高次谐波体声波谐振器的有效机电耦合系数

高次谐波体声波谐振器HBAR (High-overtone Bulk Acoustic Resonator)由基底、压电薄膜和上下电极组成,系统地研究了它们的结构参数(厚度)和性能参数(特性阻抗)对HBAR的重要性能参数有效机电耦合系数K_eff2的影响。在谐振频率附近,通过将HBAR的分布参数等效电路简化为集总参数等效电路得到了它们之间的关系表达式,分析了K_eff2在所关心频率最近谐振点的变化情况。结果表明,保持压电薄膜厚度不变,连续增加基底厚度,K_eff2呈振荡(非单调)下降,当基底厚度达到一定值时K_eff2与厚度成反比下降;保持基底厚度不变,连续增加压电薄膜厚度,K_eff2的峰值随基底和压电层的特性阻抗之比增加快速下降,到达极小值后缓慢增加;选择低阻抗的熔融石英作为基底可以获得较大的K_eff2;与Al电极相比,Au电极选择适当厚度可以获得较高的K_eff2。上述揭示的一些规律为HBAR的优化设计提供了理论依据。      

二元合金微观偏析的相场法数值模拟

使用耦合溶质场的相场模拟研究了Ni-Cu二元合金枝晶生长过程中固相溶质扩散系数D_s 对枝晶形貌和微观偏析等的影响.计算结果表明，随着D_s的减少，固液界 面前沿溶质扩散层越薄，枝晶生长越有利，枝晶尖端生长的速度越大，侧向分支越发达；D< sub>s越小，固相中溶质成分的波动越强烈，随着D_s的增大，固相中溶质 成分的波动相应减小；溶质微观偏析程度随D_s的增大而减小. 关键词： 相场方法 微观偏析 固相扩散系数 数值模拟     

耦合强度和磁场对极化子基态的影响

利用线性组合算符和幺正变换相结合的方法,推导出极化子基态与耦合强度和磁场强度的关系。数值计算表明:当磁场强度给定时,随着耦合常数α的增加,振动频率λ先减小后增大;基态能量E₀单调下降;自陷能E_0tr单调增大;Landau能E_0L先增大,达到最大值后又下降。当耦合强度给定时,随着磁场强度的增大,λ单调增大,且α愈小,λ增加愈快;基态能量E₀随磁场强度的增大而增大;自陷能E_0tr随着磁场强度的增大而略有增加;Landau能E_0L随着磁场强度的增大先增大,达到最大值后,又开始下降。     

双量子阱结构OLED效率和电流的磁效应

通过结构为ITO/NPB(60 nm)/ Alq₃ ∶1 wt% rubrene(20 nm)/ Alq₃(3 nm)/ Alq₃ ∶1 wt% rubrene(20 nm)/ Alq₃(20 nm)/LiF/Al的双量子阱的黄色有机电致发光器件,研究了不同磁场强度下的发光效率和电流变化特性. 研究结果表明该器件的电流是随着磁场强度的增加而单调下降的,显示了器件的电阻是随着磁场强度的增加而增加的. 同时也得到了该结构有 关键词： 量子阱 磁场 OLED 磁效应     

高次谐波体声波谐振器的机械品质因数

机械品质因数Q_M是高次谐波体声波谐振器(High-overtone Bulk Acoustic Resonator,HBAR)一个关键的特性参数。首次较系统地研究了Q_M随构成HBAR的3个组成部分(基底、压电薄膜和电极)的结构参数(厚度)和性能参数(特性阻抗与机械衰减因子)的变化规律。在谐振频率附近,将HBAR的分布参数等效电路简化为集总参数等效电路,首次用解析表达式给出它们的变化规律,分析了Q_M在给定频率最近谐振点的变化情况。结果表明,固定压电层厚度,Q_M随基底厚度的连续增加略呈振荡(非单调)上升,当基底厚度很大时趋于基底材料的机械品质因数;固定基底厚度,Q_M随压电层厚度的连续增加呈波浪式下降;选择低损耗的蓝宝石或YAG作为基底可以获得较大的Q_M值;电极的损耗必须考虑,它会降低Q_M值;与Au电极相比,具有较低损耗的A1电极选择适当厚度可以获得较高的Q_M值;此外,Q_M随频率的增加呈下降趋势。上述的结果为HBAR的优化设计提供了相应的理论依据。根据我们对K_eff2的研究,Q_M与K_eff2的变化规律往往是相悖的,因此在设计HBAR时要在这两者作适当的权衡。      

