Loss of cold atoms due to collisions with residual gases in free flight in a magneto-optical trap

The loss rate of cold atoms in a trap due to residual gas collisions differs from that in a free state after the cold atoms are released from the trap. In this paper, the loss rate in a cold rubidium-87 atom cloud was measured in a magneto-optical trap(MOT) and during its free flight. The residual gas pressure was analyzed by a residual gas analyzer, and the pressure distribution in a vacuum chamber was numerically calculated by the angular coefficient method. The decay factor, which describes the decay behavior of cold atoms due to residual gas collisions during a free flight, was calculated. It was found that the decay factor agrees well with theoretical predictions under various vacuum conditions.     

电磁感应现象中“电源”的探析

在电磁感应现象中,关于回路中两点间电势高低的判断与两点间电势差的分析计算是教学中的典型问题,解决此类问题的关键是弄清楚回路中哪部分导线是"电源",再利用磁场和电场的对称性与"基尔霍夫定律"进行计算,即可解决.本文以几个例题进行探析. 1 线圈部分在磁场外 [例1]用均匀导线做成的矩形线框abcd,ab＝1/2bc＝20 cm,线框的一半放在具有理想边界的均匀强磁场中,e,f分别为bc,ad边的中点,如图1(a)所示.当磁场以10 T/s的变化率增强时,问e,f两点间的电势差多大?     

环扩张下的Gorenstein平坦模型结构

研究了环扩张下的Gorenstein平坦模型结构及其同伦范畴,设R≤S是满足一些条件的平坦扩张.我们证明了若f:M→N在S-模范畴的Gorenstein平坦模型结构中是上纤维化(纤维化,弱等价),则f:M→N在R-模范畴中亦如此;若R≤S是优越扩张,反过来也成立,即在优越扩张下Gorenstein平坦模型结构是不变的.进而,相关的稳定范畴是等价的,当且仅当对任意Gorenstein平坦S-模M,Coker(ηM)是平坦的,其中η表示S-模范畴和R-模范畴间的Quillen伴随函子的单位.     

Observation of multiple charge density wave phases in epitaxial monolayer 1T-VSe2 film

As a special order of electronic correlation induced by spatial modulation, the charge density wave (CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning—tunneling microscopy in various temperatures, we discover a hidden incommensurate stripe-like CDW order besides the ($sqrt{7}$ × $sqrt{3}$) CDW phase at low-temperature of 4 K in the epitaxial monolayer 1T-VSe₂} film. Combining the variable-temperature angle-resolved photoemission spectroscopic (ARPES) measurements, we discover a two-step transition of an anisotropic CDW gap structure that consists of two parts Δ₁ and Δ₂. The gap part Δ₁ that closes around ～ 150 K is accompanied with the vanish of the ($sqrt{7}$ × $sqrt{3}$) CDW phase. While another momentum-dependent gap part Δ₂ can survive up to ～ 340 K, and is suggested to the result of the incommensurate CDW phase. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure Δ₁ + Δ₂, which suggests different forming mechanisms between the ($sqrt{7}$ × $sqrt{3}$) and the incommensurate CDW phases. Our findings provide significant information and deep understandings on the CDW phases in monolayer 1T-VSe₂} film as a two-dimensional (2D) material.     

光电化学生物传感器研究

光电化学法是在光照射下,将化学能转换为电能的低成本方法。而光电化学生物传感技术由于具有通过生物分子氧化产生的光电流来检测生物分子的能力而引起了广泛的关注。光电化学生物传感器具有低成本、高灵敏度、高特异性、仪器操作简单以及检测背景信号低等特点,在免疫检测和生物技术等重要领域具有广泛应用前景。近年来,对于光电化学生物传感器性能和检测方法的研究也取得了颇丰的成果。本文主要介绍光电化学生物传感器的概念及基本原理、分类应用及对其未来的展望。     

钛酸盐ATiO3 钙钛矿的铁电和反铁畸变

钛酸盐钙钛矿氧化物体系的研究对理解具有铁电极化的功能材料具有非常重要的科学意义.本文对ATiO3(A= Ca,Sr,Ba,Pb,Cd)体系的低温相、结构相变、及其对应的位移模式进行综述.我们着重比较了该体系存在的两类不稳定声子模式,即铁电极化模式(Ferroelectric,FE)和反铁畸变模式(Antiferrodistortive,AFD),在不同体系中所起到的作用.我们举例阐述了晶格失配应变、人工构造超晶格和化学掺杂等方法对这两种模式的调控思路.最后我们给出本文的总结讨论及研究展望.     

