Noether-Form Invariance of Nonholonomic Controllable Mechanical Systems in Phase Space

In this paper, we study the Noether-form invariance of nonholonomic mechanical controllable systems in phase space. Equations of motion of the controllable mechanical systems in phase space are presented. The definition and the criterion for this system are presented. A new conserved quantity and the Noether conserved quantity deduced from the Noether-form invariance are obtained. An example is given to illustrate the application of the results.     

Tunable charge density wave in TiS_3 nanoribbons

Recently, modifications of charge density wave(CDW) in two-dimensional(2D) show intriguing properties in quasi-2D materials such as layered transition metal dichalcogenides(TMDCs). Optical, electrical transport measurements and scanning tunneling microscopy uncover the enormous difference on the many-body states when the thickness is reduced down to monolayer. However, the CDW in quasi-one-dimensional(1D) materials like transition metal trichalcogenides(TMTCs) is yet to be explored in low dimension whose mechanism is likely distinct from their quasi-2D counterparts.Here, we report a systematic study on the CDW properties of titanium trisulfide(TiS_3). Two phase transition temperatures were observed to decrease from 53 K(103 K) to 46 K(85 K) for the bulk and 15-nm thick nanoribbon, respectively,which arises from the increased fluctuation effect across the chain in the nanoribbon structure, thereby destroying the CDW coherence. It also suggests a strong anisotropy of CDW states in quasi-1D TMTCs which is different from that in TMDCs.Remarkably, by using back gate of-30 V ～ 70 V in 15-nm device, we can tune the second transition temperature from110 K(at-30 V) to 93 K(at 70 V) owing to the altered electron concentration. Finally, the optical approach through the impinging of laser beams on the sample surface is exploited to manipulate the CDW transition, where the melting of the CDW states shows a strong dependence on the excitation energy. Our results demonstrate TiS_3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.     

Nonlinear image encryption using a fully phase nonzero-order joint transform correlator in the Gyrator domain

A novel nonlinear image encryption scheme based on a fully phase nonzero-order joint transform correlator architecture (JTC) in the Gyrator domain (GD) is proposed. In this encryption scheme, the two non-overlapping data distributions of the input plane of the JTC are fully encoded in phase and this input plane is transformed using the Gyrator transform (GT); the intensity distribution captured in the GD represents a new definition of the joint Gyrator power distribution (JGPD). The JGPD is modified by two nonlinear operations with the purpose of retrieving the encrypted image, with enhancement of the decrypted signal quality and improvement of the overall security. There are three keys used in the encryption scheme, two random phase masks and the rotation angle of the GT, which are all necessary for a proper decryption. Decryption is highly sensitivity to changes of the rotation angle of the GT as well as to little changes in other parameters or keys. The proposed encryption scheme in the GD still preserves the shift-invariance properties originated in the JTC-based encryption in the Fourier domain. The proposed encryption scheme is more resistant to brute force attacks, chosen-plaintext attacks, known-plaintext attacks, and ciphertext-only attacks, as they have been introduced in the cryptanalysis of the JTC-based encryption system. Numerical results are presented and discussed in order to verify and analyze the feasibility and validity of the novel encryption–decryption scheme.     

非晶物质中的临界现象

非晶态材料有着复杂的原子结构(短程有序、长程无序)和特殊的物理性质,其临界现象和相变问题一直受到学术界关注.非晶合金,又称为金属玻璃,是一种新型的非晶态材料,具有很高的强度和优异的弹性.从微观的角度来看,非晶合金可以看作是一个多粒子系统.临界现象的研究对认识和理解多粒子系统之间的相互作用有深刻的意义.本文主要讨论非晶合金中的临界现象,包括非晶合金从制备过程、微观结构到宏观的力学性能以及磁性方面存在的临界现象,并分析这些临界现象之间的内在联系,进而深入理解非晶合金的微观结构对其宏观性质的影响.这为认识非晶合金的形成本质,提高服役可靠性,探索具有实际应用价值的非晶合金提供理论依据.     

稠密气固两相QL-EMMS曳力模型及改进

通过引入颗粒加速度,改进现有EMMS曳力模型中的颗粒受力平衡关系,耦合流动的整体参数、局部参数与曳力,提出了基于流态化多尺度最小能量原理的QL-EMMS曳力模型.实验检验表明,模型精度高,普适性好.进一步采用O-S经验曳力模型的单峰函数,修正了颗粒团直径公式,改进后的QL-EMMSn模型更加合理地体现了非均匀曳力变化的...     

