Dissipative quantum phase transition in a biased Tavis-Cummings model

We study the dissipative quantum phase transition(QPT)in a biased Tavis–Cummings model consisting of an ensemble of two-level systems(TLSs)interacting with a cavity mode,where the TLSs are pumped by a drive field.In our proposal,we use a dissipative TLS ensemble and an active cavity with effective gain.In the weak drive-field limit,the QPT can occur under the combined actions of the loss and gain of the system.Owing to the active cavity,the QPT behavior can be much differentiated even for a finite strength of the drive field on the TLS ensemble.Also,we propose to implement our scheme based on the dissipative nitrogen-vacancy(NV)centers coupled to an active optical cavity made from the gainmedium-doped silica.Furthermore,we show that the QPT can be measured by probing the transmission spectrum of the cavity embedding the ensemble of the NV centers.     

Improvement of high-frequency properties of Co_2FeSi Heusler films by ultrathin Ru underlayer

Heusler Co2FeSi films with a uniaxial magnetic anisotropy and high ferromagnetic resonance frequency fr were deposited by an oblique sputtering technique on Ru underlayers with various thicknesses tRufrom 0 nm to 5 nm.It is revealed that the Ru underlayers reduce the grain size of Co2FeSi,dramatically enhance the magnetic anisotropy field HK induced by the internal stress from 242 Oe(1 Oe=79.5775 A·m^-1)to 582 Oe with an increment ratio of 2.4,while a low damping coefficient remains.The result of damping implies that the continuous interface between Ru and Co2FeSi induces a large in-plane anisotropic field without introducing additional external damping.As a result,excellent high-frequency soft magnetic properties with fr up to 6.69 GHz are achieved.     

Influence of external load on friction coefficient of Fe-polytetrafluoroethylene

A coarse-grained molecular dynamics simulation model was developed in this study to investigate the friction process occurring between Fe and polytetrafluoroethylene(PTFE).We investigated the effect of an external load on the friction coefficient of Fe–PTFE using the molecular dynamics simulations and experimental methods.The simulation results show that the friction coefficient decreases with the external load increasing,which is in a good agreement with the experimental results.The high external load could result in a larger contact area between the Fe and PTFE layers,severer springback as a consequence of the deformed PTFE molecules,and faster motion of the PTFE molecules,thereby affecting the friction force and normal force during friction and consequently varying the friction coefficient.     

In-situ SiN combined with etch-stop barrier structure for high-frequency AlGaN/GaN HEMT

The etch-stop structure including the in-situ SiN and AlGaN/GaN barrier is proposed for high frequency applications.The etch-stop process is realized by placing an in-situ SiN layer on the top of the thin AlGaN barrier.F-based etching can be self-terminated after removing SiN,leaving the AlGaN barrier in the gate region.With this in-situ SiN and thin barrier etch-stop structure,the short channel effect can be suppressed,meanwhile achieving highly precisely controlled and low damage etching process.The device shows a maximum drain current of 1022 mA/mm,a peak transconductance of 459 mS/mm,and a maximum oscillation frequency(fmax)of 248 GHz.     

Investigation of active-region doping on InAs/GaSb long wave infrared detectors

The eight-band κ·p model is used to establish the energy band structure model of the type-II InAs/GaSb superlattice detectors with a cut-off wavelength of 10.5μm,and the best composition of M-structure in this type of device is calculated theoretically.In addition,we have also experimented on the devices designed with the best performance to investigate the effect of the active region p-type doping temperature on the quantum efficiency of the device.The results show that the modest active region doping temperature(Be:760℃)can improve the quantum efficiency of the device with the best performance,while excessive doping(Be:>760℃)is not conducive to improving the photo response.With the best designed structure and an appropriate doping concentration,a maximum quantum efficiency of 45% is achieved with a resistance-area product of 688?·cm^2,corresponding to a maximum detectivity of 7.35×10^11cm·Hz^1/2/W.     

