Magnetic properties of the pyrite type FexCo1−x−yNiyS2 system

The magnetic phase diagram of the Fe_xCo_1−x−yNi_yS₂ system was determined. Ferromagnetic (F)-, antiferromagnetic (AF )- and paramagnetic (P) phases were found. In the triangular diagram, the F-phase appears along the CoS₂-FeS₂ line, the AF-phase around NiS₂ and the P-phase between the F- and AF-phases. Discussions are given on the F-phase.     

Luminescence from Si-Si1−xGex/Si1−yCy-Si structures

Near band edge photoluminescence has been obtained from Si_1−yC_y quantum well (QW) and neighboring Si_1−xGe_x/Si_1−yC_y double QW (DQW) structures. Enhanced no-phonon recombination is observed from the DQW structures and it is attributed to a breaking of the k-selection rule in the presence of the heterointerface. The luminescence persists for measurement temperatures up to 30–50 K and the intensity exhibits a quenching behavior with an activation energy equal to 8–20 meV. In electroluminescence only recombination in the Si_1−xGe_x layer has been observed from neighboring Si_1−xGe_x and Si_1−yC_y DQW structures.     

Ba1-xB2-y-zO4SixAlyGaz晶体和频可调谐深紫外飞秒激光器

介绍了一种基于新型非线性晶体Ba_1-xB_2-y-zO₄Si_xAl_yGa_z 的可调谐深紫外飞秒激光光源. 从理论上分析了基频光和倍频光在通过非线性晶体时所造成的空间走离和群速度失配, 为了补偿空间走离以及波长调谐过程中晶体折射造成的光束偏离现象, 将两块相同的倍频晶体成镜像放置来产生二次谐波. 并调节延迟线的长度来补偿基频光和倍频光之间的群速度失配, 从而提高和频转换效率. 然后通过和频方式进行三倍频和四倍频来突破晶体相位匹配条件的限制, 产生了波长低于200 nm的深紫外飞秒激光. 利用钛宝石激光器提供基频光光源, 最终在250–300 nm, 192.5–210 nm 范围内获得了高重频、可调谐超短脉冲紫外和深紫外激光. 并在基频光波长为800 nm时, 得到的二倍频、三倍频和四倍频的功率分别为1.28 W, 194 mW和5.8 mW, 相对于前一级的转换效率依次为46.14%, 15.16%和3%. 采用互相关法测量得到266.7 nm紫外激光的脉冲宽度约为640.4 fs.     

Negative luminescence from In1−xAlxSb and CdxHg1−xTe diodes

Indium aluminium antimonide (In_1−xAl_xSb) and cadmium mercury telluride (Cd_xHg_1−xTe) heterostructure diodes, which comprise a near intrinsic active region bounded by more highly doped contact regions, exhibit positive or negative luminescence at medium to long infrared wavelengths when forward or reverse biased respectively at room temperature. In reverse bias, the carrier densities in the near intrinsic region are reduced below their equilibrium values by the effects of exclusion and extraction. In consequence, the radiative recombination is reduced and the devices emit less infrared radiation than the thermal equilibrium value. The observed intensity of the negative luminescence is in general agreement with expected values.     

Estimation of conductivity parameters in Sr1 − xThxF2 + 2x solid solution with fluorite-type structure

Concentration of nn-dipoles, nnn-dipoles and charge carriers as well as parameters related to their reorientation and mobility have been estimated from an analysis of a.c. conductivity data of polycrystalline samples of the Sr_{1 − x}Th_xF_{2 + 2x} solid solution. The molar fraction of charge carriers has been evaluated as a function of composition. Results obtained confirm, in agreement with the clustering process model, that the (F_i)_m ions responsible for long range motions in Sr_{1 − x}Th_xF_{2 + 2x} are interstitial fluoride ions of F type.     

