A note on the harmonic plus inverse-harmonic potential in quantum mechanics

The ground state of a class of potentials described byV(x)=a ₂₀ x ²+a ₁/x ² is investigated. This potential possesses some distinct features like that of a symmetric double-well potential, and can be used in modelling the fusion process of two identical nuclei as well.     

Crystal structure of a new superconducting high-pressure phase in the YBaCuO-System

A superconducting (T _c=40K) high-pressure phase recently discovered in the system of perovskite type YBaCuO structures is investigated by high-resolution transmission electron microscopy and electron diffraction in order to find its crystal structure. A structure model is proposed on the basis of a comparison between the observed images and image simulations based on crystal chemical considerations. The new phase has aB-centered orthorhombic cell with a monoclinic primitive cell. The primitive cell is composed of two subunits. The first of these is identical with the unit cell of orthorhombic YBa₂Cu₃O_7–x (1-2-3 structure) withT _c=92K. In contrast to the first subunit, the second one contains two adjacent Cu–O chains but is identical otherwise. This second subunit has been observed as one type of planar defect in the 1-2-3 structure. It is therefore concluded, that other stacking polytypes composed of these two different units could exist. The structure of the new phase is compared to the structures of the other known high-T _c superconductors. The chemical formula for the new phase can be written as Y₂Ba₄Cu₇O_14+x, with x0.5±0.2.     

Identification and elimination of inductively coupled plasma-induced defects in AlxGa1 - xN/GaN heterostructures

By using temperature-dependent Hall,variable-frequency capacitance-voltage and cathodoluminescence (CL) measurements,the identification of inductively coupled plasma (ICP)-induced defect states around the Al x Ga 1-x N/GaN heterointerface and their elimination by subsequent annealing in Al x Ga 1-x N/GaN heterostructures are systematically investigated.The energy levels of interface states with activation energies in a range from 0.211 to 0.253 eV below the conduction band of GaN are observed.The interface state density after the ICP-etching process is as high as 2.75×10 12 cm 2 ·eV 1.The ICP-induced interface states could be reduced by two orders of magnitude by subsequent annealing in N 2 ambient.The CL studies indicate that the ICP-induced defects should be Ga-vacancy related.     

Correlation between subband population and threshold current densities in GaAs/(Al,Ga)As quantum-cascade structures/lasers with different barrier heights

The laser parameters such as threshold current density and characteristic temperature of GaAs/Al_xGa_1-xAs quantum-cascade lasers (QCLs) with x=0.45 are superior compared to QCLs with x=0.33. This improvement is usually attributed to smaller leakage currents due to the higher conduction band offset for x=0.45, since the QCLs for the two barrier compositions are designed in such a way that the population ratios ρ_p for the laser levels are the same. The experimental investigation of undoped GaAs/Al_xGa_1-xAs quantum-cascade structures reveals a significantly smaller value of ρ_p for x=0.33 than the calculated one, while for x=0.45 it agrees with it. In the framework of a linear rate equation model, we estimate the effect of the experimentally observed reduction of ρ_p on the threshold current density. We conclude that the increased threshold current density for x=0.33 has to be attributed to both, a larger leakage current and a reduced population ratio.     

Hyperfine Field in Ferromagnetic CoPd Alloys

The magnetic hyperfine interaction of ⁶⁰Co in the completely miscible alloy Co _x Pd_1−x was investigated for different values of x by measuring the nuclear orientation of ⁶⁰Co as function of temperature and by nuclear magnetic resonance of the oriented ⁶⁰Co nuclei. A broad resonance signal of Gaussian shape could be observed down to x=60%. The dependence of the mean magnetic hyperfine field on the Co concentration was observed to be linear very similar to that of experimental values of the magnetic moment per atom in the literature. The magnetic hyperfine field and its broad distribution are discussed in a simple model with RKKY interaction. This revised version was published online in September 2006 with corrections to the Cover Date.     

