Superconductivity and x—ray photoemission study of MgB2 thin films

Highly c-axis oriented MgB2 thin films with Tc^onset of 39.6K were fabricated by magnesium diffusing into pulsedlaser-deposited boron precursors.The estimation of critical current density Jc,using hysteresis loops and the Bean model,has given the value of 10^7A/cm^2(15K,0T),which is one of the highest values ever reported.The x-ray photoemission study of the MgB2 thin films has revealed that the binding energies of Mg 2p and B 1s are at 49.4eV and 186.9eV,which are close to those of metallic Mg and transition-metal diborides,respectively.     

Symmetry dependence of x—ray absorption near—edge structure at rhe metal K edge of 3d transition metal compounds

The pre-edge features in system with “even” symmetry,apart from quadrupolar tranition contribution,are mainly dipolar in character,associated with the existence of unoccupied states made up of mixed cation np with higher-neighboring cation-(n-l)d orbitals,and reflect the density of states due to the medium-range order of the system,while in “ood” symmetry materials these pre-edge features are the result of a transition from the 1s to final denisty of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation d orbitals.In the latter case,they contain not only the p but also the d base of orbitals,similar to a tetrahedral configuration.These results are validated for Fe as a photoabsorber by comparing x-ray absorption near-edge spectra of Fe2SiO4(fayalite) and Fe2O3(hematite) to ab initio full multiple scattering calculations at the Fe K edge,but pertain to all systems containing sixfold-coordinated cations.     

Study of Ni80Fe20／Fe50Mn50 superlattice microstructures by transmission electron microscopy and x—ray diffraction

The detailed microstructures of Ni80Fe20/Fe50Mn50 superlattices have been characterized using both x-ray diffraction techniques and transmission electron microscopy.The obrivous layered structure,typical column structure and twins which exist in Ni80Fe20/Fe50Mn50 superlattices were observed through performing transmission microscopy.By combining the technique of lowangle x-ray reflectivity(specular and off-specular scans)with the anomalous scattering effect and high-angle x-ray diffraction(using conventional x-ray),wequantitatively analysed the microstructural variation as a function of annealing temperature.It is found that the lateral correlation length,the(111)peak intensity of the superlattices and the average multilayer coherence length all increase with a rise in annealing temperature annealing can decrease the rootmean-square roughness at the interfaces of Ni80Fe20/Fe50Mn50 superlattices.the obtained microstructural knowledge will be helpful in understanding the magnetic properties of the ni80Fe20/Fe50Mn50 exchange bias system.     

Influence of laser power density on damage of comb by photodynamic therapy—simulation and validation of mathematical models

Photodynamic therapy （PDT） has poor therapeutic outcomes for the treatment of port-wine stain （PWS） lesions with long drug-light intervals （DLIs）. This letter investigates the possibility of improving the treatment efficacy through increasing the laser power density using a computer simulation and a cock comb model. The computational model includes a Monte Carlo simulation for the laser distribution and a calculation of the singlet oxygen concentrations （102）. Both simulation and experimental results show that increasing the power density from i00 to 140 mW/cm^2 not only improves the PDT efficacy, but also results in the unwanted skin damage.     

Calculation of the inverse bremsstrahlung absorption in unmagnetized plasmas

Inverse bremsstrahlung is one important way to deliver laser energy to the plasma in inertial confinement fusion. In this article, we study the collisional absorption rate as obtained from the Fokker–Planck treatment of an unmagnetized plasma in harmonic laser field. The electron–ion collision rate is considered in the Krook approximation, and the electron distribution function is considered a Maxwellian function. We evaluate the inverse bremsstrahlung absorption near the irradiated plasma surface in the critical layer. We observe that absorption increases with shorter laser wavelength and lower electron temperatures. When the maximum electron velocity in the limit of q → 1 reaches infinity, the q-non-extensive distribution function reduces to the standard Maxwell-Boltzmann distribution.     

Numerical simulation of chorus-driving acceleration of relativistic electrons at extremely low L-shell during geomagnetic storms

During 2018 major geomagnetic storm, relativistic electron enhancements in extremely low L-shell regions (reaching L～3) have been reported based on observations of ZH-1 and Van Allen probes satellites, and the storm is highly likely to be accelerated by strong whistler-mode waves occurring near very low L-shell regions where the plasmapause was suppressed. It is very interesting to observe the intense chorus-accelerated electrons locating in such low L-shells and filling into the slot region. In this paper, we further perform numerical simulation by solving the two-dimensional Fokker-Planck equation based on the bounce-averaged diffusion rates. Numerical results demonstrate the evolution processes of the chorus-driven electron flux and confirm the flux enhancement in low pitch angle ranges (20°-50°) after the wave-particle interaction for tens of hours. The simulation result is consistent with the observation of potential butterfly pitch angle distributions of relativistic electrons from both ZH-1 and Van Allen probes.     

