Theoretical investigation of Mössbauer hyperfine interactions in ordered FeNi and disordered Fe–Ni alloys

Electronic structure spin-polarized calculations were performed for 79-atoms embedded clusters representing the ordered intermetallic compound FeNi, the fcc Fe-rich disordered alloy Fe₈₅Ni₁₅ in an antiferromagnetic (AFM) configuration, and the ferromagnetic (FM) disordered alloy Fe₅₀Ni₅₀. The spin-polarized discrete variational method (DVM) in Density Functional theory was employed. Spin magnetic moments, as well as the ⁵⁷Fe Mössbauer hyperfine parameters isomer shift and magnetic hyperfine fields, were obtained from the calculations. For FM Fe₅₀Ni₅₀, the effect of pressure on the hyperfine field and on the isomer shift was investigated, for three different local atomic configurations surrounding the ⁵⁷Fe probe atom. In the case of the isomer shift, the calculated values were compared to reported experimental data.     

The dynamical properties of Rydberg hydrogen atom near a metal surface

The dynamical properties of Rydberg hydrogen atom near a metal surface are presented by using the methods of phase space analysis and closed orbit theory. Transforming the coordinates of the Hamiltonian, we find that the phase space of the system is divided into vibrational and rotational region. Both the Poincaré surface of section and the closed orbit theory verify the same conclusion clearly. In this paper we choose the atomic principal quantum number asn=20. The dynamical character of the exited hydrogen atom depends sensitively on the atom-surface distanced. Whend is sufficiently large, the atom-surface potential can be expressed by the traditional van der Waals force and the system is integrable. Whend becomes smaller, there exists a critical valued _c. Ford>d _c, the system is near-integrable and the motion is regular. While chaotic motion appears ford<d _c, and the system tends to be non-integrable. The trajectories become unstable and the electron might be captured onto the metal surface.     

Mössbauer studies of mixed-valence spin-crossover iron complexes with a hexadentate tripod ligand

The spin-crossover behaviors of mixed-valence iron compounds [Fe^IIH₃L][Fe^IIIL](NO₃)₂ (1) and [Fe^IIH₃L^Me][Fe^IIIL^Me](NO₃)₂ (2) have been investigated by ⁵⁷Fe Mössbauer spectroscopy, where H₃L is a hexadentate N₆ tripod ligand containing three imidazole groups and H₃L^Me is its 2-methylimidazole derivative. Deconvolution analyses of the Mössbauer spectra revealed that a two-step SCO (LS Fe^II–LS Fe^III→HS Fe^II–LS Fe^III→HS Fe^II–HS Fe^III) proceeds in each compound on elevating the temperature. Compound 2 exhibited lower spin-transition temperatures than 1. “Frozen-in effect” was observed below 120 and 50 K for 1 and 2, respectively.     

Phase transition and optoelectronic properties of MgH2

In this article, structural and electronic properties of MgH₂ have been studied. The aim behind this study was to find out the ground state crystal structure of MgH₂. For the purpose, density functional theory (DFT)-based full-potential linearized augmented plane wave (FP-LAPW) calculations have been performed in three different space groups: P4₂/mnm (α-MgH₂), Pa3 (β-MgH₂) and Pbcn (γ-MgH₂). It has been found that the ground state structure of MgH₂ is α-MgH₂. The present study shows that α-MgH₂ transforms into γ-MgH₂ at a pressure of 0.41 GPa. After further increase in pressure, γ-MgH₂ transforms into β-MgH₂ at a pressure of 3.67 GPa. The obtained results are in good agreement with previously reported experimental data. In all the studied phases, the behavior of MgH₂ is insulating and its optical conductivity is around 6.0 eV. The α-MgH₂ and γ-MgH₂ are anisotropic materials while β-MgH₂ is isotropic in nature.     