The structures and magnetic properties of Cr/Fe and Mn/Fe thin films

Cr_1-x Fe _x and Mn_1-x Fe _x films were prepared at room temperature by thermal coevaporation at a deposition rate of about 25 Å/min. It was found that an amorphous phase can be obtained for Cr_1-x Fe _x (0.25<x<0.60) films, while a metastable-Mn-type phase was observed for Mn_1-x Fe _x (x<0.70) films. The influence of the structure on the magnetic properties has been studied. The amorphization ability of the two systems was discussed in terms of thermodynamic considerations.     

Well defined superlattice structures made by phase-locked epitary using RHEED intensity oscillations

Long continuing intensity oscillations of the RHEED pattern in the [100] azimuth on a (001) oriented substrate were observed during MBE growth of GaAs and Al_xGa_1−xAs. Using these oscillations, growth rates of GaAs and Al_xGa_1−xAs, and the Al mole fraction x of the Al_xGa_1−xAs were accurately monitored during the growth. The phase of the RHEED oscillations was analyzed by computer and molecular beam shutters were operated at a particular phase of the oscillations. This computer controlled phase-locked epitaxy (PLE) was used to grow precisely defined (GaAs)₂(AlAs)₂ bi-layer superlattices. Raman scattering spectra of the bi-layer superlattice showed split lines characteristic of superlattices. From TEM observation of a GaAs-AlAs multi-layered structure, it was verified that one cycle of oscillations corresponds to one monolayer growth of GaAs and AlAs. This PLE has a great advantage over the conventional MBE growth method for the precise control of very thin films and superlattice structures because it is invulnerable to fluctuations of molecular beam flux intensity.     

Structure, dielectric and magnetodielectric properties of Bi1-xGdxFeO3 Ceramics

Different magnetodielectric effects were observed in Bi1-xGdxFeO3 ceramics depending on gadolinium content. A positive one was observed in the samples with x ≤0.10 at 295K and 16K, and a negative one in the sample with x = 0.4 at 16 K. Structure analysis by x-ray diffraction (XRD) reveals that the samples crystallize in the R3c structure (ferroelectrics) for x<0.08 and in the Pbnm structure (paraelectrics) for x≥0.3 at room temperature. Temperature-dependent dielectric response and x-ray diffraction confirm the occurrence of a structural transition in the Pbnm phase at low temperature for the samples with x ≤0.4. While the positive magnetodielectric effects can be attributed to a coupling of magnetic and crystallographic structures of the R3c phase, the observed negative magnetodielectric effect in the Pbnm phase can be associated with a low-temperature modification of the Pbnm structure. The observed dualsigned magnetodielectric effects suggest that the Bi1-xGdxFeO3 oxides are a good prototype for understanding the magnetodielectric coupling mechanism in this kind of materials.     

Influence of stoichiometry on the luminescent properties of InAs quantum dots grown on a heterostructure

We report photoluminescence (PL) spectra of InP/In_xGa_1-xAs/InAs/InP dot-in-a-well structures grown by MOVPE, with different compositions of the ternary layer. Measurements with atomic force microscopy showed that the largest quantum dot (QD) height is obtained when the InAs QDs are grown on the In_xGa_1-xAs layer with a mismatch of 1000 ppm, and the height decreases as the mismatch departs from this value. PL spectra of the QDs showed an asymmetric band, which involves transitions between dot energy levels and can be deconvoluted into two peaks. The highest energy PL peak of this band was observed for the sample with the QDs grown on top of the lattice-matched In_xGa_1-xAs layer and it shifted to lower energies for strained samples as the degree of mismatch increased. Theoretical calculations of the energy levels of the entire structure were used to interpret the obtained PL spectra and determine the possible detection tunability range.     

Influence of calcium substitution on the phase transition and ionic conductivity in BICAVOX oxide ion conductor

Samples of Bi₄Ca _x V_{2? x} O_{11?(3 x /2)?δ} in the composition range 0.07 ≤ x ≤ 0.30 were prepared by conventional solid state reactions. The stability of different phases as a function of composition was analysed by X-ray powder diffraction, FT-IR spectra, differential thermal analysis and AC impedance spectroscopy. For the compositions x ≤ 0.10, monoclinic α-phase structure is retained at room temperature. For x = 0.13, orthorhombic β-phase is observed, whereas for x ≥ 0.17, high O^2?conducting tetragonal γ-phase is stabilised. However, the highest ionic conductivity σ_300°C = 3.27 × 10^?4 S cm^?1 was observed for x = 0.17. This higher value of conductivity of the substituted compound as compared to the parent compound can be attributed to the increased oxygen ion vacancies generated as a result of cation doping. AC impedance spectroscopy reveals the fact that this ionic conductivity is mainly due to the grain contribution.     

