Electron diffraction study of various lattice defects in a Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> crystal

Lattice defects in a scintillation detector made of Bi₄Ge₃O₁₂ (BGO) could severely impact detector efficiency via non-radiative transfer of electron excitation, thus making thorough investigations of these defects highly important. Here we present a combined experimental and theoretical study of two- and three-dimensional defects in a Czochralski-grown BGO crystal. Upon examination by transmission electron microscopy the selected-area electron diffraction (SAED) patterns in two neighboring parts of the specimen reveal different kinds of two- and three-dimensional defects. Three sub-grains misoriented at 2.47° with reference to each other and probable presence of stacking faults lying in {011} planes were observed in the first examined local area. The SAED image taken from an area in the close neighborhood is much more complicated and is explained in terms of the superposition of reflections from: (i) a partially textured GeO₂ second-phase inclusion; (ii) the basic lattice of BGO and (iii) a superlattice-like structure based on the BGO lattice. The atomic structure of such a superlattice-like structure was theoretically modeled and the corresponding simulated SAED patterns were found to be in good agreement with the experimentally observed one.     

Electronic excitation energies in TiO<Subscript>2</Subscript> in the fluorite phase

The ab initio pseudopotential method within the generalized gradient approximation (GGA) and quasiparticle approximation has been used to investigate the electronic properties of titanium dioxide in the rutile, anatase, and fluorite structures, respectively. Here we present the GW approximation for the electronic self-energy, which allows to calculate excited-state properties, especially electronic band structures. For this calculation, good agreement with the experimental results for the minimum band gaps in rutile and anatase phase is obtained. In the fluorite phase we predict that titanium dioxide will be an indirect (Γ to X) wide band-gap semiconductor (2.367 or 2.369 eV) and the properties remain to be confirmed by experiment.     

Continuous-wave intracavity Raman laser at 1179.5 nm with SrWO<Subscript>4</Subscript> Raman crystal in diode-end-pumped Nd:YVO<Subscript>4</Subscript> laser

We report a continuous-wave intracavity Raman laser at 1179.5 nm with a SrWO₄ Raman crystal in a diode-end-pumped Nd:YVO₄ laser. The highest output power of 2.23 W is obtained at the laser diode power of 21.2 W corresponding to the slope efficiency of 17.3% and a diode-to-stokes optical conversion efficiency of 10.5%. The dependence of the Raman laser performance on the pump polarization is also studied. The measured Raman thresholds are about 9.3 and 8.3 W in the diode pump laser power for the a- and b-polarized configurations, respectively. The Raman gain coefficients of the c-cut SrWO₄ crystal for a- and b-polarized pumps are estimated to be about 4.9 and 4.7 cm/GW, respectively.     

Effect of annealing temperature on microstructures and optical properties of Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> films

Evolution of microstructure and optical property with annealing temperature has been examined for Ba_0.9Sr_0.1TiO₃ films derived from one single precursor solution containing polyethylene glycol polymer. The films sintered below 750°C exhibit a uniform phase structure across the cross-sections and an ordinary optical thin film feature, while the Ba_0.9Sr_0.1TiO₃ films crystallized at 750°C or higher temperature render a lamellar texture consisting of dense and porous Ba_0.9Sr_0.1TiO₃ layers and a good performance as a one-dimensional photonic crystal. The discrepancy in cross-sectional morphology and reflectance property observed in these Ba_0.9Sr_0.1TiO₃ films has been preliminarily explained.     

Orientation dependence of electrical properties of 0.96Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-0.04BaTiO<Subscript>3</Subscript> lead-free piezoelectric single crystal

In this paper, a single crystal of 0.96Na_0.5Bi_0.5TiO₃-0.04BaTiO₃ with dimensions of Φ 30×10 mm was grown by the top-seeded-solution growth method. X-ray powder diffraction results show that the as-grown crystal possesses the rhombohedral perovskite-type structure. The dielectric, piezoelectric and electrical conductivity properties were systematically investigated with 〈001〉, 〈110〉 and 〈111〉 oriented crystal samples. The room-temperature dielectric constants for the 〈001〉, 〈110〉 and 〈111〉 oriented crystal samples are found to be 650, 740 and 400 at 1 kHz. The (T _m, ε _m) values of the dielectric temperature spectra are almost independent of the crystal orientations; they are (306°C, 3718), (305°C, 3613) and (307°C, 3600) at 1 kHz for the 〈001〉, 〈110〉 and 〈111〉 oriented crystal. The optimum poling conditions were obtained by investigating the piezoelectric constants d ₃₃ as a function of poling temperature and poling electric field. For the 〈001〉 and 〈110〉 crystal samples, the maximum d ₃₃ values of 146 and 117 pC/N are obtained when a poling electric field of 3.5 kV/mm and a poling temperature of 80°C were applied during the poling process. The as-grown 0.96Na_0.5Bi_0.5TiO₃-0.04BaTiO₃ crystal possesses a relatively large dc electrical conductivity, especially at higher temperature, having a value of 1.98×10⁻¹¹ Ω⁻¹⋅m⁻¹ and 3.95×10⁻⁹ Ω⁻¹⋅m⁻¹ at 25°C and 150°C for the 〈001〉 oriented crystal sample.     