基于原位探测的空间冷原子钟的性能分析

在空间微重力环境下,应用激光冷却技术的空间冷原子钟有望获得更高精度的时间频率基准。提出了一种基于原位探测的新型空间冷原子钟方案,在开展冷原子俘获、冷却、选态、两次微波探寻与量子态探测等过程中,冷原子都保持在微波腔中,这种设计可以使单个原子钟的周期更短,微波探寻过程有更大的时间占空比,也能使原子钟的整体结构更加紧凑。在使用Boitier a Vieillissement Ameliore (BVA)晶振作为本振的条件下,从Dick效应与量子投影噪声两方面对原子钟的稳定度进行分析预估,然后分析了影响冷原子钟不确定的来源与估值,结果表明:基于原位探测的空间冷原子钟有望达到5.9×10~(-14)τ~(-1/2)的稳定度以及1×10~(-16)的不确定度,该结果优于当前使用BVA晶振作为本振的其他冷原子微波钟的性能。     

Automatic compensation of magnetic field for a rubidium space cold atom clock

When the cold atom clock operates in microgravity around the near-earth orbit, its performance will be affected by the fluctuation of magnetic field. A strategy is proposed to suppress the fluctuation of magnetic field by additional coils, whose current is changed accordingly to compensate the magnetic fluctuation by the linear and incremental compensation. The flight model of the cold atom clock is tested in a simulated orbital magnetic environment and the magnetic field fluctuation in the Ramsey cavity is reduced from 17 nT to 2 nT, which implied the uncertainty due to the second order Zeeman shift is reduced to be less than 2×10~(-16). In addition, utilizing the compensation, the magnetic field in the trapping zone can be suppressed from 7.5 μT to less than 0.3 μT to meet the magnetic field requirement of polarization gradients cooling of atoms.     

ZnO薄膜硅微压电矢量水听器

提出了一种基于ZnO压电薄膜的硅微压电矢量水听器,其核心部件是利用微电子机械系统(MEMS)技术制作的悬臂梁结构压电加速度计。由近似解析和有限元分析,得出加速度计的灵敏度和谐振频率,并在此基础上对其进行了优化设计。研制了MEMS压电加速度计,并装配后构成MEMS矢量水听器。测试结果表明:加速度灵敏度在20~1,200 Hz范围内约为0.83 mV/(m/s2)。经过液柱法测量,在1 kHz时,MEMS矢量水听器等效声压灵敏度为-229.5 dB (ref.1V/μPa),比同类型压阻式MEMS矢量水听器的灵敏度高17 dB以上。      

钙钛矿型氧化物非常规铁电研究进展

钙钛矿型氧化物因具有丰富的磁性、铁电、力学和光学等诸多功能属性,在电子信息通信材料器件领域中有广阔的应用前景.在各种物理性质之中,铁电极化因其产生机制多样,并能与磁性和晶格应变相互耦合形成多铁性等特点,近十多年来一直被作为凝聚态物理研究的国际热点问题.与以自发极化作为初级序参量的常规铁电材料不同,非常规铁电材料中的铁电极化是被其他的序参量诱导而产生的.本综述围绕无机钙钛矿型氧化物非常规铁电体的研究进展进行了总结.回顾了该体系经典唯象理论和原子尺度的微观模型,有序排列的人工钙钛矿超晶格型结构,以及稀土正铁氧体单晶的反铁磁畴壁结构中非常规铁电的极化强度大小及其诱导机制,为系统理解非常规铁电提供了理论途径.