Room temperature femtosecond optical parametric generation in MgO-doped stoichiometric LiTaO3

We demonstrate room temperature femtosecond optical parametric generation with high average output power in periodically poled MgO-doped stoichiometric LiTaO₃. Direct pumping with 725-fs pulses from a passively mode-locked thin disk laser at 1030 nm resulted in stable 1.5 W average signal power at 1484 nm at the full laser repetition rate of 59 MHz. With this demonstration we achieved a significant simplification of our recently presented red-green-blue laser source because no temperature stabilization of any nonlinear crystal is required. PACS 42.65.Yj; 42.70.Mp; 42.79.Nv     

Hysteresis and nonlinear thermooptic waves in a semiconductor Fabry-Perot interferometer

We propose and demonstrate the existence of temperature hysteresis phenomena in a Fabry-Perot interferometer, resulting from a change in the refractive index of the interferometer material when heated by laser radiation. We also demonstrated theoretically that temperature multistability in the distributed interferometric system may cause the emergence of nonlinear thermooptic switching waves and found an approximate expression for the propagation velocity of such waves. We present the results of experimental observations of temperature multistability and thermooptic switching waves in a germanium Fabry-Perot interferometer illuminated by CO₂-laser radiation.     

Field history dependence of nonlinear dielectric properties of Ba0.6Sr0.4TiO3 ceramics under bias electric field: Polarization behavior of polar nano-regions

Nonlinear dielectric properties of Ba_0.6Sr_0.4TiO₃ ceramics prepared by citrate method were investigated under bias electric field with respect to field history. X-ray diffraction analysis and temperature dependence of the dielectric constant (ε_r) confirmed a macroscopically paraelectric state for the specimen at room temperature. A slim polarization versus electric field (P-E) hysteresis loop of the specimen at room temperature indicated the existence of polar nano-regions (PNRs) superimposed on the paraelectric background. The nonlinear dielectric properties in continuous cycles of bias field sweep displayed a strong sensitivity to the field history. This phenomenon was qualitatively explained in terms of an irreversible polarization evolution of the PNRs under the bias fields. A considerable decline of the tunability with the cycle number suggests an appreciable contribution of the PNRs to the dielectric nonlinearity. The polarization and size of the PNRs were determined by fitting the dielectric constants to a multipolarization mechanism model.     

氧离子导体La2Mo2-xMxO9(M=Cr,W)的理论研究

采用第一性原理方法研究了高温β-La₂Mo₂O₉中Mo被Cr或W取代后晶体的结构性质和高温相稳定性.研究发现Cr和W分别倾向于占据具有4配位数和5配位数的Mo位.W取代体系晶胞参数相对取代浓度非线性变化.该非线性变化与W离子取代位置配位数降低以及体系内La—O和Mo(W)—O平均键长非线性变化有关.内聚能计算表明W取代Mo后能够有效稳定高温相结构,降低相变温度.La₄Mo₃W₁关键词： 氧离子导体 第一性原理 晶体结构 相稳定性     

Nonlinear lattice dynamical theory for valence transition in rare earth compounds

A nonlinear lattice dynamical theory is proposed to explain the mode softening in intermediate valence compound Sm_0.75 Y_0.25 S. In this theory we consider the breathing motion of the electron shells of Sm-ion to be nonlinear, and depend on temperature, resulting in strong electron-ion coupling. We calculate the salient features of the nonlinear breathing potential, which resembles theφ ⁴ potential for second order ferro-electric transitions. We also calculate the temperature dependence of LO(L) frequency for this compound, showing that at transition temperature (about 200 K) this mode freezes.     

α-碘酸锂的电流弛豫和偏压场作用下表观介电常数弛豫行为

本工作较为详细地测定了静电场作用下α-碘酸锂的电流弛豫行为:在撤去c轴方向上的静电场后,从我们所用测量电流仪器的响应时间到某一t_k的区间内,放电电流服从(t/t₀)^-α的负幂次方律;而在t>t_k,其规律近似地为:(t/t₀)^{(-α′ln(t/t₀))}。相关的弛豫参数α,α′和t_k在有限的范围内,依赖于温度和施加电压的大小,也因样品不同而有差异。对将近十个样品进行了测试,结果表明,大多数晶体在尽可能小的电压下0.43≤α≤0.7,0.07<α′<0.09;上述的电流弛豫表达式与α-碘酸锂在偏压场作用下观察到的其它物理现象的弛豫行为,与在中子衍射加强和表观介电常数中的表现如出一辙;可以初步肯定各种现象的弛豫行为具有相同的物理根源。 关键词：     

Effect of Sn-doping at Mo-site on the conductivity of La2Mo2O9 series of compounds

Taking oxygen ion conductor La₂Mo₂O₉, as a base compound, a series of Sn-doped La₂Mo_2−x Sn _x O_9−δ, x = 0, 0.01, 0.02, 0.03, 0.05, 0.1, 0.15, 0.2 specimens were prepared and characterized by XRD for phase and crystal structure determination and ac impedance spectroscopy for ac and dc conductivity measurement. We have found that there is slight improvement in overall conductivity of the specimen with x = 0.03 at 800°C compared to the undoped compound at the same temperature. The value of conductivity when extrapolated to 800°C is found to be 0.055 S cm-1 for the specimen with x = 0.03, whereas conductivity of undoped specimen at the same temperature is found to be 0.033 Scm⁻¹.     