Efficient continuous-wave YVO<Subscript>4</Subscript>/Nd:YVO<Subscript>4</Subscript> Raman laser at 1176 nm

We present a simple and compact continuous-wave (CW) 1176 nm laser based on self-frequency Raman conversion in continuous-grown YVO₄/Nd:YVO₄ composite crystal. With a composite crystal 30 mm in length, a maximum output power up to 1.84 W was achieved at the incident diode pump power of 23.6 W. Corresponding to overall optical conversion, the efficiency was 7.8% and the slope efficiency was 8.5%. The conversion efficiency has been doubled compared with the conventional Nd:YVO₄ CW self-frequency Raman laser. The excellent performance of this laser shows that the long continuous-grown YVO₄/Nd:YVO₄ composite crystal is promising in the application of CW Raman lasers and ideal for miniaturization.     

Synthesis of SmAlO<Subscript>3</Subscript> nanocrystalline powders by polymeric precursor method

SmAlO₃ nanocrystalline powders were successfully synthesized by the polymeric precursor method using ethylenediaminetetraacetic acid as a chelating agent. The precursor and the derived powders were characterized by thermogravimetry analysis (TG) and differential scanning calorimetry analysis (DSC), infrared spectroscopy (IR), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The results showed that pure SmAlO₃ powder with orthorhombic perovskite structure could be synthesized at 800°C for 2 h without formation of any intermediate phase. The average particle size of the powder synthesized at 900°C was as low as 28 nm. Subsequently, the bulk SmAlO₃ ceramics were prepared at various sintering temperatures using the synthesized powders calcined at 900°C for 2 h as starting materials. The sintering experiments indicated that the sample sintered at 1550°C for 2 h exhibited the highest relative density of 97.2% and possessed the best microwave dielectric properties of ε _r=20.94, Q×f=78600 GHz and τ _f=−71.8 ppm/°C.     

Thermally activated dewetting of organic thin films: the case of pentacene on SiO<Subscript>2</Subscript> and gold

The morphology of pentacene organic thin films deposited on SiO₂ and Au(111) surfaces using organic molecular beam deposition (OMBD) has been characterized by a multi-technique approach. Among the techniques applied were X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and thermal desorption spectroscopy (TDS). Our rather detailed studies reveal that on both substrates the growth is strongly influenced by dewetting and islanding phenomena, yielding very rough surfaces. Surprisingly, substantial changes in the morphology were observed also after deposition on room-temperature samples on a time scale of several hours. The rather extensive set of in situ XPS data was analyzed in the framework of a simple model, which allows us to derive rather detailed information on the roughness parameters.     

12.95 mW sixth harmonic generation with KBe<Subscript>2</Subscript>BO<Subscript>3</Subscript>F<Subscript>2</Subscript> crystal

We obtained the second harmonic of a frequency-tripled Nd:YVO₄ laser at wavelength of 177.3 nm with 12.95 mW output by using an optically contacted KBe₂BO₃F₂-CaF₂ prism-coupled device with a 2.1 mm thick KBe₂BO₃F₂ crystal, which is to our knowledge the best result at this wavelength. PACS 42.65.Ky; 42.70.Mp; 42.55.Xi     

Effect of process parameters and post-deposition annealing on the microwave dielectric and optical properties of pulsed laser deposited Bi<Subscript>1.5</Subscript>Zn<Subscript>1.0</Subscript>Nb<Subscript>1.5</Subscript>O<Subscript>7</Subscript> thin films

Bismuth Zinc niobate (Bi_1.5Zn_1.0Nb_1.5O₇) thin films were deposited by pulsed laser deposition (PLD) method on fused silica substrates at different oxygen pressures. The structural, microwave dielectric and optical properties of these thin films were systematically studied for both the as-deposited and the annealed films at 600°C. The as-deposited films were all amorphous in nature but crystallized on annealing at 600°C in air. The surface morphology as studied by atomic force microscopy (AFM) reveals ultra-fine grains in the case of as-deposited thin films and cluster grain morphology on annealing. The as-deposited films exhibit refractive index in the range of 2.36–2.53 (at a wavelength of 750 nm) with an optical absorption edge value of 3.30–3.52 eV and a maximum dielectric constant of 11 at 12.15 GHz. On annealing the films at 600°C they crystallize to the cubic pyrochlore structure accompanied by an increase in band gap, refractive index and microwave dielectric constant.     

