Interplay between magic number stabilities and superfluidity of small parahydrogen clusters

The interplay between magic number stabilities and superfluidity of small parahydrogen clusters with sizes N=5 to 40 and temperatures 0.5 K相似文献   

Lipkin-Meshkov-Glick模型中的能级劈裂与宇称振荡研究

Lipkin-Meshkov-Glick (LMG)模型原本描述的是核物理系统,然而近年来,人们发现它广泛存在于凝聚态物理、量子信息、量子光学中,因此对其研究兴趣正在升温.本文采用精确对角化的方法以及量子微扰理论计算和分析了LMG模型在费米子数量为有限N时的能谱结构.在U(1)极限下给出它的能级精确解,发现其相互交错成渔网结构.而离开U(1)极限,系统的能级总是奇偶宇称成对地分组,形成束缚态,并且宇称会发生振荡,给出了宇称交叉点的临界塞曼场的位置.而达到Z2极限,系统能级则在零塞曼场附近形成劈裂,解析地计算了这些能隙与塞曼场之间关系,并发现对于奇数和偶数的N,各能态宇称的行为有所差别,具体而言,奇数N系统各态在零塞曼场处会发生宇称改变,而偶数N不会.     

Diffraction of neutral helium clusters: evidence for "magic numbers"

The size distributions of neutral 4He clusters in cryogenic jet beams, analyzed by diffraction from a 100 nm period transmission grating, reveal magic numbers at N=10-11, 14, 22, 26-27, and 44 atoms. Whereas magic numbers in nuclei and clusters are attributed to enhanced stabilities, this is not expected for quantum fluid He clusters on the basis of numerous calculations. These magic numbers occur at threshold sizes for which the quantized excitations calculated with the diffusion Monte Carlo method are stabilized, thereby providing the first experimental confirmation for the energy levels of 4He clusters.     

The Hubbard model extended by nearest‐neighbor Coulomb and exchange interaction on a cubic cluster – rigorous and exact results

The Hubbard model on a cube was revisited and extended by both nearest‐neighbor Coulomb correlation W and nearest‐neighbor Heisenberg exchange J. The complete eigensystem was computed exactly for all electron occupancies and all model parameters ranging from minus infinity to plus infinity. For two electrons on the cluster the eigensystem is given in analytical form. For six electrons and infinite on‐site correlation U we determinded the groundstate and the groundstate energy of the pure Hubbard model analytically. For fixed electron numbers we found a multitude of ground state level crossings depending on the various model parameters. Furthermore the groundstates of the pure Hubbard model in dependence on a magnetic field h coupled to the spins are shown for the complete U‐h plane. The critical magnetic field, where the zero spin groundstate breaks down is given for four and six electrons. Suprisingly we found parameter regions, where the ground state spin does not depend monotonously on J in the extended model. For the cubic cluster gas, i.e. an ensemble of clusters coupled to an electron bath, we calculated the density n (μ, T, h) and the thermodynamical density of states from the grand potential. The ground states and the various spin‐spin correlation functions are studied for both attractive and repulsive values of the three interaction constants. We determined the various anomalous degeneration lines, where n (μ, T = 0, h = 0) shows steps higher than one, since in this parameter regions exotic phenomena as phase separation are to expect in extended models. For the cases where these lines end in triple points, i.e. groundstates of three different occupation numbers are degenerated, we give the related parameter values. Regarding the influence of the nn‐exchange and the nn‐Coulomb correlation onto the anomalous degeneration we find both lifting and inducing of degeneracies depending on the parameter values.     

Stability and magic numbers of hetero-atomic clusters of simple metals

The main trends in the magic numbers observed for a class of simple-metal hetero-atomic clusters M_xN (M = Na or K; N = monovalent or divalent impurity; X = number of host atoms) is explained using an extension of the jellium model of clusters. The appearance of a “new” magic number, n = 10 (n = number of valence electrons), and the growing importance of n = 20 over n = 18, as Δ₊ increases (Δ₊ is the difference in the density of the jellium backgrounds of the two metals) are explained by an inversion of the order of the energy levels with respect to that of pure clusters.     

