Disorder physics in mixtures of fermionic atoms

A mixture of light and heavy fermionic atoms in an optical lattice is considered. Tunneling of the heavy atoms is neglected such that they are only subject to thermal fluctuations. There is a complex interplay between light and heavy atoms which results in an Ising-like distribution of heavy atoms with a first-order transition from homogeneous to staggered order. A second-order transition is possible between an ordered and a disordered phase of heavy atoms. Depending on the phase of heavy atoms, light atoms are scattered around and can propagate, diffuse, and localize. Due to correlations in the disordered phase of heavy atoms, there can be a gap in the spectrum of light atoms, leading to a Mottlike incompressible state.     

Protecting entanglement of atoms stored in a common nonperfect cavity without measurements

We study entanglement dynamics of two and three atoms stored in a common nonperfect cavity together with some other nonentangled atoms. It is guessed at first thought that the presence of nonentangled atoms would favor the decoherence process of the interested entangled atoms. We show, on the contrary, that it is not so. Namely, as results of a rigorous nonperturbative analysis, disentanglement rate of the interested atoms decreases with the increase of the number of nonentangled atoms. If the number of nonentangled atoms is sufficiently large, the entanglement of interested atoms could be protected efficiently.     

速度调控的双原子纠缠

当光学腔中光场处于相干态,而原子处于运动中时,双原子的纠缠演化与光学腔场模结构相关联. 假如初始时刻原子的位置固定在腔中某一位置,双原子的纠缠演化将是无序的.然而,假如一开始双原子在光学腔相干态光场中处于运动状态,则双原子的纠缠随时间的变化将变得规则有序.如此,通过适当的选择双原子的速度和初始光场,就能对双原子的周期性纠缠进行控制,让纠缠在指定时刻出现.     

Structural optimization and segregation behavior of quaternary alloy nanoparticles based on simulated annealing algorithm

Alloy nanoparticles exhibit higher catalytic activity than monometallic nanoparticles, and their stable structures are of importance to their applications. We employ the simulated annealing algorithm to systematically explore the stable structure and segregation behavior of tetrahexahedral Pt–Pd–Cu–Au quaternary alloy nanoparticles. Three alloy nanoparticles consisting of 443 atoms, 1417 atoms, and 3285 atoms are considered and compared. The preferred positions of atoms in the nanoparticles are analyzed. The simulation results reveal that Cu and Au atoms tend to occupy the surface, Pt atoms preferentially occupy the middle layers, and Pd atoms tend to segregate to the inner layers. Furthermore, Au atoms present stronger surface segregation than Cu ones. This study provides a fundamental understanding on the structural features and segregation phenomena of multi-metallic nanoparticles.     

Coherence-controlled stationary entanglement between two atoms embedded in a bad cavity injected with squeezed vacuum

We investigate the entanglement between two atoms in an overdamped cavity injected with squeezed vacuum when these two atoms are initially prepared in coherent states. It is shown that the stationary entanglement exhibits a strong dependence on the initial state of the two atoms when the spontaneous emission rate of each atom is equal to the collective spontaneous emission rate, corresponding to the case where the two atoms are close together. It is found that the stationary entanglement of two atoms increases with decreasing effective atomic cooperativity parameter. The squeezed vacuum can enhance the entanglement of two atoms when the atoms are initially in coherent states. Valuably, this provides us with a feasible way to manipulate and control the entanglement, by changing the relative phases and the amplitudes of the polarized atoms and by varying the effective atomic cooperativity parameter of the system, even though the cavity is a bad one. When the spontaneous emission rate of each atom is not equal to the collective spontaneous emission rate, the steady-state entanglement of two atoms always maintains the same value, as the amplitudes of the polarized atoms varies. Moreover, the larger the degree of two-photon correlation, the stronger the steady-state entanglement between the atoms.     

碱土金属原子与Ne原子间相互作用势的理论计算

本文基于无任何可调参数的势模型计算了碱土金属原子(Be、Mg、Ca、Sr、Ba)与Ne原子间相互作用势,得到的势能曲线及势阱位置和深度与现有的从头计算结果符合较好.本文的计算结果进一步验证了碱土金属原子与稀有气体原子间交换能主要来自碱土金属原子最外层s电子与稀有气体原子最外层p电子之间的交换作用.     