氮化硼纳米管阵列储氢的计算机模拟

采用巨正则蒙特卡罗方法模拟常温、中等压强下单壁氮化硼纳米管阵列的物理吸附储氢,重点研究压强、纳米管阵列的管径和管间距对单壁氮化硼纳米管阵列物理吸附储氢的影响.计算结果表明,氮化硼纳米管阵列的储氢性能明显优于碳纳米管阵列,在常温和中等压强下的物理吸附储氢量(质量百分数)可以达到和超过美国能源部提出的商业标准.并给出相应的理论解释.     

Stabilizing the intensity of a wave amplified by a beam of particles

The intensity of an electromagnetic wave interacting self-consistently with a beam of charged particles as in a free electron laser, displays large oscillations due to an aggregate of particles, called the macro-particle. In this article, we propose a strategy to stabilize the intensity by re-shaping the macro-particle. This strategy involves the study of the linear stability (using the residue method) of selected periodic orbits of a mean-field model. As parameters of an additional perturbation are varied, bifurcations occur in the system which have drastic effect on the modification of the self-consistent dynamics, and in particular, of the macro-particle. We show how to obtain an appropriate tuning of the parameters which is able to strongly decrease the oscillations of the intensity without reducing its mean-value.     

TDDFT study on the excited‐state hydrogen bonding of N‐(2‐hydroxyethyl)‐1,8‐naphthalimide and N‐(3‐hydroxyethyl)‐1,8‐naphthalimide in methanol solution

The time‐dependent density functional theory method was performed to investigate the excited‐state hydrogen‐bonding dynamics of N‐(2‐hydroxyethyl)‐1,8‐naphthalimide (2a) and N‐(3‐hydroxyethyl)‐1,8‐naphthalimide (3a) in methanol (meoh) solution. The ground and excited‐state geometry optimizations, electronic excitation energies, and corresponding oscillation strengths of the low‐lying electronically excited states for the complexes 2a + 2meoh and 3a + 2meoh as well as their monomers 2a and 3a were calculated by density functional theory and time‐dependent density functional theory methods, respectively. We demonstrated that the three intermolecular hydrogen bonds of 2a + 2meoh and 3a + 2meoh are strengthened after excitation to the S₁ state, and thus induce electronic spectral redshift. Moreover, the electronic excitation energies of the hydrogen‐bonded complexes in S₁ state are correspondingly decreased compared with those of their corresponding monomer 2a and 3a. In addition, the intramolecular charge transfer of the S₁ state for complexes 2a + 2meoh and 3a + 2meoh were theoretically investigated by analysis of molecular orbital. Copyright © 2014 John Wiley & Sons, Ltd.     

Local structure of amorphous tellurium studied by EXAFS

The local structure of an amorphous tellurium (a‐Te) film was investigated by extended X‐ray absorption fine‐structure analysis. The covalent bond length shortens and the Debye–Waller factor of the intra‐chain decreases in a‐Te compared with trigonal Te (t‐Te). The value of the intra‐chain coordination number is close to two, which is the value for t‐Te, and the inter‐chain interaction weakens. These results suggest that the primary chain structure remains intact, but the secondary structure is disrupted. The decrease in the inter‐chain interaction strengthens the intra‐chain interaction.     

Small amplitude ion-acoustic double layers with cold electron beam and q-nonextensive electrons

Small amplitude ion-acoustic double layers in an unmagnetized and collisionless plasma consisting of cold positive ions, q-nonextensive electrons, and a cold electron beam are investigated. Small amplitude double layer solution is obtained by expanding the Sagdeev potential truncated method. The effects of entropic index q, speed and density of cold electron beam on double layer structures are discussed.     

Post collision interactions and polarization effect in (e, 2e) collisions of helium

A modified distorted-wave Born approximation (DWBA) method is used to calculate the triple differential cross sections (TDCSs) in a coplanar asymmetric geometry for the electron impact single ionization of a He (1s²) atom at intermediate and lower energies. The post-collision interaction and the polarization effect in (e, 2e) collisions of helium are considered in the calculations. The polarization potentials from the damping method and density functional theory (DFT) are compared. Theoretical results are compared with the recent experimental data.