原子团簇的稳定结构和幻数

原子团簇介于原子分子和宏观凝聚物质之间 ,一般产生于非平衡条件 ,其结构和性质随所含原子数目而变化。当含有某些特定原子数时 ,团簇特别稳定 ,这就是“幻数”。本文重点讨论几种典型团簇的幻数及其演变规律 ,说明“幻数”是团簇的基本属性之一及其与键合方式的关系     

The eigenspectrum of the relativistic two-center Hamiltonian in the diatomic basis of bound hydrogenlike Dirac-spinors

During heavy ion collisions below the Coulomb barrier adiabatic molecular orbitals are formed for the inner electrons. For systems with total changes exceending Z=Z₁ + Z₂ 60 relativistic effects are important and we, therefore, use the two-center Dirac Hamiltonian to describe the dynamics. We investigate the method to approximate these molecular orbitals by an expansion in a di-atomic basis of Dirac wave functions and determine the binding energies by a variation of the expansion coefficients. Deviations from adiabaticity are taken into account to first order in the relative velocityv/c 0.1) of the nuclei which is considerably smaller than the electron velocity.     

Alpha Decay,Shell Structure,and New Elements

We systematically analyze the experimental data of alpha decay in even-even heavy nuclei far from stability and find that the Geiger-Nuttall law brea~s for an isotopic chain when its neutron number is across a marc number or there is a deformed subshell. This break can be used to identify new magic numbers of superheavy nuclei. It is also discovered that there is a new linear relation between the logarithm of half-life and the reciprocal of the square root of decay energy for N = 126 and N = 152 isotones. It could be a new law of alpha decay for nuclei with magic neutron numbers but the physics behind it is to be explored. The significance of these researches for the search of new elements is discussed.     

Magic numbers in exotic nuclei and spin-isospin properties of the NN interaction

The magic numbers in exotic nuclei are discussed, and their novel origin is shown to be the spin-isospin dependent part of the nucleon-nucleon interaction in nuclei. The importance and robustness of this mechanism is shown in terms of meson exchange, G-matrix, and QCD theories. In neutron-rich exotic nuclei, magic numbers such as N = 8, 20, etc. can disappear, while N = 6, 16, etc. arise, affecting the structure of the lightest exotic nuclei to nucleosynthesis of heavy elements.     

Representation of the quantum Fourier transform on multilevel basic elements by a sequence of selective rotation operators

To experimentally realize quantum computations on d-level basic elements (qudits) at d > 2, it is necessary to develop schemes for the technical realization of elementary logical operators. We have found sequences of selective rotation operators that represent the operators of the quantum Fourier transform (Walsh-Hadamard matrices) for d = 3–10. For the prime numbers 3, 5, and 7, the well-known method of linear algebra is applied, whereas, for the factorable numbers 6, 9, and 10, the representation of virtual spins is used (which we previously applied for d = 4, 8). Selective rotations can be realized, for example, by means of pulses of an RF magnetic field for systems of quadrupole nuclei or laser pulses for atoms and ions in traps.     

OsnN0,±(n=1—6)团簇几何结构与稳定性的理论研究

采用密度泛函理论(DFT)中的B3LYP方法,在Lanl2dz基组水平上对(Os_nN)^0,±(n=1—6)团簇的各种可能构型进行了几何参数全优化,得到了它们的基态构型;并对基态构型的平均结合能(E_b) 、二阶能量差分(Δ₂E_n) 、解离能(E_d)和能隙(E_g)进行了理论研 关键词： nN^{0')" href="#">Os_nN0 (n=1—6)团簇')" href="#">±(n=1—6)团簇 几何结构 稳定性 密度泛函理论}     