旋转磁场对凝固组织形成的影响

研究了旋转磁场作用下Pb-45%Sn亚共晶合金的凝固组织.实验发现,旋转磁场的频率恒定时,凝固组织的晶粒尺寸随着磁场强度的增强而线性减小,同时,初生相的生长形态从枝晶转变为椭球状.X射线测试结果表明,初生相Pb发生了点阵膨胀,并且晶格常数随着磁场强度的增强先变大后减小,磁场强度在此存在一个临界值.能谱分析显示,随着磁场强度的增强,初生相Pb内Sn的含量逐渐降低.根据电磁场理论和扩散定律,对上述现象进行了理论分析,揭示出旋转磁场引起了液相强烈流动,加快了溶质原子的扩散以及对熔体的加热效应,导致了形核率的提高和长大速度的降低. 关键词： 旋转磁场 液相流动 晶格常数 溶质分配     

磁场对石墨烯中弱耦合极化子性质的影响

研究了磁场作用下石墨烯中电子与表面光学声子弱耦合情况下的极化子的性质。采用线性组合算符和Pekar变分法分别推导出了石墨烯中弱耦合极化子的基态能量E₀、第一激发态能量E₁和跃迁频率ω随磁场强度B和德拜截止波数k_d之间的变化关系。数值计算结果表明,极化子的基态能量E₀随磁场强度B变化的曲线（k_d一定时）和E₀随k_d的变化曲线（B一定时）均会分裂成对称的两条,并且当B一定时E₀的绝对值随k_d的增加而增加。在k_d一定时,极化子的第一激发态能量E₁和跃迁频率ω均为磁场B的增函数;在B一定时,E₁和ω均随k_d的增加而增大。     

磁场对基于Co铁磁薄膜的有机发光器件效率和电流的影响

制备结构为ITO/Co/NPB/Alq₃/LiF/Al的有机发光器件,测量了室温下磁场对器件发光效率和电流的影响.发现磁场强度小于80 mT时,器件发光效率随磁场强度的增加而增大,最大为18.8%,随磁场强度的继续增加发光效率的增强趋于饱和.效率的增加是Co的自旋极化的注入和磁场效应共同作用的结果,其中自旋极化注入起主要作用.在磁场强度小于60 mT时电流随磁场增强而增加,最大为6.9%,随磁场强度的进一步增加电流的增加有所减弱.产生这种现象的原因可归结为磁场相关的单线态极化子对的解 关键词： 有机电致发光 自旋极化 磁场效应     

KENO Ⅳ交互绘图功能的配制

介绍了为KENOⅣ配制的交互绘图功能的有关内容,说明了用户界面、几何可视化(绘制二维剖面图和三维立体图)、K_eff结果图、运行进程显示等功能。     

恒流应力下E2PROM隧道氧化层的退化特性研究

在电容测量的基础上研究了薄隧道氧化层在恒定Fowler-Nordheim(F-N)隧穿电流下的退化情况. 这种退化是恒流应力和时间的函数，对恒流应力大小的依赖性更加强烈，隧道氧化层在F-N电流下的退化是注入电荷密度(Q_inj)的函数. 在较低Q_inj下氧化层中发生正电荷俘获，在较高Q_inj下发生负电荷俘获，导致栅压变化的反复. 关键词： 2PROM')" href="#">E²PROM 隧道氧化层 退化 恒流应力     

Effective diffusivity of solute in multiphase materials with segregation

The effective diffusion coefficient D_eff for solute in a multiphase microstructure in which solute segregation can occur is related to D_eff for an isomorphic microstructure in which no segregation occurs. This permits analytical expressions (approximations, bounds, etc.) and methods of numerical calculation for D_eff that neglect solute segregation to be applied to systems that exhibit segregation. As an example, exact results are obtained for solute diffusion and segregation to grain boundaries in an idealized polycrystalline microstructure.     

