tm3_－Ho3+离子间能量转移效率和转移几率．讨论了（Cr，Tm．Ho）：YAG晶体中Tm3+的能量转移中介作用．     

γ-CuI晶体的发光衰减时间和对X射线的能量响应

采用溶剂蒸发法生长出透明的带隙宽度为2.96 eV的γ-CuI晶体。在紫外光激发下, 该晶体在410、430 nm处分别呈现有近带边发射峰, 另在720 nm附近还出现一个与样品碘缺陷有关的宽发射带。经碘退火后, 样品720 nm发射带被基本抑制, 而在420 nm处出现了一个更强的近带边发射峰。使用扫描相机分别测量了γ-CuI晶体各发射峰(带)的衰减时间谱, 其中近带边发射峰的发光衰减时间常数均在数十皮秒量级, 表明γ-CuI晶体具有极快的时间响应特性; 而720 nm发射带的发光衰减时间常数主要在数十纳秒量级。X射线激发下, γ-CuI晶体具有435 nm近带边发射峰和680 nm发射带, 其近带边发射对X射线能量响应的测量结果表明, 当E_X<49.1 keV时, γ-CuI晶体闪烁光快分量对X射线的探测效率相对较高。     

Dynamics of quantum crystals

The phonon spectrum in a quantum crystal, where short-range correlations are important, is obtained by adapting the general relations between the one-phonon response and the thermodynamic properties of an anharmonic crystal proposed by Götze. Specifically, the one-phonon self-energy is obtained by requiring that the elastic constants obtained by the method of long waves equal those obtained as the strain derivatives of the ground-state energy. In practice this means that the atomic force constants can be obtained by formal differentiation of the ground-state energy. We explore the effects of this conserving approximation for theT-matrix and Jastrow methods and implement it numerically for bcc helium.     

凝胶玻璃网络稳定性与析晶分析

利用溶胶-凝胶方法制备了：BaO-TiO2-Al2O3-SiO2体系的凝胶玻璃，以及其不同晶相的玻璃陶瓷。利用X-衍射分析确定凝胶玻璃析出的晶相。利用晶格能对凝胶玻璃的稳定性与析晶特性进行了定性分析。在凝胶玻璃的析晶过程中，品格能大者优先析晶。BaO-TiO-Al2O3-SiO2体系凝胶玻璃析出离子晶体的晶格能由大到小依次为BaTiO3、Ba2TiSi2O8、BaAl2Si2O8，这与实验结果的析晶次序一致。     

Does antisymmetry matter in b.c.c. 3He crystals?

By comparing the results of both variational and exact Diffusion Monte Carlo (DMC) results for states of different symmetries we conclude that antisymmetry plays a fundamental role in stabilizing the b.c.c. ³ He crystal. We performed calculations for a system of 54 particles of mass 3 at density = 0.02557 Å, just above the experimental freezing point. Symmetric (Jastrow and Shadow wave functions) and unsymmetrized wave functions (of the Nosanow-Jastrow type), fail to describe the system. In particular, a shadow wave function predicts a fluid as lowest energy state at the density considered, and this is confirmed by the computation of the exact symmetric ground state with DMC, which predicts an energy well below the experimental energy of the crystal. On the other hand, DMC calculations projecting the ground state in the space of the Nosanow–Jastrow functions, give an energy which is much above the experimental energy. The use of antisymmetric functions, and in particular of the recently introduced Fermionic Shadow Wave Function (FSWF), leads to the prediction that the b.c.c. crystal is the stable ground state. Antisymmetry plays therefore a fundamental role in this system. FSWF calculations also demonstrate the peculiar characteristics of this crystal (very low order parameter, a non Gaussian density profile around the lattice sites, and very wide vibrations of the atoms around the lattice sites, small dependence of the energy with respect to the magnetic order), which cannot be seen in the Nosanow framework.     

Optical excitation of space-charge waves involving interband transitions in Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> Crystals

The excitation of space-charge waves in a semiconductor (B₁₂GeO₂₀) crystal using light with photon energy close to the bandgap width has been studied. The sample crystal has been optically excited by exposure to an oscillating interference pattern. The existing model notions are consistent with the experimental results.