黄酮类化合物的原子电距矢量表达及核磁共振碳谱

提出以原子电性距离矢量(VAED)描述19种黄酮醇和21种黄酮中不同等价碳原子的化学微环境，结合碳原子类型，建立核磁共振碳谱(¹³C NMR)化学位移(CS)的四 参数线性模型, 对于19种黄酮醇285个碳原子其回归方程：Y(1)=72.097+20.803 X(1)+14.395 X(2)+ 12.030 X(3)；对于21种黄酮315个碳原子其回归方程：Y(1)=85.530+19.174 X(1)+15.565 X (2)+5.495 X(3)；黄酮醇和黄酮一起建模的回归方程为：Y(1)=77.679+20.373 X(1)+15.481 X(2)+9.004 X(3). 用于黄酮醇和黄酮分子等价碳原子化学位移的估计. 样本数目，相关系数，标准偏差，F统计值，回归平方和和残差平方和分别为：N=285，R=0.796，SD=14.734，F=162.315，U=105 705.359，Q=60 999.156 ；N=31 5，R=0.884，SD=12.381，F=369.114，U=169 745.141，Q=47 673.375和N=600，R=0.831，SD =14.122， F=443.449，U=265 310.563，Q=118 859.906. 经交互校验，模型稳定性较好. 并综合几种处理方法，找到一种较好的建模方法，作为初次有益的探讨，对黄酮和黄酮醇核磁共振碳谱建模效果尚好.     

A transition in the spectral statistics of quantum optical model by different electromagnetic fields

In this paper, we have considered the effects of different quantized electromagnetic fields on the spectral statistics of two-level atoms. The Berry-Robnik distribution and the maximum likelihood estimation technique are used to analyze the effect of the mean photon numbers, the two level atoms numbers and also the quantum number of considered states on the fluctuation properties of different systems which are described by different sets of the Dicke Hamiltonian’s parameters. Our results describe the obvious effect of mean photon number on the spectral statistics and show more regular dynamics when this quantity reaches 700. Also, we observed universality in the spectral statistics of considered systems when the number of two level atoms approaches an unrealistic limit (N _A ~ 200) and there are some suggestions about the effect of the quantum number of selected levels and the atom-field coupling constant on level statistics.     

Self-organization of ultra-thin uranium film

We have studied the adsorption behaviors of uranium (U) atoms coated graphene at different coverage ratios using first-principles calculations. Isolated U atom is demonstrated to be more likely to stay on the hollow site of graphene with a large adsorption energy of 2.80 eV and high magnetic moment of 5.07 ${\mu }_{\text{B}}$. It seems that two U atoms tend to adsorb on the nearest neighbor hollow sites of graphene. As the concentration of U atom increases, no U-dimer or U-cluster appears with increasing coverage ratio. In the case of coverage ratio of 2/3, U atoms on graphene is the most stable configuration, where each U atoms are uniformly-distributed in U network with honeycomb lattice. The adsorption energy is as large as 2.57 eV per U atom. Moreover, the spin–orbit coupling effect on electronic band structures is outstanding, which induces the degeneracy bands splitting. Our calculations will provide profound background to understand the adsorption behaviors of U atoms on graphene.     

三维量子点的三电子系统

由量子力学对称性研究了三维量子点三电子系统的结构，指出了存在“幻数”的可能性，并与二维量子点三电子系统的“幻数”特征作了比较．在有效质量近似下，计算了抛物阱（?ω₀=2meV）三电子系统的能谱．结果表明在三维三电子量子点中存在“幻数”的迹象，但很少．理论分析和计算结果都表明三维量子点三电子系统的基态是1^-态 关键词：     