Species and temperature predictions in a semi-industrial MILD furnace using a non-adiabatic conditional source-term estimation formulation

A Reynolds-Averaged Navier–Stokes (RANS) simulation of the semi-industrial International Flame Research Foundation (IFRF) furnace is performed using a non-adiabatic Conditional Source-term Estimation (CSE) formulation. This represents the first time that a CSE formulation, which accounts for the effect of radiation on the conditional reaction rates, has been applied to a large scale semi-industrial furnace. The objective of the current study is to assess the capabilities of CSE to accurately reproduce the velocity field, temperature, species concentration and nitrogen oxides (NO_x) emission for the IFRF furnace. The flow field is solved using the standard k–ε turbulence model and detailed chemistry is included. NO_x emissions are calculated using two different methods. Predicted velocity profiles are in good agreement with the experimental data. The predicted peak temperature occurs closer to the centreline, as compared to the experimental observations, suggesting that the mixing between the fuel jet and vitiated air jet may be overestimated. Good agreement between the species concentrations, including NO_x, and the experimental data is observed near the burner exit. Farther downstream, the centreline oxygen concentration is found to be underpredicted. Predicted NO_x concentrations are in good agreement with experimental data when calculated using the method of Peters and Weber. The current study indicates that RANS-CSE can accurately predict the main characteristics seen in a semi-industrial IFRF furnace.     

Absence of morphotropic phase boundary effects in BiFeO<Subscript>3</Subscript>–PbTiO<Subscript>3</Subscript> thin films grown via a chemical multilayer deposition method

We report an unusual behavior observed in (BiFeO₃)_1−x –(PbTiO₃) _x (BF–xPT) thin films prepared using a multilayer chemical solution deposition method. Films of different compositions were grown by depositing several bilayers of BF and PT precursors of varying BF and PT layer thicknesses followed by heat treatment in air. X-ray diffraction showed that samples of all compositions show mixing of two compounds resulting in a single-phase mixture, also confirmed by transmission electron microscopy. In contrast to bulk compositions, samples show a monoclinic (M_A-type) structure suggesting disappearance of the morphotropic phase boundary (MPB) at x=0.30 as observed in the bulk. This is accompanied by the lack of any enhancement of the remanent polarization at the MPB, as shown by the ferroelectric measurements. Magnetic measurements showed an increase in the magnetization of the samples with increasing BF content. Significant magnetization in the samples indicates melting of spin spirals in the BF–xPT films, arising from a random distribution of iron atoms. Absence of Fe²⁺ ions was corroborated by X-ray photoelectron spectroscopy measurements. The results illustrate that thin film processing methodology significantly changes the structural evolution, in contrast to predictions from the equilibrium phase diagram, besides modifying the functional characteristics of the BP-xPT system dramatically.     

Mg-Ni-Nd非晶合金晶化温度与晶化驱动力的预测

通过对Buschow提出的预测二元非晶态合金晶化温度的“最小空位”模型进行扩展,并进一步结合Miedema理论得到了一种预测三元非晶态合金晶化温度和晶化驱动力的理论方法.利用该方法计算了(Mg_70.6Ni_29.4)_1-xNd_{x(x＝5,10,15)非晶态合金的晶化温度、晶化驱动力以及晶化焓.其中晶化温度和晶化焓的理论预测值与实验值的相对误差分别小于8%和7%.同时发现较高的晶化驱动力会降低 关键词： 非晶态合金 晶化温度 晶化驱动力}     

First-principles studies of Mn-doped LiCoPO4

This paper investigates Mn-doped LiCoPO₄ material using first-principles calculations. Results indicate that the volume change of LiMn_xCo_1-xPO₄ to Mn_xCo_1-xPO₄ is smaller than that of undoped LiCoPO₄, which is responsible for the excellent tolerance of repeated cycling in lithium ion batteries. Combining first-principles calculations with basic thermodynamics, we calculate the average intercalation voltage of Mn-doped LiCoPO₄. It is shown that the redox couple Mn³⁺/Mn²⁺ can be observed with increasing Mn content. Therefore, the Mn ion displays some electrochemical activity during discharge/charge of LiMn_xCo_1-xPO₄ due to the coexistence of Co and Mn.     