Dimensional optical resonances in atomic nanostructures

A nanostructural object consisting of two atoms is proven to contain two or four dimensional resonances depending on whether these atoms are identical or different. Each of the atoms is characterized by linear isotropic polarizability and contains one isolated resonance. The frequency of the dimensional resonances considerably differs from the resonance frequencies of the isolated atoms, and the effective resonance polarizabilities of the atoms in the nanostructure approximately coincide with the resonance values of polarizabilities of the isolated atoms. It is shown that, in the nanostructure consisting of two different atoms, two of four resonances have a negative dispersion. It is also shown that the nanostructural object has an induced anisotropy in the case of a fixed position of the object axis relative to the direction of incidence of the external wave.     

Effect of pairwise dipole–dipole interaction among three-atom systems

We present an analysis of a system of three two-level atoms interacting with one another through dipole–dipole interaction. The interaction manifests between the excited state of one of the atoms and the ground state of its nearest neighbour. Steady-state populations of the density matrix elements are presented and are compared with a situation when only two atoms are present. It can be noticed that the third atom modifies the behaviour of the three atoms. Two configurations are analysed, one in which the three atoms are in a line, with no interaction between atoms at the end points and the other in which the atoms form a closed loop with one atom interacting with both its neighbours.     

Spontaneously created entanglement between two atoms

Spontaneous emission as a potential tool for creation of entanglement between two atoms is investigated. We assume that the atoms are coupled to the same environment and study entanglement engineering between the atoms and its transfer between different states. The role of the atomic coherence induced by spontaneous emission will be explored which, in contrast to what is generally believed, can create entanglement between initially unentangled atoms. We quantify entanglement by the concurrence and find that it exhibits threshold properties that can lead to interesting noncontinuous phenomena of sudden birth and sudden death of entanglement. In addition, we consider the mechanism involved in creation of entanglement between distant atoms coupled to a single-mode cavity field. We include a possible variation of the coupling constants between the atoms and the cavity mode with location of the atoms in a standing-wave cavity mode. Effectively, we engineer two coupled atoms whose the dynamics are analogous to that of interacting and collectively damped two nonidentical atoms. We illustrate the interesting result that spatial variations of the coupling constants can lead to a stationary entanglement between the atoms. We explain this effect in terms of the trapping phenomenon of atomic population in a non-decaying entangled state.     

Simultaneous magneto-optical trapping of a boson-fermion mixture of metastable helium atoms

We simultaneously confine fermionic metastable 3He atoms and bosonic metastable 4He atoms in a magneto-optical trap. The trapped clouds, containing up to 1.5 x 10(8) atoms of each isotope, are characterized by measuring ions and metastable helium atoms escaping from the trap. Optical pumping of 3He atoms to a nontrapped hyperfine state is investigated and it is shown that large atom numbers can be confined without additional repumping lasers. Unique possibilities for quantum degeneracy experiments with mixtures of spin-polarized metastable 3He and 4He atoms are indicated.     

Bombardment induced surface chemistry on Si under keV C60 impact

Molecular dynamics simulations of the sputtering of Si by C₆₀ keV bombardment are performed in order to understand the importance of chemical reactions between C atoms from the projectile and Si atoms in the target crystal. The simulations predict the formation of strong covalent bonds between the C and Si atoms, which result in nearly all of the C atoms remaining embedded in the surface after bombardment. At low incident kinetic energies, little sputtering of Si atoms is observed and there is a net deposition of solid material. As the incident kinetic energy is increased, the sputtering yield of Si atoms increases. At 15 keV, the yield of sputtered Si atoms is more than twice the number of C atoms deposited, and there is a net erosion of the solid material.     

铝中氦原子行为的密度泛函研究

用密度泛函理论计算了大量He原子存在时He在金属铝中不同位置的能量,并在理论上预测了铝中的氦原子行为。结果表明,铝晶胞内He原子择优占位区是空位,而在整个晶体范围,最有利于容纳He原子的区域是晶界,其次是空位和位错。在fcc-铝的两种间隙位中,He原子优先充填四面体间隙位。间隙He原子的迁移能很小,易于通过迁移在晶内聚集,或被空位、晶界、位错等缺陷束缚。     