Effective magnetic anisotropy of nanocrystalline Nd-Fe-Ti-N hard magnetic alloys

The intermetallic compound Nd-Fe-Ti-N has been successfully synthesized by a mechanical alloying process. The structure and magnetic properties of the sample have been studied using X-ray diffraction and magnetic measurements. It is found that alloy exhibits a nanocrystalline ThMn₁₂-type tetragonal structure with lattice parameters of a=0.8723 nm and c=0.4896 nm. The saturation magnetization M_S and effective magnetic anisotropy K_eff of the compound have been determined by investigating magnetization processes. The calculated results based on the law of approach to magnetic saturation have been successfully used to determine the constant K_eff. The difference between observed and calculated values in magnetization is lower than 3%. Of all terms in the law of approach to saturation, it is the 1/H ² term, which is attributed more to non-compensated anisotropy energy, that has the prevailing effect for the compound. The absorption of nitrogen is found to increase unit cell volume, M_S and K_eff. Received: 28 October 1996 / Revised: 14 March 1997 and 4 August 1997 / Accepted: 8 August 1997     

Optical orientation of nuclei in nitrogen alloys GaAsN at room temperature

The intensity and giant circular polarization of edge luminescence in a longitudinal magnetic field have been measured in nitrogen alloys GaAsN under circularly polarized pumping. It has been found that these dependences are shifted with respect to zero field by a value B _eff. The magnitude of the internal field B _eff increases with the pumping intensity and reaches saturation (≈250 G) at large excitation densities. The saturation of the B _eff field with growth of pumping indicates that this is a field of nuclei, polarized dynamically due to hyperfine interaction with optically oriented deep paramagnetic centers, rather than a field of exchange interaction created on the center by spin-polarized photo-excited conduction electrons. The short time of nuclear polarization by electrons (<15 μs), measured under modulation of circular polarization of the exciting light with high frequency, points to a small number of nuclei undergoing hyperfine interaction with an electron localized at a center.     

The two-body multipole problem of electrodynamics

A 2-body system composed of two objects having arbitrary distributions of charge and current is discussed. An expression for the velocity dependent potential between these two objects has been obtained in the non-relativistic approximation. This potential consists of two parts viz. a velocity independent scalar potential Φ_eff and another part which is linearly dependent on the relative velocity between the objects. The second part naturally suggests a vector potential A_eff. The potentials have been expanded into multipole terms. It has been found that Φ_eff is a sum of two components viz. Φ_EE and Φ_MM such that each multipole term in Φ_EE represents an interaction between the electric multipoles of the two systems, each term in Φ_MM represents an interaction between their magnetic multipoles whereas each term in A_eff represents an interaction between an electric multipole of one and a magnetic multipole of the other. The results have been applied to the interaction between an electric dipole and a magnetic dipole. The symmetry among the multipole terms in A_eff suggests vanishing vector potential between two identical objects. A corollary of this appears to be absence of spin orbit interaction between two identical particles in the same spin state.     

Solid-state reactions in Ga/Mn thin films: Formation and magnetic properties of the ϕ-Ga<Subscript>7.7</Subscript>Mn<Subscript>2.3</Subscript> phase