Effect of C_(60) nanoparticles on elasticity of small unilamellar vesicles composed of DPPC bilayers

The interaction between C60 nanoparticles and biomembranes has been of great interest in researches over the past decades due to their novel applications as well as potential cytotoxicity.In this work,we study the deformation of the small unilamellar vesicles composed of dipalmitoylphosphatidylcholine(DPPC)lipid bilayers infiltrated with C60 nanoparticles of different molecular concentrations through coarse-grained molecular dynamics simulations.By employing the Helfrich spontaneous curvature model,the bending modulus and the spontaneous curvature of the vesicles with C60 nanoparticles of different concentrations are obtained according to the simulation data.The results show that the bending modulus and the spontaneous curvature of pure DPPC vesicle membranes are approximately 1.6×10^-19J and 1.4 nm^-1,respectively.Both of them increase linearly approximately as the C60 concentration increases from 0 to 16.3%.The density profiles of vesicles,the order of lipid packing and the diffusion characteristics of DPPC and C60 are also investigated.     

喷流冷却复合陶瓷薄片激光器的热特性仿真分析及实验研究

为分析喷流冷却复合陶瓷薄片激光器的热特性,设计用于冷却复合陶瓷薄片的喷流冷却系统.利用湍流换热理论和计算流体动力学仿真方法建立喷流冷却复合陶瓷薄片激光器的流固耦合热仿真模型,定义评价其冷却能力和冷却均匀性的定量参数.根据该仿真模型得到喷流冷却系统的最优设计参数,并进行实验验证.使用163孔喷板,流量为0.2kg/s,入口温度为20℃,在1200 W泵浦时获得359 W激光输出功率,并测得复合陶瓷薄片上表面的最高温度为92℃.激光输出功率与复合陶瓷薄片上表面温度均与泵浦功率呈近似正线性关系,且温度的实验值与仿真值相符度较高.     

G波段500 W带状注扩展互作用速调管设计研究

针对太赫兹频段实现高功率面临物理机制上的难题,设计了一个G波段带状注速调管,展示了基于非相对论带状电子注和扩展互作用技术所能达到的功率水平以及影响性能的物理因素。文中设计基于电压24.5 kV、电流0.6 A,1 mm×0.15 mm的椭圆电子注,以及与之相匹配的互作用系统,即横向过尺寸哑铃型多间隙谐振腔,可以实现高功率和高增益。三维PIC仿真结果显示,在考虑实际腔体损耗的情况下,能够获得超过500 W的功率,电子效率和增益分别达到3.65%和38.2 dB。研究发现,输出功率和效率的提升很大程度上受到多间隙腔模式稳定性以及电路欧姆损耗的制约;输出腔的欧姆损耗对输出功率影响尤为显著,工程设计需要特别考虑。本文的研究为高频段带状注扩展互作用器件的研发打下了良好的基础。     

氢原子束在大气长程传输中的剥离效应

考虑到中性粒子束对近地轨道太空垃圾的清理作用以及在太空探索中的潜在应用前景,研究了中性粒子束在亚轨道空间长程传输过程中影响束流能量和密度损失的主要物理机制,重点分析了剥离效应对束流损耗的影响。中性束剥离效应包括束流粒子之间碰撞导致的自剥离效应和其与大气粒子碰撞导致的剥离效应。以束密度随传播距离变化的方程为基础,通过引用几何因子来表征束流的自剥离效应强度,建立了剥离效应机制下束流的传输模型,由此评估了束流自剥离效应在中性束长程传输中对传输距离的影响关系。研究结果表明,在固定的高度,当中性束密度大于空气粒子密度时,自剥离效应的影响将非常突出,随着传输高度的升高,即使束密度和空气密度同时降低维持量级一致,自剥离效应对传输距离的影响在大几何因子的情况下仍会增强。     

高功率微波作用下限幅器尖峰泄漏特性

针对限幅器在高功率微波（HPM）作用下的尖峰泄漏问题,基于搭建的HPM注入实验平台和电路仿真开展了研究。研究结果表明：当注入功率超过6 dBm时,限幅器会出现尖峰泄漏现象,限幅器泄漏尖峰的上升沿与脉宽随着注入功率的增加而减小,而绝对尖峰泄漏功率随注入功率的增加呈增长趋势,平顶泄漏功率呈近似“线性增加-缓慢下降-小幅增长”趋势。并且,实验结果显示：HPM脉宽与重频对限幅器尖峰泄漏特性基本无影响,其泄漏特性变化规律与单次脉冲的基本一致;尖峰泄漏能量随注入功率的增加而降低。