Noise behavior of amorphous GexSi1−xOy for microbolometer applications

High resistivity sputtered a-Ge_xSi_1−xO_y compound was investigated for application to microbolometer fabrication for thermal imaging. Noise behavior of the fabricated bolometers was measured, showing no evidence of random telegraph switching (RTS) noise. 1/f noise was measured at several measuring currents, resulting in a 1/f noise factor of 2.9 × 10⁻¹¹ that can be used for further design and modeling.     

ALT in the nucleon-nucleon polarized Drell-Yan process

We present a leading order (LO) estimate for the longitidinal-transverse spin asymmetry (A ) in the nucleon-nucleon polarized Drell-Yan process at RHIC and energies in comparison with A and A . A receives contribution from g₁, the transversity distribution h₁, and the twist-3 distributions g and h . For the twist-3 contribution we use the bag model prediction evolved to a high energy scale by the large-N_c evolution equation. We found that A (normalized by the asymmetry in the parton level) is much smaller than the corresponding A . Twist-3 contribution given by the bag model turned out to be negligible.     

多铁材料Bi1-xCaxFeO3的介电、铁磁特性和高温磁相变

采用溶胶凝胶法制备Bi_1-xCa_xFeO₃ (x=0, 0.05, 0.1, 0.15, 0.2)陶瓷样品. X衍射图谱表明所有样品的主衍射峰均与纯相BiFeO₃相符合且具有良好的晶体结构. 随着x的增大, Bi_1-xCa_xFeO₃样品的主衍射峰由双峰(104)与(110) 逐渐重叠为单峰(110), 当x ≥0.15时, 样品呈现正方晶系结构; 扫描电镜形貌分析可知, 晶粒由原来的0.5 μm逐渐增大到2 μm. Bi_1-xCa_xFeO₃样品介电常数和介电损耗随着x 的增加先增大而后减小. 当f=1 kHz, Bi_0.9Ca_0.1FeO₃ 的介电常数达到最大值, 是BiFeO₃的7.5倍, 而Bi_0.8Ca_0.2FeO₃的介电常数达到最小值, 仅仅是BiFeO₃的十分之一. Bi_1-xCa_xFeO₃样品所呈现的介电特性是由偶极子取向极化和空间电荷限制电流两种极化机理共同作用的结果. 随着Ca²⁺ 的引入, BiFeO₃ 样品的铁磁性显著提高. X射线光电子能谱图表明Fe²⁺和Fe³⁺ 共存于Bi_1-xCa_xFeO₃ 样品中, Fe²⁺/Fe³⁺比例随着Ca²⁺ 掺杂量的增加而增大, 证明Ca²⁺掺杂增加了Fe²⁺的含量, 增强BiFeO₃的铁磁特性. 从M-T曲线观察到BiFeO₃样品在878 K附近发生铁磁相变, 示差扫描量热法测试再次证明BiFeO₃ 在878 K发生相变. Ca²⁺掺杂使BiFeO₃样品的T_N略有变化而T_M基本不变, 其主要原因是Fe-O-Fe反铁磁超交换作用的强弱和磁结构相对稳定.     

Superconductivity in Ba2−xSrxYCu3Oz prepared at GPa

The compounds Ba_1.4Sr_0.6YCu₃O_z (7.0≤z<7.4) were synthesized at 2 GPa to clarify the effect of high temperature and pressure on the superconducting properties. The structure as observed by powder X-ray diffraction was tetragonal for z≥7.2, and orthorhombic or tetragonal for z<7.1. It has been revealed that high-pressure syntheses suppress the superconductivity in the compounds with z<7.1. Recovery of the superconducting properties occurs above z≥7.2. The effect of Sr substitution on the superconducting properties was also studied. The magnitude of diamagnetism at 5–30 K decreases monotonically from −0.003 to −0.0002 emu/g Oe with increasing x in Ba_2−xSr_xYCu₃O_z (x≥1.2).     