铁基超导体中的化学掺杂研究

目前已经发现的绝大部分铁基超导体都是通过化学掺杂而得到的。铁基超导体的母体一般在200K以下经历自旋密度波（SDW）转变：即其基态是一类巡游电子反铁磁不良导体。通过适当的元素替代可以在FeAs层产生额外的电子、空穴、巡游性或化学压力,从而有效地抑制SDW序,实现超导电性。本文侧重作者所在小组的相关研究结果,将铁基超导体中的元素替代研究分为FeAs层外和FeAs层内掺杂两大类,依次介绍和评述两年来国际上对4种主要铁基化合物中的化学掺杂研究进展。     

Mechanisms of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation

For the last decade, a variant of pulsed laser ablation, Resonant-Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE), has been studied as a deposition technique for organic and polymeric materials. RIR-MAPLE minimizes photochemical damage from direct interaction with the intense laser beam by encapsulating the polymer in a high infrared-absorption solvent matrix. This review critically examines the thermally-induced ablation mechanisms resulting from irradiation of cryogenic solvent matrices by a tunable free electron laser (FEL). A semi-empirical model is used to calculate temperatures as a function of time in the focal volume and determine heating rates for different resonant modes in two model solvents, based on the thermodynamics and kinetics of the phase transitions induced in the solvent matrices. Three principal ablation mechanisms are discussed, namely normal vaporization at the surface, normal boiling, and phase explosion. Normal vaporization is a highly inefficient polymer deposition mechanism as it relies on collective collisions with evaporating solvent molecules. Diffusion length calculations for heterogeneously nucleated vapor bubbles show that normal boiling is kinetically limited. During high-power pulsed-FEL irradiation, phase explosion is shown to be the most significant contribution to polymer deposition in RIR-MAPLE. Phase explosion occurs when the target is rapidly heated (10⁸ to 10¹⁰ K/s) and the solvent matrix approaches its critical temperature. Spontaneous density stratification (spinodal decay) within the condensed metastable phase leads to rapid homogeneous nucleation of vapor bubbles. As these vapor bubbles interconnect, large pressures build up within the condensed phase, leading to target explosions and recoil-induced ejections of polymer to a near substrate. Phase explosion is a temperature (fluence) threshold-limited process, while surface evaporation can occur even at very low fluences.     

GaN氢化物气相外延生长系统的设计与制作

根据GaN氢化物气相外延生长(HVPE)的原理,设计制作了双温区卧式HVPE系统．根据实际生长中出现的问题和CaN样品的测试情况,对系统进行了逐步的调试和改进．     

大非线性相移下光学非线性Z扫描特性的研究

采用高斯分解法(GD)对大非线性相移下的Z扫描特性进行了分析，通过对数值算法的优化，将GD推广到对脉冲入射激光下大非线性相移下的Z扫描理论分析.对不同条件下大非线性相移Z扫描曲线峰谷结构的比较，发现在大非线性相移的情况下，Z扫描曲线的峰和谷随透过光阑或入射光强变化表现出某些新的特性.随着透过光阑孔径的增加，Z扫描曲线峰的变化要明显快于谷的变化，而且在谷明显存在的情况下，峰很快消失.采用皮秒脉冲激光下的纯二硫化碳实验对理论结果加以验证，实验结果和理论分析相一致.我们的分析结果对大非线性相移下Z扫描测量有一定的指导性意义，避免在大非线性相移下对Z扫描结果产生错误的分析. 关键词： 大非线性相移 高斯分解法 Z扫描     

克尔介质中超连续谱的产生与自聚焦的抑制

 从（3+1）维非线性薛定谔方程出发,理论上分析了超短脉冲频谱展宽与自聚焦的影响因素。分析得出：通过改变泵浦光的功率和光束口径,可以实现光谱的极大展宽并避免自聚焦成丝。数值模拟了小口径强泵浦光束在BK7玻璃中的传输过程并进行了实验验证。模拟结果显示在超连续谱产生的同时小尺度调制被完全抑制。实验结果表明：降低泵浦光功率,使光束不会因为全光束自聚焦而发生塌陷,同时还能控制除自聚焦外的其它非线性效应,进而改善近场光束质量。由于自相位调制是超短超强脉冲产生超连续谱的重要机制之一,需要维持传输过程中的泵浦光功率,由此最佳的入射光功率应选在全光束自聚焦功率阈值附近。