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.     

Influence of elastic strain onμ + SR in α-iron single crystalsSR in α-iron single crystals

The spin-precession frequencies and the transverse spin relaxation rates of positive mouns (μ ⁺) have been measured on two elastically strained α-Fe single crystal platelets as well as on an unstrained reference α-Fe crystal at temperatures down to 2.7 K in applied magnetic field 0≤B _appl≤3 T. The drastic effects of the strains may be qualitatively understood in terms of their influence on both the magnetic domain structure and theμ ⁺ energies at the various interstitial sites. This leads to the conclusion that at low temperaturesμ ⁺ in α-Fe occupy configurations related to octahedral interstitials with dipolar fieldB _dip=0.70 T.     

Electron paramagnetic resonance of deep boron acceptors in 4H-SiC and 3C-SiC crystals

EPR spectra of deep boron in 4H-SiC and 3C-SiC crystals have been observed and studied. Two sites in 4H-SiC produced deep-boron EPR signals, quasi-cubic k and hexagonal h. In both cases the deep-boron center symmetry is close to axial along the c crystal axis, and the g factor anisotropy is about an order of magnitude larger than that for shallow boron centers. In the 3C-SiC crystal, the deep-boron symmetry is also close to axial along one of the four 〈111〉 directions. The model proposed for the deep boron center with acceptor properties is B_Si-v _C, where B_Si is the boron substituting for silicon, and v _C is the carbon vacancy, with the B_Si-v _C direction coinciding in 4HSiC with the hexagonal axis of the crystal for both k and h positions. In the cubic 3C-SiC crystal, there are four equivalent deep boron centers, which represent B_Si-v _C pairs with the bond directed along one of the four 〈111〉 crystal directions. Fiz. Tverd. Tela (St. Petersburg) 40, 36–40 (January 1998)     

Segregation during crystal growth from melt and absorption cross section determination by optical absorption method

Segregation during crystal growth from melt under two conditions is studied by using crystal mass, which can be measured easily, as an independent variable, and a method to determine the effective segregation coefficient and absorption cross section of optical dopant is given. When the segregated solute disperses into the whole or just a part of melt homogenously, the concentration C _s in solid interface will change by different formulas. If the crystal growth interface is conical and segregated solute disperses into melt in total or part, the solute concentration at r = 2/3R, where r is the distance from the growth cross section center and R the crystal radius, is independent on the shape of the crystal growth interface, and its variation at r = 2/3R can be regarded as the result from crystal growth in flat interface. With C _s variation formula in solid and absorption cross section σ for optical dopant, the absorption coefficients along the crystal growth direction can be calculated, and the corresponding experimental value can be obtained through the crystal optical absorption spectra. By minimizing the half sum, whose independent variables are k, ΔW or σ, of the difference square between the calculated and experimental absorption coefficients from one or more absorption peaks along the crystal growth direction, k and σ, or k and ΔW, can be determined at the same time through the Levenberg-Marquardt iteration method. Finally, the effective segregation coefficient k, ΔW and absorption cross sections of Nd:GGG were determined, the results fitted by two formula gave more closed effective segregation coefficient, and the value ΔW also indicates that the segregated dopant had nearly dispersed into the whole melt. Experimental results show that the method to determine effective segregation coefficient k, ΔW and absorption cross sections σ is convenient and reliable, and the two segregation formulas can describe the segregation during the crystal growth from melt relatively commendably. Supported by the National Natural Science Foundation of China (Grant No. 50772112)     

Size effects under plastic deformation of microcrystals and nanocrystals

A theoretical analysis of size effects in plastically deformed crystals with transverse sizes in micro and nanometer ranges has been performed in the framework of the dislocation-kinetic approach. The analysis is based on the evolution equation of the dislocation density in these crystals and takes into account the generation of dislocations from surface dislocation sources and the escape of dislocations from the crystal through the crystal surface. It has been established that the generation of dislocations from the sources leads to a strong strain hardening of the crystal and that the escape of dislocations through the crystal surface results in a fast equilibration of these two kinetic processes. As a result, there occurs a strong “exhaustion” of strain hardening of thin crystals at the early stage of their plastic deformation in accordance with experiments. According to the theory, the flow stresses σ and transverse sizes D of microcrystals and nanocrystals are related by the expressions σ ∼ D ⁻ⁿ (n = 0.625–1.0), which are in agreement with the experiment.     