Experimental results concerning the solid-state synthesis of the ϕ-Ga_7.7Mn_2.3 phase in Ga/Mn thin films are presented. A ferromagnetic (or ferrimagnetic) state is observed in the samples annealed at temperatures above 250°C. The X-ray diffraction studies demonstrate the formation of the ϕ-Ga_7.7Mn_2.3 phase, which is poly-crystalline being grown on glass substrates and exhibits the preferential cube-on-cube orientation on MgO(001) substrates. A strong dependence of the perpendicular anisotropy constant K _⊥ and of the effective biaxial anisotropy constant K ₁^eff on the magnetic field H has been found. Owing to such dependence, the easy axis of magnetization lying in the plane of the film changes its direction approaching the film normal when the increasing magnetic field exceeds 8 kOe. The anomalous behavior of K _⊥ and K ₁^eff constants is explained both by the in-plane stresses arising in the course of the formation of the ϕ-Ga_7.7Mn_2.3 phase and by the direct dependence of magnetostriction constants on the magnetic field. For the ϕ-Ga_7.7Mn_2.3 phase, the saturation magnetization M _S has been determined and the first magnetocrystalline anisotropy constant K ₁ has been estimated.     

Mössbauer study of the invar Fe–Ni and Fe–Ni–C alloys in magnetic field

F.C.C. Fe–30.3%Ni and Fe–30.5%Ni–1.5%C (wt.%) alloys were studied by means of Mössbauer spectroscopy in external magnetic field B _ext?=?2.5, 5, 7 T parallel to the gamma-beam. It is shown that distribution of effective magnetic field in the alloys is broad and that carbon expands the range of B _eff. The external magnetic field increases B _eff in the Fe–Ni alloy and decreases it more evidently in the Fe–Ni–C alloy. Antiferromagnetic spin coupling along the ferromagnetic component is proposed to explain data.     

The influence of reactor core parameters on effective breeding coefficient Keff

The values of effective breeding coefficient Keff in a reactor core of nuclear power plant are calculated for different values of parameters （core structure, fuel assembly component） by using the Monte Carlo method. The obtained values of Keff are compared and analysed, which can provide theoretical basis for reactor design.     

Negative magnetoresistance and nonlinear conduction in Ti0.99V0.01Se2

Both magnetic and electric field dependences of transport coefficients are investigated on the layered material Ti_1-xV_xSe₂ (x = 0.01). In contrast to semimetallic TiSe₂, the resistivity of the V-doped samples increases with decrease of temperature even in the low temperature region. At liquid helium temperatures it is found that the resistivity is strongly dependent on electric field strength. The behaviour of the nonlinear conduction is similar to that observed in 1T-TaS₂. In the low field (Ohmic) region anomalously large negative mangetoresistance is observed, Δ?/?₀ = -80% at 1.6 K and 60 KOe. Moreover the Hall coefficient is also found to depend on both magnetic and electric fields. All the experimental data suggests that mobile carriers are excited by the applied fields.     

Different temperature dependences of photorefractive parameters of Ce-doped and Rh-doped BaTiO3

eff and the electro-optic coefficient r₃₃ in Ce-doped and Rh-doped BaTiO₃ were determined by two-beam coupling measurements. It was found that the effective trap density N_eff of BaTiO₃:Ce increases whereas that of BaTiO₃:Rh decreases with increasing temperature. The electro-optic coefficient r₃₃ of both crystals increases with temperature. The photorefractive response times were also measured and found to decrease with different rates as temperature increases in the two crystals. The results were discussed by using the two-centre model for BaTiO₃:Ce and three-charge-state model for BaTiO₃:Rh. We found that the different temperature dependence of N_eff in the two crystals was due to the fact that the deep- and shallow-trap levels in BaTiO₃:Ce are caused by different impurity centres whereas those in BaTiO₃:Rh are caused by different charge states of the same impurity centre. Received: 16 July 1998/Revised version: 26 October 1998     

Features of low-temperature thermal expansion of n-type Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals in magnetic field

Tensometric study of n-type Bi₂Se₃ single crystals in dc magnetic fields to 6 T in a temperature range of 7–23 K detected a weak negative thermal expansion (NTE) in the basal plane. The NTE increases with the field strength and depends on its orientation with respect to the trigonal c axis. In a magnetic field of 6 T, parallel to the c axis, the linear NTE coefficient reaches ?7 · 10^?7 K^?1, and a minimum sample length is reached at a temperature of 13 K, where a Hall carrier concentration maximum is also detected. The found magnetoelastic anomaly can be associated with the topological insulator state.