原子核基态性质的系统研究

在Skyrme-Hartree-Fock-Bogoliubov（SHFB）理论框架下,利用SkOP1,SkOP2,SKC和SKD 4套新的Skyrme相互作用参数系统地研究了Ca,Ni,Sn和Pb同位素链上原子核的结合能、电荷半径等基态性质,并重点讨论了丰中子Ca核的新中子幻数以及Pb的同位素位移现象。通过与实验数据和SLy5相互作用参数的结果对比,发现这4套相互作用参数都能很好地再现结合能的实验数据,其预言精度比SLy5要高。对于丰中子Ca核,只有SKC和SKD相互作用参数能够再现N=28处的壳效应,而对于实验上发现的新幻数N=32和34,所有的相互作用参数均不能再现这一结果。对于电荷半径,发现所有的相互作用参数均不能很好地预言Ca同位素链电荷半径的演化规律以及Pb的同位素位移现象。另外,还将这些相互作用参数推广至远离β稳定线原子核的单粒子能级结构研究,发现其不适用于描述其随同位旋的演化行为。因此,为了更好地描述远离β稳定线原子核的宏观性质及单粒子能级,建议在拟合Skyrme相互作用参数时,除自旋-轨道耦合项包括合理的同位旋依赖外,还要考虑张量力成分。The nuclear ground state properties of Ca, Ni, Sn and Pb isotopes, such as the binding energies, the charge radii, are studied systematically by 4 sets of new Skyrme parametrizations SKC, SKD, SkOP1 and SkOP2 in the framework of the Skyrme-Hartree-Fock-Bogoliubov (SHFB) method. The new magic numbers of neutronrich Ca isotopes and the isotopic shift of Pb isotopes are discussed emphatically. By the comparisons between the calculations and the experimental data and results from the SLy5 interaction parametrization, it is found that the experimental binding energies can be reproduced accurately by all parametrizations. The calculated accuracies of SKC, SKD SkOP1 and SkOP2 parametrizations are higher than the ones of SLy5 parametrization. For the neutron-rich Ca nuclei, the shell effect of N=28 can be reproduced by the SKC and SKD parametrizations, but the magic numbers at N=32 and 34 are not found by the calculations of all the parametrizations. For the charge radii, the experimental evolution tendency of Ca isotopes and isotopic shift of Pb isotopes can not be reproduced by all the parametrizations. In addition, all Skyrme parametrizations are extended to study the structure of the nuclei far from the β stability line, it is shown that the single-particle energy evolutions with the isospin are not suitable for being studied by these parametrizations. Thus the tensor force component should be considered besides the isospin dependence in spin-orbit coupling term when the Skyrme interaction parametrizations are fitted.     

The impact of dense plasma environments on the 1s3l fine structure levels of He-like ions

Finite-temperature dense plasma relativistic atomic structure calculations are employed to study the energy eigenvalues and transition probabilities related to the 1s3l fine structure levels of He-like ions. We discovered a particular response of the level structure that is distinctly different from simple nuclear charge screening effects that exists also for atoms and ions in vacuum as Z decreases. It is demonstrated that characteristic level crossings are driven by the free electron screening due to a particular dependence on the quantum numbers of each level of the 1s3l-configuration. Levels with large l shift stronger to the continuum than levels with small l whereas spin dependent dense plasma effects are relatively small. As a result, the known energy level fine structure in vacuum is subject to characteristic changes (level crossings, level order). Numerical calculations performed for a wide range of density, temperature and different chemical elements indicate that induced level crossings and change in level order are general characteristics of dense plasma effects.     

Magic numbers in small clusters made up of two kinds of alkali atoms

The magic numbers in small clusters made up of two kinds of alkali atoms are theoretically studied and new experiments are proposed. These clusters exhibit two series of almost identical magic numbers, one for each kind of atoms, which are further identical to those for protons and neutrons in nuclear physics (i.e.,2,8,20,50,82).     

Shell evolution in neutron-rich nuclei: the single particle perspective

The isospin dependence of spin-orbit(SO)splitting becomes increasingly important as N/Z increases in neutron-rich nuclei.Following the initial independent-particle strategy toward explaining the occurrence of magic numbers,we systematically investigated the isospin effect on the shell evolution in neutron-rich nuclei within the Woods-Saxon mean-field potential and the SO term.It is found that new magic numbers N=14 and N=16 may emerge in neutron-rich nuclei if one changes the sign of the isospin-dependent term in the SO coupling,whereas the traditional magic number,N=20,may disappear.The magic number N=28 is expected to be destroyed despite the sign choice of the isospin part in the SO splitting,corresponding to the strength of the SO coupling term.Meanwhile,the N=50 and 82 shells may persist within the single particle scheme,although there is a decreasing trend of their gaps toward extreme proton-deficient nuclei.Besides,an appreciable energy gap appears at N=32 and 34 in neutron-rich Ca isotopes.All these results are more consistent with those of the interacting shell model when enhancing the strength of the SO potential in the independent particle model.The present study may provide a more reasonable starting point than the existing one for not only the interacting shell model but also other nuclear many-body calculations toward the neutron-dripline of the Segrèchart.     

Structural and electronic properties of metal-encapsulated silicon clusters in a large size range

Structural and electronic properties of metal-doped silicon clusters MSi(n)s (M=W, Zr, Os, Pt, Co, etc.) in a large size range of 8相似文献