Temperature dependence of thermal properties of Ag<Subscript>8</Subscript>In<Subscript>14</Subscript>Sb<Subscript>55</Subscript>Te<Subscript>23</Subscript> phase-change memory materials

The dependence of thermal properties of Ag₈In₁₄Sb₅₅Te₂₃ phase-change memory materials in crystalline and amorphous states on temperature was measured and analyzed. The results show that in the crystalline state, the thermal properties monotonically decrease with the temperature and present obvious crystalline semiconductor characteristics. The heat capacity, thermal diffusivity, and thermal conductivity decrease from 0.35 J/g K, 1.85 mm²/s, and 4.0 W/m K at 300 K to 0.025 J/g K, 1.475 mm²/s, and 0.25 W/m K at 600 K, respectively. In the amorphous state, while the dependence of thermal properties on temperature does not present significant changes, the materials retain the glass-like thermal characteristics. Within the temperature range from 320 K to 440 K, the heat capacity fluctuates between 0.27 J/g K and 0.075 J/g K, the thermal diffusivity basically maintains at 0.525 mm²/s, and the thermal conductivity decreases from 1.02 W/m K at 320 K to 0.2 W/m K at 440 K. Whether in the crystalline or amorphous state, Ag₈In₁₄Sb₅₅Te₂₃ are more thermally active than Ge₂Sb₂Te₅, that is, the Ag₈In₁₄Sb₅₅Te₂₃ composites bear stronger thermal conduction and diffusion than the Ge₂Sb₂Te₅ phase-change memory materials.     

Strain-balanced Si<Subscript>1-x</Subscript>Ge<Subscript>x</Subscript>/Si type II quantum wells for 1.55 μm detection and emission

This work deals with the optoelectronic properties of heterostructures built on type II Si_1-xGe_x/Si strained quantum wells grown on relaxed Si_1-yGe_y/Si (001) pseudo-substrates. To limit the intrinsic problem due to the real-space indirect nature of the interface, we propose and model three heterostructures having three different potential profiles of the valence and conduction bands which consist in various arrangements of Si and Si_1-xGe_x barriers of different Ge contents. The proposed stacks are designed in a pragmatic way for a pseudomorphic growth on relaxed Si_1-yGe_y assuming individual layer thickness being smaller than the known critical thickness and an overall compensation of the strain. Variation of thickness and compositions (x>y) permits to optimize i) the quantum confinement of electrons and heavy-hole levels and ii) the wave function's overlap and the out-of-plane oscillator strength. The optimum parameters satisfy a fundamental emission at a key 1.55 μm wavelength below the absorption edge of each layer constitutive of the stacks. A comparison between the characteristics of the three heterostructures brings out the superior advantages of the W architecture.     

Terahertz spectroscopy of central and soft phonon modes in LuFe<Subscript>2</Subscript>O<Subscript>4</Subscript>

The terahertz dielectric response of LuFe₂O₄ is investigated by terahertz time-domain spectroscopy over a temperature range of 6–290 K. It is revealed that besides the central mode associated with the charge-ordered state, a soft TO₁ mode at below ∼240 K is identified indicating the existence of displacing ferroelectricity, in addition to the charge-ordering-induced ferroelectricity at below 320 K. The anomaly of the soft mode at ∼180 K reflects the magnetoelectric correlation between the soft TO₁ mode and the spin/charge fluctuations revealed recently. Finally, the magnetic property at below ∼240 K is discussed.     

Comment on the recent COMPASS data on the spin structure function <Emphasis Type="Italic">g</Emphasis><Subscript>1</Subscript>

We examine the recent COMPASS data on the spin structure function g ₁ singlet. We show that it is rather difficult to use the data in the present form in order to draw conclusions on the initial parton densities. However, our tentative estimate is that the data better agree with positive than with negative initial gluon densities.     

Role of A-site deficiency in the magneto-transport properties of Pr<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> relaxed films

The magneto-transport properties of thick relaxed A-site deficient films having the composition Pr Sr_0.3MnO₃ (P_□SMO with the □ symbol for the Pr vacancy) and Pr_0.7Sr MnO₃ (PS_□MO) are studied. A direct comparison with a Pr_0.7Sr_0.3MnO₃ (PSMO)completely relaxed film, deposited under the same growth conditions, shows a reduction of the in-plane parameter a₁₀₀ associated to an enhancement of the out-plane parameter. The strains (bulk strain ε_B and biaxial Jahn-Teller strain ε_J-T) do vary with the nature of the cationic vacancy. For example, an enhancement of ε_B of 9% in the PS_□MO film (Sr deficient) produces a decrease of T_C of 30 K, whereas the Pr deficient P_□SMO film exhibits a large reduction of both ε_B (-16%) and ε_J-T (divided by a factor of 5), which enhances T_C of 12 K, similarly to previous observations on bulk ceramics. With a reduced resistivity (ρ<0.02 Ω cm), the obtained Pr-deficient film, P_□SMO, exhibits the best magneto-transport properties with a decreasing magnetoresistance sensitivity at low field.     