Magnetic properties of the Bi1.95Sr2.05−xLaxCuOy (Bi-2201) superconducting phase

We have investigated the reversible mixed-state magnetization M of three lanthanum substituted Bi_1.95Sr_2.05−xLa_xCuO_y (Bi-2201) ceramic samples having different critical temperatures T_c ranging from 20.0 to 35.5 K. As for the Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) phase, we found that anisotropy of Bi-2201 is large. A manifestation of this anisotropy is the field independent magnetization M^* observed at a temperature T^*. In the framework of the London model, and including thermal fluctuations of vortices, we found for the temperature dependence of the penetration depth λ_ab(T) = λ_ab(0)[1 − (T/T_c0)ⁿ]^−1/2, with n 1.7 and λ_ab (T = 0) 4000 Å. The estimated upper critical fields μ₀H_c2,c are of the order of 10 T. We observe a peculiar negative slope M/T at low temperature and sufficiently high external magnetic field. This feature seems to be a characteristic of the Bi-2201 phase. However, we do not know whether it is associated with the superconducting mixed-state. A small amount of magnetic impurities could also be responsible for this behavior. Finally, the behavior of the reversible magnetization of the Bi-2201 samples investigated, which are situated at the optimal and in the overdoped region, did not indicate any unusual temperature dependence for the upper critical field H_c2,c.     

纤锌矿结构ZnO/Mg_xZn_(1-x)O量子阱中带间光吸收的尺寸效应和三元混晶效应

对于纤锌矿结构ZnO/MgxZn1-xO有限深单量子阱结构,考虑内建电场、导带弯曲及材料掺杂对实际异质结势的影响,利用有限差分法和自洽法数值求解Schr?dinger方程和Poisson方程,获得电子(空穴)的本征能级和本征波函数.进而,采用费米黄金法则讨论带间光吸收的尺寸效应和三元混晶效应.结果表明:三元混晶材料MgxZn1-xO中Mg组分的增加会增强垒层和阱层的内建电场强度,使得电子(空穴)平均位置靠近左(右)垒,导致带间跃迁吸收峰呈指数减小且发生蓝移;ZnO/MgxZn1-xO量子阱带间跃迁吸收峰随阱宽增大而减小,吸收峰发生红移.所得结果可为改善异质结构材料和器件的光电性能提供理论指导,以期获得实际应用所需的光学吸收频谱和波长.     

Vortex core physics in the cuprate superconductor Nd2−xCexCuOy

Flux-flow voltage steps and negative differential resistivity in the cuprate Nd_2−xCe_xCuO_y are explained in terms of subbands between the Fermi energy and the energy gap affecting the quasiparticle dynamics.     

Synthesis and characterization of the Aurivillius phases Bi2 − xPbxSr1 − xNdxNb2O9

The n = 2 Aurivillius phase Bi_{2 − x}Pb_xSr_{1 − x}Nd₂O₉ was successfully synthesized as a ceramic material over the whole range of simultaneous, charge compensated substitution x = 0–1.0. Structural investigations were performed by Rietveld refinement applying different space groups Fmmm and A2₁am, and additionally by X-ray absorption spectroscopy (EXAFS) on the Nd L_III-edge, confirming the accommodation of Nd on the atomic sites of Sr, which implies the substitution of Bi³⁺ by the isoelectronic Pb²⁺. The ferroelectric transition temperature T_c = 270 °C of the substituted powders with x = 0.4 and 1.0 is distinctly reduced compared to the unsubstituted sample with T_c = 450 °C. In temperature resolved powder X-ray diffraction patterns no structural phase transition could be detected.     

Charge trapping in n-AlxGa1-xAs “insulators” and related device instabilities

In some devices based on GaAs/Al_xGa₁-_xAs heterostructures, the Al_xGa₁-_xAs plays the role of a wide band gap “insulator”. These devices are therefore excellent systems for studying charge trapping in Al_xGa₁-_xAs. It is a poorly understood property of Al_xGa₁-_xAs that incorporation of any n-type dopant results in the formation of a deep electron trap, the DX center. Recent experiments on heterostructure devices have probed both thermal and athermal (hot electron) capture and emission by the DX center. By observing the trapping behavior as the composition (Al mole fraction) of the alloy is varied, the relationship between the trap level and the band structure of the host material has been clarified. A remarkable result is the observation of electron trapping at alloy compositions where the trap state is resonant with the conduction band.     