Temperature-dependent dispersion relations for RbTiOPO<Subscript>4</Subscript> and RbTiOAsO<Subscript>4</Subscript>

We have measured the first and second derivatives of the refractive index with respect to temperature along the z- and y-axes for two nonlinear optical materials, RbTiOPO₄ and RbTiOAsO₄. The measurements were done using an interferometric setup in the wavelength range of 532–1580 nm and in the temperature range of 25–200 °C. In addition, the thermal expansion coefficients were measured in the x direction. Experimental results of nonlinear frequency conversion in RbTiOAsO₄ were used to explore the temperature dependence of the refractive index in the z-axis near 3.5 microns. We have derived thermal-dependent dispersion equations for the two materials. Calculations based on these dispersion equations are in good agreement with previously reported measurements. The thermal dependence of the refractive index of RbTiOAsO₄ is greater than that of RbTiOPO₄ by approximately 15% and 70% in the z- and y-axes, respectively. Hence RbTiOPO₄ is more appropriate for applications that require immunity to thermal lensing, whereas RbTiOAsO₄ is suitable for realizing temperature-tuned nonlinear devices. PACS 42.65.Ki, 78.20.Cy, 78.20.Nv     

准晶非线性光子晶体中二次谐波波长和温度调谐的研究

在八重准周期极化的铌酸锂非线性光子晶体中,通过调节基频光波长实现了多个波长的同时共线准相位匹配倍频,最高转换效率达36%.同时,测量了准晶非线性光子晶体中二次谐波转换效率随晶体温度以及入射波长的变化,结果表明二次谐波在长波处具有更宽的温度以及波长调谐带宽.该项研究对于准周期非线性光子晶体在实际工作中的运用具有重要的指导意义. 关键词： 准周期 非线性光子晶体 温度调谐 波长调谐     

La2/3Ca1/3MnO3薄膜的光致电阻率变化特性

射频磁控溅射法制备了La_2/3Ca_1/3MnO₃纳米薄膜(LCMO).该薄膜发生FM-PM相变的转变点温度为T_c≈308K（近似为电阻峰值温度T_p）；在不同温度下的光电导性质实验表明所制备的LCMO薄膜在连续激光作用时低温段（Tc）表现为光致电阻率增大效应，即ΔR/R＞0，并在R-T曲线拐点附近取得极大值，（ΔR/R）_max=43.5%；当T>T关键词： 钙钛矿薄膜 光响应 电子自旋 小极化子     

High-resolution X-ray diffraction study of the cubic-to-tetragonal transition in BaTiO3

A drastic change is found in the diffraction pattern of a single-crystal specimen of BaTiO₃. At and just below the transition temperature, the original single peak of 004 of the tetragonal phase splits or collapses into a group of small peaks. They appear around the positions of 004 and 400, and probably correspond to different orientations of grains and domains. The positions and relative intensities of the peaks randomly vary with time and temperature. These features are observed for both heating and cooling processes.     

57Fe high pressure Mössbauer and X-ray studies of Gd2Fe17C x interstitial compounds

The unit cell volume dependence of the Curie temperature of Gd₂Fe₁₇ is studied by means of high-pressure Mössbauer spectroscopy. From the pressure dependence of the average isomer shift of Ge₂Fe₁₇ at room temperature, the contribution of the pure magnetovolume effect to the change of the average isomer shift is derived. By combining the Mössbauer measurements of Gd₂Fe₁₇C _x with different C concentrations at room temperature, the change ofIS in Gd₂Fe₁₇C _x is dominated by the magnetovolume effect. The contribution of the chemical bonding effect is about 24% of that of the magnetovolume effect, but with negative sign.     

Thermodynamic critical field of superconducting ErRh4B4 as a function of temperature

Continuous magnetization curves for a polycrystalline specimen of ErRh₄B₄ are presented for a complete hysteresis cycle. A method is given to construct the diamagnetic contribution from the total magnetization curve. This method is used to obtain the values of the thermodynamic critical field H_c for the specimen as a function of temperature in the superconducting state.     

Anomalous phenomenon of conductivity of the amorphous fast ionic conductor B2O3−0.7Li2O−0.7LiCl during the crystallization process

This paper presents a new experimental method for studying the effect of crystallization of amorphous fast ionic conductors on conductivity, which includes heating the specimen to the selected temperature and annealing the specimen within a fixed time, quenching the specimen in order to freeze its high temperature structure, then returning to a fixed temperature and measuring the conductivity. This method gets rid completely of the effect of temperature on conductivity and thus the effect of structure on conductivity can be studied. It has been found with this method for the first time that during the crystallization process the conductivity of amorphous fast ionic conductor B₂O₃-0.7Li₂O-0.7LiCl increases at the beginning with temperature, and then after passing a maximum, the conductivity decreases. This result coincides with that of the relative rate of crystallization obtained by the author with X-ray diffraction technique at the same temperature range. The interface at the crystalline and amorphous two-phase boundary is thought to be responsible for the anomalous phenomenon.     

