Semiclassical calculation of ionization rate for Rydberg hydrogen atoms near a metal surface

The ionization of Rydberg hydrogen atoms near a metal surface at different scaled energies above the classical saddle point energy has been discussed by using the semiclassical method. The results show that the atoms ionize by emitting a train of electron pulses. In order to reveal the chaotic and escape dynamical properties of this system in detail, the sensitive dependence of the ionization rate upon the scaled energy is discussed. As the scaled energy is close to the saddle point energy, the ionization process of the hydrogen atom is nearly the same as the case of hydrogen atom in an electric field. There is only a single pulse of electrons, with an exponentially decaying tail. With the increase of the scaled energy, the ionization rates are similar to the case of the hydrogen atom in parallel electric and magnetic field, a series of electron pulses appear in the ionization process. This is caused by classical chaos, which occurs for the metal surface. Our studies also suggest that the metal surface can play the role of both the electric and the magnetic fields. Our theoretical analysis will be useful for guiding experimental studies of the ionization of atoms near the metal surface.     

Up-conversion emissions of Er<Superscript>3+</Superscript>-Yb<Superscript>3+</Superscript> codoped Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles by the arc discharge synthesis method

The Er³⁺-Yb³⁺ codoped Al₂O₃ nanoparticles with an average particle size of about 50 nm have been synthesized by an arc discharge synthesis method. The green and red up-conversion emissions centered at about 526, 547 and 677 nm, corresponding respectively to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, were detected by a 978-nm semiconductor laser diode excitation. The Annealing has evident effect on the up-conversion emissions of the samples: The red up-conversion emission is noticeable before annealing; however, the green up-conversion emission becomes predominant after annealing. The mixture of (Er,Yb)₃Al₅O₁₂ and α-(Al,Er,Yb)₂O₃ phases is more favorable for green up-conversion emissions due to an enhancement of the ESA (I) of ⁴I_11/2+a photon→⁴F_7/2 and ET (III) of ²F_5/2(Yb³⁺)+⁴I_11/2(Er³⁺)→²F_7/2(Yb³⁺)+⁴F_7/2(Er³⁺) processes. The two-photon absorption up-conversion process is involved in the green and red up-conversion emissions. The results have proved that arc discharge synthesis is a new promising preparation technology for optical materials. Supported by National Natural Science Foundation of China (Grant No. 10804015), the Scientific Research Foundation for Doctor of Liaoning Province (Grant No. 20071095), and the Educational Committee Foundation of Liaoning Province (Grant No. 2008123)     

Comparative atomistic and coarse-grained study of water: What do we lose by coarse-graining?

We employ the inverse Boltzmann method to coarse-grain three commonly used three-site water models (TIP3P, SPC and SPC/E) where one molecule is replaced with one coarse-grained particle with isotropic two-body interactions only. The shape of the coarse-grained potentials is dominated by the ratio of two lengths, which can be rationalized by the geometric constraints of the water clusters. It is shown that for simple two-body potentials either the radial distribution function or the geometrical packing can be optimized. In a similar way, as needed for multiscale methods, either the pressure or the compressibility can be fitted to the all atom liquid. In total, a speed-up by a factor of about 50 in computational time can be reached by this coarse-graining procedure.     

高温超导电机电枢线圈临界电流密度研究

高温超导线材的临界电流密度受磁场的影响,在设计计算时,通常假设在线圈截面上电流密度是均匀分布的。但是,实际上在线圈截面上,电压是一致的。由于线圈中的磁场不均匀分布,各处的临界电流密度是不同的。采用数值模拟的方法,分析了高温超导线圈截面上的临界电流分布。结果显示,临界电流密度的分布是不均匀的,总的临界电流可比按均匀分布时的计算结果提高25%以上。     

退火时间对MgB_2超导薄膜的影响

在多晶A l2O3衬底上,以B2H6作为硼源,化学气相沉积先驱B薄膜,采用Mg扩散方法,在不同退火时间条件下制备了MgB2超导薄膜。通过电阻-温度曲线测量、X射线衍射分析和扫描电子显微镜形貌观测方法,研究了退火时间对MgB2薄膜的超导特性、晶体结构、表面形貌的影响。     

Mechanical stability and thermodynamic properties of OsC from first-principles calculations

The elastic and thermodynamic characteristics of OsC crystal have been predicted through a method of density functional theory within the generalized gradient approximation (GGA). Compared with WC-type OsC, NaCl-type OsC is not only energy unfavorable but also mechanics unstable. The five independent elastic constants (C_ij), bulk modulus (B₀), the dependence of bulk modulus on temperature and pressure as well as the thermal expansion coefficient (α_V) at various temperatures for WC-type OsC are discussed. According to our calculations, WC-type OsC should be an ultra-incompressible material with high bulk modulus about 381 GPa. In addition, the bulk modulus will increase with increasing pressure while decrease with increasing temperature. The researches on the thermal expansion coefficient indicate that there will be a knee point during the process of thermal expansion coefficient variation versus increasing temperature. Our results may provide useful information for theoretical and experimental investigation of OsC.     

Vibration and instability analysis of tubular nano- and micro-beams conveying fluid using nonlocal elastic theory

A nonlocal Euler–Bernoulli elastic beam model is developed for the vibration and instability of tubular micro- and nano-beams conveying fluid using the theory of nonlocal elasticity. Based on the Newtonian method, the equation of motion is derived, in which the effect of small length scale is incorporated. With this nonlocal beam model, the natural frequencies and critical flow velocities for the case of simply supported system and for the case of cantilevered system are obtained. The effect of small length scale (i.e., the nonlocal parameter) on the properties of vibrations is discussed. It is demonstrated that the natural frequencies are generally decreased with increasing values of nonlocal parameter, both for the supported and cantilevered systems. More significantly, the effect of small length scale on the critical flow velocities is visible for fluid-conveying beams with nano-scale length; however, this effect may be neglected for micro-beams conveying fluid.     

有机半导体的物理掺杂理论

基于最低未被占据分子轨道（LUMO）和最高被占据分子轨道（HOMO）的高斯态密度分布与载流子在允许量子态中的费米-狄拉克（Fermi-Dirac）分布,提出有机半导体中物理掺杂的理论模型;研究了掺杂浓度、温度和禁带宽度对载流子浓度的影响,并与一些报道的实验结果做了比较.研究发现无论是否掺杂,温度升高时,有机半导体中的载流子浓度都会增大,并且随温度倒数的线性减小而指数增大;对于本征有机半导体,载流子浓度随禁带宽度的增大而指数下降,随高斯分布宽度的平方指数增加;对杂质和主体不同能级关系的掺杂情形下掺杂浓度对载 关键词： 有机半导体 掺杂 高斯态密度 载流子浓度     

First-principles study of structrural and corrected band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO systems

A first-principles method based on density functional theory(DFT),a generalized gradient approximation(GGA),and a projector-augmented wave(PAW) are used to study the structual and band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO(0 ≤x≤1) ternary alloys.By taking into account all of the possible structures,the band gaps of Zn1-xCdxO and Zn1-xMgxO alloys are corrected and compared with experimental data.     

Elastic stability and electronic structure of low energy tetragonal and monoclinic PdN2 and PtN2

This paper studies the elastic and electronic structure properties of two new low-energy structures of PdN2 and PtN2 by first-principles calculations.It finds that tetragonal and monoclinic structures are more stable than a pyrite one.The always positive eigenvalues of the elastic constant matrix confirm that both the tetragonal and monoclinic structures are elastically stable.The origin of the low bulk modulus of the two structures is discussed.The results of the calculated density of states show that both of the two low-energy structures are metallic.