Moisture barrier of AlxOy coating on poly(ethylene terephthalate), poly(ethylene naphthalate) and poly(carbonate) substrates

The moisture barrier property of Al_xO_y coated poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN) and poly(carbonate) (PC), have been investigated. The differences in the morphology of the Al_xO_y sputtered grown on these substrate were investigated using atomic force microscopy (AFM). The initial growth of the Al_xO_y followed closely the topology of the substrate and an amplified roughness was observed. In the fully grown Al_xO_y, the comparative roughness followed that of the substrates. It has been found that a single layer Al_xO_y improved the moisture barrier of PET by an order of magnitude, PC by two orders of magnitude while no improvement was observed for PEN. UV-ozone treatment on PC further improved the moisture barrier, while no improvement was observed for PET and PEN. The comparative effects of the substrate surface roughness and surface energy on the moisture barrier are discussed.     

Crystal structures of stage-n iodine-intercalated compounds IBi2nSr2nCanCu2nOx

The crystal structure of stage-3 iodine-intercalated superconducting IBi₆Sr₆Ca₃Cu₆O_x has been determined by transmission electron microscopy to belong to the space group Pma2 with lattice parameters a=5.4Å, B=5.4Å and C=49.4Å. Ioidine atoms intercalated between every three Bi---O bilayers expand the distance between the Bi---O layers by 3.6Å and alter the atomic stacking across Bi---O layers from the staggered configuration characteristic of host superconducting Bi₂Sr₂CaCu₂O_x to an aligned configuration characteristic of stage-1 iodine-intercalated superconducting IBi₂Sr₂CaCu₂O_x. Higher-stage intercalation has also been observed as stacking faults which predominantly contain both stage-2 and stage-3 phases. The space groups and c-axis dimensions of the higher-stage phases have been deduced to be Pma2 with c=3.6+15.3n Å when stage number n is odd, and Bbmb with c=2(3.6+15.3n) Å when n is even.     

(Fe1-xCox)3BO5纳米棒磁性的研究

本文采用高温有机溶剂法制备了(Fe_1-xCo_x)₃BO₅纳米棒, 通过控制反应物中乙酰丙酮钴的含量合成了不同Co含量的(Fe_1-xCo_x)₃BO₅. 利用高分辨透射电子显微镜(HRTEM)、超导量子干涉磁强计(SQUID)对其形貌和磁性能进行了表征. 高分辨透射电子显微镜结果表明制备出的纳米(Fe_1-xCo_x)₃BO₅为多晶棒状, 且具有多折孪晶结构; 磁性测量的结果表明,(Fe_1-xCo_x)₃BO₅纳米棒在室温下表现出铁磁性, 随着Co含量的增加, 纳米棒的铁磁性逐渐增加, 该纳米棒有望用来研究生物大分子的机械性能.     

Phonon Raman scattering in Nd1+xBa2−xCu3O7−δ and Pr1+xBa2−xCu3O7−δ single crystals

The polarized Raman spectra of Nd_1+xBa_2−xCu₃O_7−δ (−0.023≤x≤0.107) and Pr_1+xBa_2−xCu₃O_7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) A_g mode softens by 6 cm⁻¹ in Nd 1:2:3 and 4 cm⁻¹ in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.     

Oxygen overdoping in superconducting and non-superconducting Y1−xPrxBa2Cu3Oy

We present the evolution of magnetic and structural properties of Y_1−xPr_xBa₂Cu₃O_y (x0.5 and x=1) single crystals and polycrystalline materials when the oxygen concentration y is varied from under- to overdoping. We have found a monotonous evolution of the Pr Néel temperature for x=1 samples and a maximum of the superconducting critical temperature for the x0.5 samples. The structural properties as detected by X-ray diffraction and Raman spectroscopy show no instabilities when crossing the optimal doping region as was found in the x=0 material.