Hyperfine Interactions of N Isotopes in TiO2 and Quadrupole Moment of 16N

Hyperfine interactions of ^12,14N in BN(hexagonal) crystal were studied by detecting β-NQR(¹²N) and FT-NMR(¹⁴N). A β-NMR of ¹⁶N (I ^π=2⁻,T _1/2=7.13 s) in MgO crystal was detected to determine the magnetic moment of ¹⁶N to be |μ(¹⁶N:2⁻)|=(1.986±0.001)μ_N. Also, the β-NQR of ^12,16N in TiO₂ crystal were detected to determine |Q(¹⁶N:2⁻)|=(17.9±1.7) mb. An abnormally small effective charge for neutrons is required to account for |Q(¹⁶N:2⁻)|. This revised version was published online in September 2006 with corrections to the Cover Date.     

Kinetics of crystal growth in superfluid helium at high temperatures

The growth rate of ⁴He crystals from superfluid is measured in the temperature range 1.2–1.75 K at supersaturations up to 40 mbar. The growth rate is observed to decrease at high supersaturations: above 5 mbar in the bcc phase and above 20 mbar in the hcp phase. The temperature dependence of the kinetic growth factor K is measured in the low-supersaturation limit. In the vicinity of the superfluid transition the kinetic growth factor exhibits critical behavior: K ∝ (Tλ−T)^ε with the exponent ε=0.743±0.123. A jump in the growth factor is observed at the bcc-hcp transition point. The crystal growth kinetics problem is solved in the hydrodynamic approximation, explaining both the temperature behavior of K and the existence of the jump in the modification of the crystal structure. Zh. éksp. Teor. Fiz. 114, 1313–1328 (October 1998)     

Spectrum and losses of surface magnetostatic waves in a 1D magnon crystal

The spectrum and propagation losses of a surface magnetostatic wave in a 1D ferrite magnon crystal are experimentally studied as a function of angle φ between the wavevectors of the surface magnetostatic wave and periodic crystal lattice. Variation in the positions of Bragg forbidden bands with angle φ is described. A wide transmission band is discovered near long-wave frequency boundary f ₀ of the surface magnetostatic wave in a narrow interval of angles around φ ≈ 57°. In the interval 70° < φ < 90° and at frequencies below f ₀, there exists a magnetostatic wave with a wavevector that is normal to the bias field.     

Synthesis, crystal structure, thermal and dielectric properties of bis(p-phenylenediammonium) chloride hexachlorobismuthate(III) monohydrate [C6H4(NH3)2]2ClBiCl6.H2O

Synthesis, crystal structure, and dielectric properties of [C₆H₄(NH₃)₂]₂ClBiCl₆.H₂O are reported. The compound crystallizes in the monoclinic system with space group P2₁/n. The unit cell dimensions are a = 9.836(5), b = 19.582(5), c = 13.082(5) ?, β = 104.731(5)° with Z = 4. The atomic arrangement can be described by an alternation of organic and inorganic layers. The anionic layer is built up of octahedral of [BiCl₆]^3- arranged in sandwich between the organic layers. The crystal packing is governed by means of the ionic N–H···Cl hydrogen bonds, forming a three-dimensional network. The dielectric properties have been investigated at temperature range from 297 to 410 K at various frequencies (10 Hz–100 kHz). Dielectric studies were performed to confirm results obtained with thermal analysis. The evolution of dielectric constant as a function of temperature and frequency of single crystal has been investigated in order to determine some related parameters.     

Dendritic and eutectic growth in Sb<Subscript>60</Subscript>Ag<Subscript>20</Subscript>Cu<Subscript>20</Subscript> ternary alloy

The rapid solidification of Sb60Ag20Cu20 ternary alloy was realized by high undercooling method, and the maximum undercooling is up to 142 K (0.18TL). Within the wide undercooling range of 40-142 K, the solidified microstructures are composed of (Sb), θand ε phases. High undercooling enlarges the solute solubility of (Sb) phase, which causes its crystal lattice to expand and its crystal lattice constants to increase. Primary (Sb) phase grows in two modes at small undercoolings non-faceted dendrite growth is the main growth form; whereas at large undercoolings faceted dendrite growth takes the dominant place. The remarkable difference of crystal structures between (Sb) and θphases leads to (θ Sb) pseudobinary eutectic hard to form, whereas strips of θform when the alloy melt reaches the (θ Sb) pseudobinary eutectic line. The cooperative growth of θand ε phases contributes to the formation of (ε θ) pseudobinary eutectic easily. In addition, the crystallization route has been determined via microstructural characteristic analysis and DSC experiment.     