Optical anisotropy of GaNAs grown on GaAs(0 0 1) substrate

In this paper GaN_xAs_1−x surfaces during growth are observed using reflectance difference or reflectance anisotropy spectroscopy (RDS or RAS). The epi-layer was grown by solid-source molecular beam epitaxy (MBE) system with a RF nitrogen prasma source. RD spectra showed broader structure and reduced amplitude compared to those of GaAs surfaces; GaAs(2 × 4)-like features were still observed with weak and blue-shifted peaks. In the low growth temperature region, an extra structure was also observed around 3.02 eV. We proposed that GaN_xAs_1−x surface can be classified into three types of the surface.     

Pb1－xMnxSe薄膜的光学特性

采用分子束外延的方法在BaF₂(111)衬底上制备出了高质量的Pb_1-xMn_xSe(0≤x≤0.0681)薄膜.X射线衍射结果表明，Pb₁-xMn_xSe薄膜为立方相NaCl型结构，没有观察到MnSe相分离现象，薄膜的取向为平行于衬底(111)晶面.晶格常数随着Mn含量的增加逐渐减小，Mn含量由Vegard公式得到.通 关键词： 1-xMn_xSe外延薄膜')" href="#">Pb_1-xMn_xSe外延薄膜 透射光谱 带隙 折射率     

脉冲激光沉积法制备的Ni1-xFexO稀磁半导体的结构和磁性研究

利用脉冲激光沉积方法制备出了具有室温铁磁性的Ni_1-xFe_xO(x=0.02,0.05)稀磁半导体.X射线衍射(XRD)结果表明Ni_1-xFe_xO的晶体结构为NaCl结构,并且在Fe含量较高的Ni_0.95Fe_0.05O中出现了少量的α-Fe₂O₃物相.X射线吸收近 关键词： 1-xFe_xO')" href="#">Ni_1-xFe_xO XANES 脉冲激光沉积方法     

四方晶系应变Si空穴散射机制

基于Fermi黄金法则及Boltzmann方程碰撞项近似理论, 推导建立了(001)弛豫Si_1-xGe_x衬底外延四方晶系应变Si空穴散射几率与应力及能量的理论关系模型, 包括离化杂质、声学声子、非极性光学声子及总散射概率(能量40 meV时)模型. 结果表明: 当Ge组分(x)低于0.2时, 应变Si/(001)Si_1-xGe_x材料空穴总散射概率随应力显著减小. 之后, 其随应力的变化趋于平缓. 与立方晶系未应变Si材料相比, 四方晶系应变Si材料空穴总散射概率最多可减小66%. 应变Si材料空穴迁移率增强与其散射概率的减小密切相关, 本文所得量化模型可为应变Si空穴迁移率及PMOS器件的研究与设计提供理论参考.     

Comparative evaluation of methods for precise determination of oxygen stoichiometry in highT c oxides

A comparative study of the methods for oxygen stoichiometry—direct inert gas fusion method and two indirect iodometric methods—has been carried out. The indirect iodometric method in which Cu(III) is directly estimated, has been found to be more precise. Several samples of YBa₂Cu₃O _x of varyingx, their fluorine-substituted counterparts and also Bi₂Sr₂Ca₁Cu₂O _x and Tl₂Ba₂Ca₁Cu₂O _x have been analysed.     

Thermovoltaic effect in samarium sulfide-based heterostructures

The thermovoltaic effect in samarium sulfide-based bulk heterostructures with compositions Sm_{1 − x} Eu _x S and Sm_{1 − x} Gd _x S is considered in the temperature interval 300–460 K. It is shown that this effect can be observed when an external temperature gradient is absent, i.e., when the sample temperatures near the contacts from which a generated voltage is tapped off are the same. The temperature is measured with thermocouples and also by a contactless method using a thermal imager.