Inverse Bremsstrahlung absorption coefficient in inhomogeneous plasma with nonextensive electron velocity distribution and Tsallis exponential electron density profile

Inverse bremsstrahlung (collisional) absorption of the laser beam is studied in plasma with a generalized (q-nonextensive) electron velocity distribution and some kind of generalized electron density profile. It is shown that for some values of parameters designating the q-nonextensive electron velocity distribution function and its generalized density profile, the calculated absorption coefficient reduces to the already known cases with Maxwellian velocity distribution with linear and exponential density profiles.     

碱金属原子的光激发与光电离

本文研究了碱金属原子在三步激光脉冲作用下的光激发和光电离过程的动力学特性, 重点关注和比较了锂和铯原子的异同. 针对多种激发模式, 本文不但建立了其原子布居数在各个跃迁态的速率方程组, 还给出了各相关态的光激发和光电离过程的解析解. 通过精心设计并选择了特殊情况, 显著简化了解析解的数学表达式, 从而凸显和讨论了其物理内涵. 通过自行编程, 系统地计算和观察了各种激发模式对锂原子的光激发和光电离过程的可能影响, 研究和讨论了电离率随激光参数的变化规律. 在相同激发模式下, 比较和分析了采用两种不同激发路径所导致的各态原子布居率的变化, 凸显了改变原子参数所产生的作用. 探讨了锂和铯原子在类似的激发条件下在电离率方面的差别. 最后, 基于本文的研究结果, 本文指出了优化电离率的多种途径.     

Nb,Ag原子的K壳层和L壳层电离截面的理论计算

电子离子碰撞电离截面是模拟激光等离子体的超热电子的能谱和产额的主要过程之一.基于相对论性的电子离子碰撞的K壳层电离截面理论,计算了Nb、Ag的K壳层和L壳层电子碰撞电离截面,结果和最近的文献实验数值和其它理论数值进行了比较,计算结果比其它模型更加准确,与最近的实验结果也吻合较好,该结果可用来模拟激光等离子体的超热电子能谱和产额.     

正电子碰撞Ag，In，Sn原子L壳层电离截面的理论计算

在David Botz分析模型的基础上,综合考虑正电子及电子碰撞电离的库仑效应和电子交换效应,引入离子效应和相对论效应修正因子,计算了Ag,In,Sn原子的L壳层电离截面.计算结果表明,引入了修正因子的计算结果明显优于平面波波恩近似和扭曲波波恩近似的计算结果,并和最近文献的实验值符合得较好.其计算结果可为激光等离子体模拟提供准确参数.     

正电子碰撞Ag，In，Sn原子L壳层电离截面的理论计算

在David Botz分析模型的基础上,综合考虑正电子及电子碰撞电离的库仑效应和电子交换效应,引入离子效应和相对论效应修正因子,计算了Ag,In,Sn原子的L壳层电离截面.计算结果表明,引入了修正因子的计算结果明显优于平面波波恩近似和扭曲波波恩近似的计算结果,并和最近文献的实验值符合得较好.其计算结果可为激光等离子体模拟提供准确参数.     

Cluster-assisted generation of multi-charged ions in nanosecond laser ionization of pulsed hydrogen sulfide beam at 1064 and 532nm

The multi-charged sulfur ions of S^q= (q\le 6) have been generated when hydrogen sulfide cluster beams are irradiated by a nanosecond laser of 1064 and 532,nm with an intensity of 10¹⁰\sim 10¹²W1\cdotcm^-2. S⁶⁺ is the dominant multi-charged species at 1064nm, while S⁴⁺, S³⁺ and S²⁺ ions are the main multi-charged species at 532nm. A three-step model (i.e., multiphoton ionization triggering, inverse bremsstrahlung heating, electron collision ionizing) is proposed to explain the generation of these multi-charged ions at the laser intensity stated above. The high ionization level of the clusters and the increasing charge state of the ion products with increasing laser wavelength are supposed mainly due to the rate-limiting step, i.e., electron heating by absorption energy from the laser field via inverse bremsstrahlung, which is proportional to \lambda ², \lambda being the laser wavelength.     

Z-scan analytical theory for material with saturable absorption and two-photon absorption

By using the Adomian decomposition method, we present the transmitted optical intensity of a nonlinear medium with the concurrence of saturable absorption (SA) and two-photon absorption (TPA) processes. We obtain the analytical expression of the open-aperture Z-scan and investigate the interplay between SA and TPA in the Z-scan trace. Through Z-scan measurements at different levels of laser intensities, both the saturable intensity and TPA coefficient could be obtained quickly yet unambiguously.     

相对论电子的逆韧致吸收

在激光强度大于１０＾１８Ｗ／ｃｍ＾２的条件下，利用强激光场中完全相对论的电子运动轨迹，得到了稀薄等离子体中的韧致吸收系数计算公式，并与原有的逆韧致吸收系数的计算公式进行了比较。