Theory of the electroacoustic echo in single crystals of order-disorder ferroelectrics

The mechanism governing the shaping of the echo response in a monodomain order-disorder ferroelectric crystal to excitation by two pulses of a microwave electric field at the frequencies ω and 2ω [(ω, 2ω) echo] and at the frequency ω [(ω, ω) echo] is described in the framework of the pseudospin formalism (S=1/2). The parameters of the echo response are calculated, and the results obtained are in good qualitative agreement with experiment. Fiz. Tverd. Tela (St. Petersburg) 39, 697–703 (April 1997)     

Analysis of photonic band gaps in a two-dimensional photonic crystal with centrifugal core-shell rods

The absolute photonic band gap (PBG) in two-dimensional (2D) photonic crystal with excentric core-shell rods is studied in this paper. The core rod shifts away from the core-shell rod center, and its position is decided by two new introduced parameters — the shift angle θ and the offset ρ. We use the FDTD algorithm to calculate the photonic bands of the photonic crystal, and analyze how the offset and shift angle affect the photonic bang gap of excentric core-shell photonic crystal for different core rod size. It has been shown that the variation of the photonic band gap is quite peculiar.     

Phase and shape controlling of MnS nanocrystals in the solvothermal process

MnS nanocrystals with different phases and shapes were prepared through solvothermal synthesis. The products were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), UV–vis absorption and photoluminescence (PL) spectra. The solvent and reaction time played an important role in controlling the phase and shape of MnS nanocrystals. The possible mechanism of the shape evolution was investigated, which revealed that the crystal growth along the unique c axis of γ-MnS resulted in the rod-like MnS at the primary period, and the tetrahedral crystal seed of β-MnS with zincblende structure resulted in the interlinking of MnS rods, so the zigzag and three-branched and palm-like MnS appeared; with increasing reactive time the thermodynamically stable spherical α-MnS was favored through the Ostwald ripening process. The PL results showed that the intensity of γ-MnS was much weaker than that of α-MnS, and the trap state emissions of γ-MnS at 470 and 482 nm, respectively, disappeared, which might be ascribed to the difference of the shapes between the sphere and the rod or branch.     

Effect of gamma irradiation on the dielectric properties and electrical conductivity of the TlInS<Subscript>2</Subscript> single crystal

The effect of gamma irradiation on the dielectric properties and ac conductivity of a TlInS₂ single crystal with a layered structure has been investigated in the frequency range from 5 × 10⁴ to 3.5 × 10⁷Hz. It has been shown that gamma irradiation of the TlInS₂ single crystal with a dose of 10⁴–2.25 × 10⁶ rad leads to a considerable increase in the dielectric loss tangent tanδ, the real part ɛ′ and imaginary part ɛ″ of the complex permittivity, and the ac conductivity σ _ac across the layers. It has been established that, for all gamma irradiation doses, the TlInS₂ single crystal is characterized by the dielectric loss due to electrical conduction up to a frequency of 10⁷ Hz and by the relaxation loss at a higher frequency. Irradiation of the TlInS₂ single crystal results in an increase in the dispersion of tan δ, ɛ′, and ɛ″. It has been demonstrated that, as the gamma irradiation dose is accumulated in the TlInS₂ single crystal, the density of localized states near the Fermi level N _F increases (from 5.2 × 10¹⁸ to 1.9 × 10¹⁹ eV⁻¹ cm⁻³).     

Ultrasound and vortex oscillations in high-temperature superconductors

An HTSC powder sample with grain (particle) diameter of 20–50 μm placed in a dc magnetic field B ₀ and cooled to a temperature below the superconducting transition temperature was exposed to the radiofrequency (rf) pulsed magnetic field B _∼ (B _∼ ⊥ B ₀) at a carrier frequency of 30.7 MHz. Stable echo signals were recorded which followed different rf-pulse trains. This phenomenon has the following mechanism. The rf magnetic field stimulates fluxoid oscillations on the HTSC grain surface, which are transformed into lattice oscillations through the pinning centers and induce a propagating sound wave. The second-order nonlinearity with respect to the gradient of the crystal lattice deviation from the equilibrium position taken into account in the sound wave equation yields the dependence of the crystal lattice natural frequency on the amplitude and length of the pulses which excite these oscillations. This dependence is responsible for the emergence of echo signals.     

