[en] Total photodetachment cross section of H^- near a reflecting wall was studied recently by Yang et al using closed-orbit theory (2006 J. Phys. B: At. Mol. Opt. Phys. 39 1855). The main effect of a reflecting wall was shown to induce an oscillation in the cross section. In this paper, we study the photodetachment of H^- near a reflecting wall using a theoretical imaging method, which is very different from the method of closed-orbit theory. The theoretical imaging method enables us to derive the total and differential photodetachment cross sections in a straightforward way. We considered both a soft wall case and a hard wall case, they differ by the values of phase loss when reflecting the detached-electron waves. The result of Yang et al for the total photodetachment cross section is the asymptotic limit of our formula for the soft wall case. We also calculated the detached-electron flux distributions on a screen placed at a large distance from the negative ion. The distributions display strong interference patterns. Such interference patterns are similar to those in the photodetachment microscopy experiments

[en] Theoretical and interpretative study on the subject of photodetachment of H⁻ near a partially reflecting surface is presented, and the absorption effect of the surface is investigated on the total and differential cross sections using a theoretical imaging method. To understand the absorption effect, a reflection parameter K is introduced as a multiplicative factor to the outgoing detached-electron wave of H⁻ propagating towards the wall. The reflection parameter measures, how much electron wave would reflect from the surface; K = 0 corresponds to no reflection and K = 1 corresponds to the total reflection

[en] Highlights: • These alloys are dynamically stable. • These Heusler alloys can be used in fabrication of thermoelectric devices with minimization of thermal loss. • There is no optical band gap exist in these alloys. • The main contribution in phonon density of states in Ru2 FeSi is due to Si atom and in Ru2FeGe is due Ge atom. • Reststrahlen band for Ru2FeSi is found smaller than Ru2FeGe. - Abstract: In present work, the lattice dynamics of Ru₂FeX (X = Si, Ge) full Heusler alloys are investigated using density functional theory (DFT) within generalized gradient approximation (GGA) in a plane wave basis, with norm-conserving pseudopotentials. Phonon dispersion curves and phonon density of states are obtained using first-principles linear response approach of density functional perturbation theory (DFPT) as implemented in Quantum ESPRESSO code. Phonon dispersion curves indicates for both Heusler alloys that there is no imaginary phonon in whole Brillouin zone, confirming dynamical stability of these alloys in L2₁ type structure. There is a considerable overlapping between acoustic and optical phonon modes predicting no phonon band gap exists in dispersion curves of alloys. The same result is shown by phonon density of states curves for both Heusler alloys. Reststrahlen band for Ru₂FeSi is found smaller than Ru₂FeGe.

[en] The electron flux distributions in the photodetachment of HF⁻ near an interface are studied using a two-center model and the theoretical imaging method. An analytical expression for electron flux distributions is derived, which displays oscillations on an observation plane similar to the recent results published by Wang but in the presence of a static electric field. We also discuss the expressions for soft and hard wall cases in detail. A comparison is made with the previous work. The expression is a more general result, and we can deduce from it the electron flux distributions for the photodetachment of H⁻₂ near an interface. Finally, we show that the expression reveals similar results as those in [Chin. Phys. B 19 020306 (2010)] when the wall effect is neglected. (atomic and molecular physics)

[en] The thermodynamic properties of ZnSe are obtained by using quasi-harmonic Debye model embedded in Gibbs-code for pressure range 0–10 GPa and for temperature range 0–1000 K. Helmholtz free energy, internal energy, entropy, Debye temperature, and specific heat are calculated. The thermal expansion coefficient along with Grüneisen parameter are also calculated at room temperature for the pressure range. It is found that internal energy is pressure dependent at low temperature, whereas entropy and Helmholtz free energy are pressure sensitive at high temperature. At ambient conditions, the obtained results are found to be in close agreement to available theoretical and experimental data. (paper)

[en] Photo-detached electron spectra from a hypothetical linear tetra-atomic negative ion is obtained. A plane polarized laser parallel to the axis of the molecular ion is used to knock off the loosely bound electron. The spectrum of the detached-electron flux shows strong interference peaks, while the number of peaks increases with the increase in the photon energy. Strong oscillations are also observed in the total photodetachment cross section spectrum. The frequency of the oscillations increases with the increase in the distance between the successive atoms d in the linear chain. These quantum interference effects vanish for very large d or very high photon energy. (authors)