Effect of the ceramic grain size and concentration on the dynamical mechanical and dielectric behavior of poly(vinilidene fluoride)/Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript> composites

Laser-induced backside wet and dry etching (LIBWE and LIBDE) methods were developed for micromachining of transparent materials. Comparison of these techniques is helpful in understanding the etching mechanism but was not realized due to complications in setting up comparable experimental conditions. In our comparative investigations we used a solid tin film for dry and molten tin droplets for wet etching of fused-silica plates. A tin–fused-silica interface was irradiated through the sample by a KrF excimer laser beam (λ=248 nm, FWHM=25 ns); the fluence was varied between 400 and 2100 mJ/cm². A significant difference between the etch depths of the two investigated methods was not found. The slopes of the lines fitted to the measured data (sl_LIBDE=0.111 nm/mJ cm⁻², sl_LIBDE=0.127 nm/mJ cm⁻²) were almost similar. Etching thresholds for LIBDE and LIBWE were approximately 650 and 520 mJ/cm², respectively. To compare the dependence of etch rates on the pulse number, target areas were irradiated at different laser fluences and pulse numbers. With increasing pulse number a linear rise of depth was found for wet etching while for dry etching the etch depth increase was nonlinear. Secondary ion mass spectroscopic investigations proved that this can be due to the reconstruction of a new thinner tin-containing surface layer after the first pulse.     

110 W ceramic Nd<Superscript>3+</Superscript> : Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> laser

A high-power, continuous-wave 0.6% Nd³⁺-doped ceramic Y₃Al₅O₁₂ (Nd:YAG) laser has been developed. 110 W laser output at 1064 nm was obtained, with a slope efficiency of about 41%. The M² factor was found to be around 6. The laser performance of the ceramic laser material was found to compare favorably with that obtained with single crystal Nd:YAG. PACS 42.55.-f; 42.55.Xi; 42.70.Hj     

Photorefraction in LiNbO<Subscript>3</Subscript>:Fe crystals with femtosecond pulses at 532 nm

Photorefractive index gratings are written into iron-doped lithium niobate crystals using femtosecond laser pulses and cw light, both having a wavelength of 532 nm. The saturation value of the refractive index changes in as-grown samples is found to decrease with increasing peak pulse intensity. Furthermore, in oxidized crystals, writing with femtosecond pulses is much faster than with cw light and retains about the same writing speed than in as-grown crystals. We propose a charge transport model that addresses the special case of recording with high intensity femtosecond pulses.     

Mass and kinetic energy distribution of the species generated by laser ablation of La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>

The mass distributions of the species generated by laser ablation from a La_0.6Ca_0.4MnO₃ target using laser irradiation wavelengths of 193 nm, 266 nm and 308 nm have been investigated with and without a synchronized gas pulse of N₂O. The kinetic energies of the species are measured using an electrostatic deflection energy analyzer, while the mass distributions of the species were analyzed with a quadrupole mass filter. In vacuum (pressure 10⁻⁷ mbar), the ablation plume consists of metal atoms and ions such as La, Ca, Mn, O, LaO, as well as multiatomic species, e.g. LaMnO⁺. The LaO⁺ diatomic species are by far the most intense diatomic species in the plume, while CaO and MnO are only detected in small amounts. The interaction of a reactive N₂O gas pulse with the ablation plume leads to an increase in plume reactivity, which is desired when thin manganite films are grown, in order to incorporate the necessary amount of oxygen into the film. The N₂O gas pulse appears to have a significant influence on the oxidation of the Mn species in the plume, and on the creation of negative ions, such as LaO⁻,O⁻ and O₂⁻.     

Recent developments in the doping of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> cathode material for 5 V lithium-ion batteries

LiMn₂O₄ has been considered a promising cathode material for lithium-ion batteries in electric vehicles. However, there are still a number of problems of severe capacity fading before any materials modifications. Among all doped LiMn₂O₄, spinel LiNi_0.5Mn_1.5O₄ material is seen as a potential cathode material for use in electric vehicles and energy storage systems in the future because of its high working potential (4.7 V), high energy density (the energy density of LiNi_0.5Mn_1.5O₄ is 20% higher than that of LiCoO₂), acceptable stability, and good cycling performance. In the presented paper, the structure and electrochemical performance of doped LiNi_0.5Mn_1.5O₄ are reviewed. The rate capability, rate performance and cyclic life of various doped LiNi_0.5Mn_1.5O₄ materials are described. This review also focuses on the present status of doped LiNi_0.5Mn_1.5O₄, then on its near future developments.