SrTiO3(001)表面上Au和N原子相互作用的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势方法研究了SrTiO₃(001)表面上Au和N原子间相互作用的微观机理.通过比较分析N置换表面层O原子前后SrTiO₃(001)表面吸附Au原子体系的相关能量和电子结构,发现SrTiO₃(001)表面吸附Au原子和N替代表面层O原子的置换过程二者之间存在明显的"协同效应",即N原子置换SrTiO₃(001)表面层O原子的过程增强了相应表面吸附Au原子的稳定性,而SrTiO关键词： 表面结构 相互作用 第一性原理     

(Ga,Mn)As 体系中Mn自补偿效应的第一性原理研究

基于密度泛函理论的赝势平面波方法计算了处于填隙位置磁性原子(MnI)对(Ga,Mn) As体系电子结构和磁性的影响. 计算结果表明MnI在GaAs中是施主；代替Ga位的MnGa 与MnI的自旋按反铁磁序排列；静电相互作用使MnGa，MnI倾向于形成MnGa_MnI对. MnI的存在一方面补偿了(Ga,Mn)As中的空穴，降低了空穴浓度；同时还使邻近的MnG a失活. MnI的存在对获得高居里温度的(Ga,Mn)As是极为不利的. 关键词： (Ga Mn)As 稀磁半导体 密度泛函理论     

Indirect decoherence in optical lattices and cold gases

The interaction of two–level atoms with a common heat bath leads to an effective interaction between the atoms, such that with time the internal degrees of the atoms become correlated or even entangled. If part of the atoms remain unobserved this creates additional indirect decoherence for the selected atoms, on top of the direct decoherence due to the interaction with the heat bath. I show that indirect decoherence can drastically increase and even dominate the decoherence for sufficiently large times. I investigate indirect decoherence through thermal black body radiation quantitatively for atoms trapped at regular positions in an optical lattice as well as for atoms at random positions in a cold gas, and show how indirect decoherence can be controlled or even suppressed through experimentally accessible parameters.     

激光驻波场作用下原子束横向速度聚束

分别从理论和实验上研究了在负失谐强激光驻波场作用下，原子速度聚集在某一特定速度附近的现象，即速度聚束，产生速度聚束的原因是，当原子的速度小于某一临界速度时，激光对原子的辐射压力加热原子，而当原子速度大于这个临界速度时则冷却原子，因此原子将伙集于这个临界速度附近。     

通过对腔外原子的操作控制腔内原子的发射性质

考虑初始处于纠缠态的三个双能级原子,将其中两个原子分别注入处于真空态的两个腔中,发生共振相互作用。通过对腔外原子进行操作,可使腔内两个原子的发射性质受到很大影响。     

原子在单色辐射作用下长时间内的行为研究

建立了原子在单色辐射作用下的模型，用微扰理论方法求解出原子处于两能级的几率.讨论了原子在单色辐射作用下长时间内的行为，得到了原子因辐射场的影响而变化的速率比能级的衰减速率大的情况下，原子在两能级间振荡，原子和辐射场相互作用越强，辐射场能够与原子发生作用的频率区间越宽,而且原子在单色光长时间辐射作用下高、低能级跃迁的速率不随时间而改变的结论.     

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采用分子动力学模拟方法研究了样品温度对Ar+与SiC样品表面相互作用的影响。由模拟结果可知,SiC样品中Si原子的溅射产额随着温度的升高而增加,而温度对C原子的溅射产额影响不大。在相同温度下,Si原子的溅射产额要高于C原子的溅射产额。溅射出来的Si原子和C原子主要来源于样品的表层区域,样品中的Si和C原子密度、键密度及它们的成键方式也发生了较大的变化。初始样品中Si和C原子的密度是均匀的,而被轰击过后的样品表面Si原子的密度要高于C原子,而样品中部C原子的密度要高于Si原子。初始样品都是Si-C键,成键方式为Si-Csp3;被轰击过后又有Si-Si和C-C键,成键方式也发生了变化,还有Si-Csp1和Si-Csp2。     

测定含重原子晶体结构时出现多解情况的一般规律

在测定含重原子晶体的结构时,往往由于重原子分布的特殊性,使轻原于的位置不能唯一地确定,这种情况不仅常常出现在Patterson分析中,而且也常常出现在重原子法和直接法中,因此它是单晶体结构分析中常常遇到的一个困难问题。作者以前曾经系统地提出过处理这个问题的办法,本文则阐明用各种方法测定含重原子的晶体结构时,轻原子位置出现多解情况的一般规律。 关键词：