Nonmagnetic impurity effects and neutron scattering spectrum in iron pnictides

We study the effect of local impurity and the neutron scattering spectrum based on the five-orbital model obtained by the first principle calculation for iron pnictides. We find that the interband impurity scattering is induced by the complex multiorbital structure. This fact means that the fully-gapped sign-reversing s-wave state, which is predicted by spin-fluctuation theories, is very fragile against impurities. The result suggests a reasonable possibility that the fully-gapped s-wave state without sign reversal (s₊₊-wave) would be realized in dirty iron pnictides. We also find that broad peak structure observed in the neutron scattering measurements can be explained by the s₊₊-wave state.     

扭曲向列相液晶电光效应的微观机理研究

为了探究液晶电光效应的微观机理,以扭曲向列相液晶为研究对象,对液晶分子的变化情况进行了理论分析。运用连续体弹性理论分析了扭曲向列相液晶分子的指向矢取向变化,并对液晶电光效应曲线不同区间范围液晶微观结构的变化进行了系统分析。结果表明:液晶分子在零电场条件下的扭曲是均匀的;当外加电场大于某一值时,液晶分子开始转向,液晶产生电光效应。研究结果对液晶显示器件设计和深入理解液晶电光效应机理具有一定的指导意义。     

Synthesis of quantum-confined CdS nanotubes

CdS nanotubes with wall thickness comparable to excitonic diameter of the bulk material are synthesized by a chemical route. A change in experimental conditions result in formation of nanowires, and well-separated nanoparticles. The diameter and wall thickness of nanotubes measured to be 14.4 ±  6.1 and 4.7 ± 2.2 nm, respectively. A large number of CdS nanocrystallites having wurzite structure constitute these nanotubes. These nanotubes show high energy shifting of optical absorption and photoluminescence peak positions, compared to its bulk value, due to quantum confinement effect. It is proposed that nucleation and growth of bubbles and particles in the chemical reaction, and their kinetics and interactions are responsible for the formation of nanotubes.     

Metastable state and macroscopic quantum tunneling of binary mixtures

We provide a mixing model to explore the metastable state and the macroscopic quantum tunneling (MQT) of binary mixtures. For , we first observe the two condensates form the symmetry-breaking state (SBS) and then suddenly transfer to the symmetry-preserving state (SPS) through the MQT. The SBS is shown to be the metastable state in our system. We find the MQT does not spontaneously arise. The inducement mechanism is the damping but not the excitations. The damping mechanism can also control the lifetime and the tunneling decay rate of the SBS. Finally, we further present the origin of these phenomena by examining the energy of the system.     

Photochromism of new unsymmetrical isomeric diarylethenes bearing a methoxyl group

Three new unsymmetrical isomeric diarylethenes having a methoxyl substituent at ortho‐, meta‐, and para‐position of the terminal benzene ring, namely 1‐(2,5‐dimethyl‐3‐thienyl)‐2‐[2‐methyl‐5‐(2‐methoxylphenyl)‐3‐thienyl]perfluorocyclopentene ( 1o ), 1‐(2,5‐dimethyl‐3‐thienyl)‐2‐[2‐methyl‐5‐(3‐methoxylphenyl)‐3‐thienyl]perfluorocyclopentene ( 2o ), and 1‐(2,5‐dimethyl‐3‐ thienyl)‐2‐[2‐methyl‐5‐(4‐methoxylphenyl)‐3‐thienyl]perfluorocyclopentene ( 3o ), have been synthesized. The substituent position effect of methoxyl group on their properties, including photochromism and fluorescence both in hexane solution and in PMMA film, and their electrochemical properties, were investigated in detail. These diarylethenes showed good photochromism both in solution and in PMMA film. For the same photochromic diarylethene backbone, the electron‐ donating methoxyl substituent can effectively depress the cyclization quantum yields and increase the cycloreversion quantum yields compared to those of diarylethenes bearing chlorine atoms reported previously. Diarylethenes 1o – 3o showed clear fluorescent switches by photoirradiation both in hexane and in PMMA film. In addition, cyclic voltammetry tests showed that the electron‐donating methoxyl group at different position on the terminal benzene ring had a significant effect on the electrochemical properties of these isomeric diarylethenes. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of concentration and particle size on nonlinearity of Au nano-fluid prepared by γ (Co) radiation

Nonlinear properties of Au nano-fluid prepared by γ-radiation method at different concentrations were investigated. Measurements of nonlinear refractive index and nonlinear absorption coefficient were carried out using a single beam Z-scan technique. A green CW laser beam operated at 532 nm was used as excitation source. The Au nano-fluid shows a good third order nonlinear response. The sign of the nonlinear refractive index is found to be negative and the magnitude is in the order of 10⁻⁷ cm²/W. This nonlinear effect increases as the concentration increases from 3.119 × 10⁻⁴ to 2.354 × 10⁻³ M which correspond to particle sizes of 4.0-30.5 nm, respectively. A good linear relationship was obtained between nonlinear refractive index and concentration. However the relationship between nonlinear refractive index and particle size was nonlinear behavior.     

Error and jitter effect studies on the SLED for the BEPC Ⅱ-linac

An RF pulse compressor is a device used to convert a long RF pulse to a short one with a much higher peak RF magnitude. SLED can be regarded as the earliest RF pulse compressor to be used in large-scale linear accelerators. It has been widely studied around the world and applied in the BEPC and BEPC linac for many years. During routine operation, error and jitter effects will deteriorate the performance of SLED, either on the output electromagnetic wave amplitude or phase. The error effects mainly include the frequency drift induced by cooling water temperature variation and the frequency/Q 0 /β unbalances between the two energy storage cavities caused by mechanical fabrication or microwave tuning. The jitter effects refer to the PSK switching phase and time jitters. In this paper, we re-derive the generalized formulae for the conventional SLED used in the BEPC linac, and the error and jitter effects on SLED performance are also investigated.     

Magnetocaloric effect and transition order of Mn5Ge3 ribbons

Single phase Mn₅Ge₃ ribbons were successfully synthesized by single roller melt-spinning method followed by an annealing procedure at 800 °C for 1 h. The magnetocaloric effect and transition order were investigated by dc magnetization measurement. A maximum entropy change of 4.92 J/kg K and a refrigerant capacity of 201.3 J/kg were observed at an external field change of 30 kOe. The Banerjee criterion was adopted to discriminate the order of the transition, indicating a second order transition. The calculated entropy changes were also obtained by Landau theory, which are in agreement with the experimental values at the temperature region above the Curie temperature. This phenomena implies a potential transition of the magnetic interaction in the vicinity of the Curie temperature. Universal behavior was also indicated in that all of the experimental entropy change curves collapse into one universal curve, which is consistent with the Banerjee criterion.     

Current-induced colossal electroresistance in La0.8Ba0.2MnO3 films fabricated by sol–gel method

Transport properties of La_0.8Ba_0.2MnO₃ thin films deposited by the sol–gel method were investigated. It has been found that resistivity plateaus occurred in the ρ–T curves after application of a dc bias current over a critical value. A current of 200 μA could induce a huge resistance variation ∼1200% in these La_0.8Ba_0.2MnO₃ films near room temperature, demonstrating a colossal electroresistance effect. Such strange transport behavior suggests the formation of conductive filaments and that the multi-phase coexistence is sensitive to external stimuli. This phenomenon may find applications in sensing and logic devices.