高压下LiF和NaF的结构稳定性及其电子和光学性质的第一性原理研究

基于密度泛函理论，采用全势线性缀加平面波加局域轨道方法，计算模拟了LiF高压下的相变行为，预测其在450GPa附近发生由NaCl结构(B1)到CsCl结构(B2)的结构相变.同时还计算了高压下LiF不同相的电学特性，LiF的复介电函数以及介电常数随压强变化关系.通过比较能带结构的变化行为，得出LiF在53GPa附近还存在等结构相变，即由直接带隙结构变为间接带隙结构.将LiF的计算结果与另外一个同构化合物NaF进行了比较讨论. 关键词： LiF 压致相变 从头计算     

Irreversible structural transformation of Si(1 1 4)-2 × 1 induced by subsurface carbon

Using scanning tunneling microscopy (STM), it has been found that the reconstruction of Si(1 1 4) is transformed irreversibly from a 2 × 1 structure composed of dimer (D), rebonded atom (R), and tetramer (T) rows (phase A) to a different kind of 2 × 1 structure composed of D, T, and T rows (phase B) by C incorporation. It has been confirmed by high-resolution synchrotron core-level photoemission spectroscopy (PES) that such an irreversible structural transformation is due to stable subsurface C atoms. They induce anisotropic compressive stress on the surface, which results in insertion of Si dimers to an R row to form a T row.     

Sr位Nd掺杂对SrBi$lt;sub$gt;2$lt;/sub$gt;Nb$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;9$lt;/sub$gt;性能的影响及机理研究

采用传统固相法制备了（Sr_1-3x/2A_x/2Nd_x）Bi₂Nb₂O₉（x=0,0.05,0.1和0.2）陶瓷,并系统研究了Nd离子取代Sr离子对SrBi₂Nb₂O₉性能的影响及其作用机理.研究结果表明：Sr_1-3x/2A_x/2Nd_xBi₂Nb₂O₉的介电常数和介电损耗随温度变化的行为具有明显的离子松弛极化特征.Nd³⁺对Sr²⁺的部分取代,导致Sr_1-3x/2A_x/2Nd_xBi₂Nb₂O₉剩余极化强度P_r稍有下降,但其压电系数d₃₃却有所增加,根据铁电热力学理论,这是Nd³⁺对Sr²⁺取代导致材料介电常数增大所致.Sr_1-3x/2A_x/2Nd_xBi₂Nb₂O₉的居里温度（T_C）没有随Nd含量的增加而变化,拉曼光谱技术分析表明这是其NbO₆八面体畸变程度没有发生变化所致.Nd³⁺取代Sr²⁺提高了材料的介电常数ε_r、压电系数d₃₃、机电耦合系数K_p,同时降低了机械品质因数Q_m,但是谐振频率温度系数C值没有改变. 关键词： 压电陶瓷 介电性能 压电性能 拉曼光谱     

Doughnut-like beam generation of Laguerre-Gaussian mode with extremely high mode purity

An apparatus to achieve a single doughnut-like Laguerre-Gaussian (LG) mode with extremely high mode purity higher than 99.6% is reported. The proposed apparatus is a conceptual combination of a high-order HG mode selector and an astigmatic mode converter, which consists of only four simple and easily-made optical elements: three lens elements and one binary phase element. Theoretical simulations show that the apparatus has a good tolerance for both system production and alignment errors. With such high mode purity, the doughnut-like LG beam generated by the apparatus carrying an orbital angular momentum will be beneficial to numbers of studies including the trapping and rotating of micro and nano-particles, non-linear optics, and atom-light interaction.     

Beam concentration and homogenization for high power laser diode bar

A novel optical element is presented and applied for beam concentration and homogenization of laser diode (LD) bar. It consists of a tapered SiO₂-rod with twisted surfaces which is designed and optimized by the optical system design software and fabricated by a curved surface grinder and an optical polishing lathe. Results show that a rectangular output beam spot with uniform intensity distributions both at slow and fast axis is obtained at the output facet of the rod and the beam size is only 1.3 mm ∗ 0.8 mm.     

Measurements of the cross section for e(+)e(-) --> hadrons at center-of-mass energies from 2 to 5 GeV

We report values of R = sigma(e(+)e(-)-->hadrons)/sigma(e(+)e(-)-->mu(+)mu(-)) for 85 center-of-mass energies between 2 and 5 GeV measured with the upgraded Beijing Spectrometer at the Beijing Electron-Positron Collider.     

Wavelength dependent damage in biological multi-photon confocal microscopy: A micro-spectroscopic comparison between femtosecond Ti:sapphire and Cr:forsterite laser sources

Molecular excitation by the simultaneous absorption of two photons provides intrinsic three-dimensional resolution in laser scanning fluorescence microscopy. Thus induced two-photon absorption and the accompanied multi-photon absorption/ionization not only cause photo-bleaching but also cell damage in the vicinity of the focal point. In this paper, we study the wavelength dependent cell damage induced by high intensity femtosecond near infrared lasers. The study was performed with a Ti:sapphire laser and a Cr:forsterite laser. With a longer output wavelength from a Cr:forsterite laser, multi-photon absorption and auto-fluorescence were found to be significantly suppressed and the destructive plasma formation was found to be greatly reduced. Sustained multi-photon spectra can be observed in most plant specimens with a tightly focused Cr:forsterite laser beam under long term irradiation with more than 100 mW laser average power. In contrast, multi-photon absorption induced destructive plasma formation were frequently observed with a tightly focused Ti:sapphire laser beam within seconds with more than 10 mW laser average power.     

Ultrathin nanosheets of Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>: A new two-dimensional ferromagnetic material with strong magnetocrystalline anisotropy

Two-dimensional (2D) materials with robust ferromagnetism have played a key role in realizing nextgeneration spin-electronic devices, but many challenges remain, especially the lack of intrinsic ferromagnetic behavior in almost all 2D materials. Here, we highlight ultrathin Mn₃O₄ nanosheets as a new 2D ferromagnetic material with strong magnetocrystalline anisotropy. Magnetic measurements along the in-plane and out-of-plane directions confirm that the out-of-plane direction is the easy axis. The 2D-confined environment and Rashba-type spin-orbit coupling are thought to be responsible for the magnetocrystalline anisotropy. The robust ferromagnetism in 2D Mn₃O₄ nanosheets with magnetocrystalline anisotropy not only paves a new way for realizing the intrinsic ferromagnetic behavior in 2D materials but also provides a novel candidate for building next-generation spin-electronic devices.     

激光脉冲波形对烧蚀Si靶表面温度的影响

利用双温方程对激光烧蚀Si靶的过程进行了数值模拟,并结合合适的初始条件和边界条件,研究了在飞秒、皮秒激光作用下,脉冲波形(矩形、梯形、三角形和高斯形)对Si靶表面载流子和晶格温度分布的影响。结果表明:激光功率密度是影响载流子温升的主要因素,矩形脉冲激光烧蚀Si靶表面载流子的峰值温度最高,而高斯分布的脉冲引起靶面载流子峰值温度最低。可见,激光脉冲波形对Si靶表面载流子的温度分布具有重要影响。所得结果可为制备高质量的薄膜提供理论依据。