A Possible Nuclear Spin Dewar. Hyperfine Interactions of Short-Lived β Emitter 8Li and 12B in TiO2

It was confirmed by detecting the β-NMR of ¹²B (I ^π=1⁺, T _1/2=20.2 ms) in a TiO₂ (rutile) crystal that the nuclear spin polarization of ¹²B was totally maintained in the crystal as produced through a nuclear reaction before implantation. Two locations, site 1 and site 2, were found with the relative populations 9 and 1, respectively, and the electric field gradients (EFGs) at those sites were obtained to be q(site 1)=+(37.1±0.5)10¹⁵ V/cm², η(site 1)<0.03, q(site 2)=+(185±5)10¹⁵ V/cm² and η(site 2)=0.62±0.02. We also found that about 30% of the initial polarization of ⁸Li (I ^π=2⁺, T _1/2=838 ms) was maintained in the crystal. Since the polarizations of other β emitting nuclei, ^12,16N, ^13,19O, and ⁴¹Sc were also totally maintained in the crystal, the crystal can be a “Spin Dewar” in which many short-lived nuclides can be implanted with their polarizations totally maintained during their lifetimes for the studies not only on the electronic structure of impurities in it but also on the nuclear properties. This revised version was published online in September 2006 with corrections to the Cover Date.     

On the technique of absolutization of diffuse scattering intensity measurements based on thermal diffuse scattering measurements

Features of microdefect (MD) formation in GaAs(Si) single crystals grown by horizontally oriented crystallization were studied by X-ray diffuse scattering (XRDS). Measurements were performed at room temperature (∼298 K) and near the liquid nitrogen evaporation temperature (∼85 K) using an open-flow cooling nitrogen cryostat. A practical technique for measuring XRDS using a triple-axis X-ray diffractometer was developed and applied to separate scattering on defects and thermal diffuse scattering. For a crystal with n = 2.0 × 10¹⁸ cm⁻³, the radius of detected nonspherical MDs was determined as ∼0.2 μm; thermal diffuse scattering (TDS) was experimentally separated. For a crystal with n = 3.9 × 10¹⁸ cm⁻³, nonspherical MDs ∼0.5 μm in radius were detected; TDS was found to be a negligible fraction of total XRDS. At the same time, in the case of coinciding crystal orientations and identical experimental conditions, TDS measurement data for one crystal can be used for other GaAs(Si) crystals with the same orientation.     

Characteristic features of the formation of the resistive state in a Bi2Sr2CaCu2Oy crystal

The effect of a magnetic field H⊥(ab) on the transverse current-voltage characteristics (IVCs) of the mixed state of a single crystal of the layered superconductor Bi₂Sr₂CaCu₂O_y (BSCCO) is investigated. It is established that in a wide range of temperatures and fields above the irreversibility line the initial part of the IVC is described by the law V∝I ^γ with γ≃1. As the current increases further, this law is replaced by a section where V∝exp(I). It is established that the multivalued, multibranch characteristics, interpreted as a manifestation of an internal Josephson effect, do not change appreciably when the crystal passes into a state with nonzero linear resistance. The character of the dependence of the characteristic switching current on the first resistive branch, I _J (H,T), is determined. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 543–548 (25 October 1999)     

Laser radiation tunable within the range of 4.35–5.45 <Emphasis Type="Italic">μ</Emphasis>m in a ZnTe crystal doped with Fe<Superscript>2+</Superscript> ions

We realized laser generation on a Fe²⁺:ZnTe crystal for first time. The crystal was pumped at room temperature by 40 ns pulses of an Er:YAG laser operating at a wavelength of 2.94 μm in the Q-switching mode. The output energy of the Fe²⁺:ZnTe laser was 0.18 mJ at a slope efficiency of 2.4% with respect to absorbed pumping energy. We achieved tuning of the Fe²⁺:ZnTe laser generation wavelength within the range of 4.35–5.45 μm using a prism-dispersion cavity.     

Muon spin relaxation study on magnetism in high quality single crystal of a high transition temperature superconductor La2−xSrxCuO4−σ (0.11≤x≤0.14)

The positive muon spin relaxation method is applied to probe magnetic ordering in the superconducting phase of a high quality single crystal of La_2−xSr_xCuO_4−σ (0.11≤x≤0.14). The well characterized crystal ofx=0.11 (T _c=34.5 K) with nearly complete flux exclusion exhibits spin freezing at 8K(T _f) with significant spin fluctuation up to 20 K. The onset of spin fluctuation andT _f decrease against increasingx towardsx=0.15, suggesting an existence of a magnetic phase boundary aroundx whereT _c becomes maximum.