[en] Highlights: • These compounds are mechanically stable. • These compounds are ductile, ionic and elastic anisotropic in nature. • High-temperature acoustical devices can be fabricated by using these compounds along the [111] longitudinal direction. • Reststrahlen band for LiMgGa is found greater than NaMgGa. -- Abstract: In present study, thermo-elastic and vibrational properties of XMgGa (X = Li, Na) half Heusler compounds are investigated using density functional theory implemented in WIEN2k and Quantum ESPRESSO codes. Generalized gradient approximation as an exchange correlation functional is used in Kohn-Sham equations to find three elastic constants C₁₁, C₁₂ and C₄₄ for C1_b type structures. First, we optimized the structures of these Heusler compounds. These three elastic constants are then used to determine different elastic modulii like bulk modulus, shear modulus, Young modulus and other mechanical parameters like Pugh's ratio, Poisson's ratio, anisotropic ratio, sound velocities, Debye temperature and melting temperature. On behalf of these mechanical parameters, the brittle/ductile nature and isotropic/anisotropic behavior of the materials are investigated. Different regions of vibrational modes in the materials are also discussed on behalf of Debye temperature calculations. The vibrational properties of the half Heusler compounds are computed using Martins-Troullier pseudo potentials implemented in Quantum ESPRESSO. The phonon dispersion curves and phonon density of states in first Brillion zone are obtained and discussed. Reststrahlen band of LiMgGa is found greater than NaMgGa.

[en] Structural, elastic and mechanical properties of CuCoMnZ (Z=Si, Sn, Sb) Equiatomic quaternary Heusler alloys are studied by Density Functional Theory (DFT) with Predew, Burke and Ernzerhof Generalized Gradient Approximation (PBE-GGA). On the basis of the atomic positions there are three types of structures of these alloys named as Y-type I, Y-type II and Y-type III. Structural optimization of these alloys confirms that Y-type III is the most stable structure than other two possible structures. Three cubic elastic constants C ₁₁, C ₁₂ and C ₄₄ are obtained by using self-consistent-field calculations. Elastic moduli like bulk modulus, shear modulus, Young’s modulus and other mechanical parameters such as Pugh’s ratio, Poisson’s ratio, anisotropic ratio, Cauchy pressure, velocity of sound and Debye temperature are then calculated using these elastic constants to determine the mechanical behavior of the alloys. It is noticed that all three Heusler alloys are physically stable materials and they are ionic in nature. (paper)

[en] Thermodynamic properties of Cd_0.25Zn_0.75Se alloy are studied using quasi harmonic model for pressure range of 0 GPa−10 GPa and temperature range 0 K−1000 K. The structural optimization is obtained by self-consistent field calculations and full-potential linearized muffin-tin orbital method with GGA+U as an exchange correlation functional where U = 2.3427 eV is Hubbard potential. The effects of temperature and pressure on bulk modulus, Helmholtz free energy, internal energy, entropy, Debye temperature, Grüneisen parameter, thermal expansion coefficient, and heat capacities of the material are observed and discussed. The bulk modulus, Helmholtz free energy, and Debye temperature are found to be decreased on increasing temperature while there is an increasing behavior with rise of the pressure. Whereas the internal energy has increasing trend with the rise in temperature and it almost remains insensitive to pressure. The entropy of the system increases (decreases) with rise of pressure (temperature). (paper)

[en] Thermodynamic properties of $\text{C}{\text{d}}_{0.50}$ $\text{Z}{\text{n}}_{0.50}$Se alloy are studied using quasi harmonic model for pressure range 0–10 GPa and temperature range 0–1000 K. The structural optimization is obtained by self consistent field calculations and full-potential linear muffin-tin orbital method with GGA+U as an exchange correlation functional where $U=2.3427$ eV is the hubbard potential. The effects of temperature and pressure on the bulk modulus, Helmholtz free energy, internal energy, entropy, Debye temperature, Grüneisen parameter, thermal expansion coefficient and heat capacities of the material are observed and discussed. The bulk modulus, Helmholtz free energy and Debye temperature are found to decrease with increasing temperature while there is an increasing behavior when the pressure rises. Whereas internal energy has increasing trend with rises in temperature and it almost remains insensitive to pressure. The entropy of the system increases (decreases) with a rise of pressure (temperature). (paper)