液态结构与性质关系Ⅱ——Mg-9Al熔体的运动黏度及与熔体微观结构的关系

采用坩埚扭摆振动法测量Mg-9Al熔体的运动黏度,得到890—1190 K温区内高精度的黏度-温度关系曲线ν(T),发现升温过程中黏度随温度升高发生异常变化,当温度升高至1000—1075 K时,黏度由快速增大转变为逐渐减小,即发生转折变化;在随后的降温和第二次升温过程中,黏度随温度变化呈指数规律单调递增(减),符合Arrhenius方程式.在实验研究基础上,采用剩余键结构模型和"平均原子集团"演变行为的计算模型讨论Mg-9Al熔体的黏度与微观结构之间的相关性,结果表明:类关键词： 合金熔体 结构与黏度相关性 剩余键结构 平均原子集团模型     

Charge density wave transport in a novel inorganic chain compound, (TaSe4)2I

We report the observation of nonlinear conductivity with a well-defined threshold electric field E_T, and frequency dependent ac conductivity, in a novel linear chain compound, (TaSe₄)₂I. The material undergoes a phase transition to a semiconducting phase at T ～ 260 K, and nonlinear and frequency dependent transport is observed below this temperature. We argue that the material is a new example of collective mode transport provided by a Peierls-Fröhlich charge density wave condensate.     

Negative evidence of any history dependence of the néel temperature of a Dy crystal from neutron scattering and ultrasonic measurements

A simultaneous measurement of the sublattice magnetization of helical ordered Dy with neutrons and of the C₁₁, C₃₃ and C₄₄ elastic constants, indicate a Néel temperature of (177.3±0.2) K. This value is not noticeably changed by repeated cycling of the specimen through T_N, in contrast to a previous observation of a large change in T_N for a Dy crystal (Palmer and Greenough, 1976).     

INFLUENCE OF SEED CRYSTAL DIMENSION ON CRYSTAL GROWTH FROM SOLUTION UNDER MICROGRAVITY

The influence of the dimension of seed crystal on the characteristics of crystal growth from solution under microgravity is studied. The dimensionless average velocity of fluid V_av, the dimensionless maximum velocity of fluid V_max, the temperature distribution index S_θ, the concentration distribution index S_φ and the dimensionless average growth rate of crystal \overline{V}_cg are calculated by only taking into account the variation of the solution density caused by the temperature change, that caused by the concentration change being neglected. In certain regions of the parameter (Ra, Pr,Sc and μ) space, some scaling laws are generated: the scales of S_φ and \overline{V}_cg are given by power functions of μ with negative exponents. It is shown that the characteristics of crystal growth for small seed crystal are different from those for large seed crystal.     

Positive muon spin precession in magnetic oxides MnO and V2O3; local fields and phase transition

Muon spin precession is observed in the ordered state of antiferromagnetic oxides MnO and V₂O₃. An anomalous temperature dependence of muon precession frequency and the absence of critical behavior atT _N are found in MnO and a hysterisis of the transition temperature is observed in V₂O₃. These results are explained in terras of the crystal distortion of these materials belowT _N.We would like to thank Dr. T. Suzuki, Dr. S. Takagi, Dr. Y. Kuno and Dr. C.Y. Huang for their help in the measurements, and Dr. K. Kosuge and Dr. S. Kachi for providing the specimen of V₂O₃. We are grateful to the financial support from Japan Society for Promotion of Science.     

Effect of the microstructure of epitaxial YBa2Cu3O7 − x films on their electrophysical and nonlinear microwave properties

The local nonlinear microwave response of YBa₂Cu₃O_{7 ? x} films was measured by near-zone field microscopy with a spatial resolution of 50 μm, and YBa₂Cu₃O_{7 ? x} film microbridges were locally studied by low-temperature scanning microscopy with a spatial resolution of 4 μm. The microstructure of epitaxial YBa₂Cu₃O_{7 ? x} films was examined using x-ray diffraction and electron microscopy. A correlation is detected between the average crystallite size and the half-widths of the temperature dependences of the third-harmonic power (W _TH) and the electron-beam-induced voltage (W _EBIV). The experimental results are described in terms of a model of a two-phase medium taking into account the nonlinear I-V characteristic of the superconductor. For large crystallites, the nonlinear microwave response is shown to be caused by intracrystallite vortex pinning. As the average crystallite size decreases, an additional contribution to the nonlinear response appears due to the pinning of a magnetic flux by the Josephson network of crystallite boundaries. Calculations show that a three-fold increase in the crystallite size decreases the nonlinearity coefficient of YBa₂Cu₃O_{7 ? x} films by two orders of magnitude.