Atomistic simulation of the vacancy diffusion in (0 0 1) surface of MoTa alloy

The formation and diffusion of a single Mo or Ta vacancy in the (0 0 1) surface of the B₂-type MoTa alloy have been investigated by using modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface on the vacancy is only down to the sixth layer. It is easier for the vacancy to form in the first layer. Comparing the migration energy of the vacancy migrating in the intra-layer, to the upper layer and to the nether layer via 2NN jump, we find that the vacancy in the first or second layer is preferred to migrate in intra-layer, and that in the third or fourth layer is favorable to migrate to the upper layer. Although 1NN jump will result in an anti-site so that a disorder in the order alloy, it may also occur due to the much lower migration energy especially for the vacancy in the second and third layer to migrate to the first and second layer, respectively.     

Diffusion of single adatom Cu on Cu (0 0 1) and (1 1 0) surfaces

With static relaxation, the surface diffusion activation energies of a single Cu adatom migrated by both atomic exchange and hopping mechanisms and the forces acted on the diffusing adatom from other atoms of Cu (0 0 1) or (1 1 0) surface are calculated by using the MAEAM. When adatom migrated on Cu (0 0 1) or (1 1 0) surface, the increment curves of the system energy by hopping mechanism are symmetrical and the saddle points are in the midpoints of the migration path, but the ones by the exchange mechanism are dissymmetrical and the saddle points are always close to the initial hole positions of the adatom and away from the initial equilibrium positions of the exchanged atom. From minimization of both the diffusion activation energy and the force acted on the diffusing adatom from other atoms, we found that, on Cu (0 0 1) surface the favorable diffusion mechanism is hopping mechanism, however, on Cu (1 1 0) surface, hopping via long bridge is easier than the exchange mechanism but the hopping via short bridge is more difficult than the exchange mechanism.     

Atomistic simulation of the vacancy in Ni (1 1 0) surface

Both the formation energies and the intra- and inter-layer diffuse activation energies of a vacancy in the first six atomic planes of Ni (1 1 0) surface have been investigated by means of molecular dynamics (MD) in conjunction with the semi-experiential many-body potential of the modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface is only down to the fifth-layer. It is easer for a vacancy in the first or second layer to form and to migrate in intra-layer. For the inter-layer migration, a vacancy in the second or third layer is favorable to migrate to the upper layer, this is not the case for a vacancy in the fourth or fifth layer.     

Atomistic simulation of the self-diffusion in Fe (1 1 1) surface

The formation energies, the intra- and inter-layer self-diffusion activation energies of a single vacancy in Fe (1 1 1) surface have been investigated with the modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface is down to the sixth layer for the formation and intra-layer migration of the vacancy. It is easier for a vacancy to form and to migrate in intra-layer in the first (especially), the second and the third layer. For inter-layer migration, a single vacancy in each of the first six layers is favorable to migrate to the upper layers. On the contrary, a single vacancy in the seventh, the eighth and the ninth layers is favorable to migrate to the lower layers.     

Co-energy of surface and grain boundary in Ag film

A model is proposed for calculating the co-energy of surface and grain boundary (GB) by the modified analytical embedded atom method (MAEAM). As example, the energy densities E_d of the two adjacent grains are obtained when their (0 0 1) twist GB meets the free surface (h₁ k₁ 0)/(h₂ k₂ 0) of Ag film. The period along the boundary line on the surface is found and the energy density is calculated for the situations either with or without periodicity. The results show that, the energy value achieved via this model can be stable even for most grains with boundary line smaller than 100 nm. Among the grains with (h k 0) surface, (1 0 0) grains should be favored and grow fastest when they meet (1 1 0) grains.     

Quantum interferences from cross talk in J = 1/2 ↔ J = 1/2 transitions

We consider the possibility of a control field opening up multiple pathways and thereby leading to new interference and coherence effects. We illustrate the idea by considering the J = 1/2 ↔ J = 1/2 transition. As a result of the additional pathways, we show the possibilities of nonzero refractive index without absorption and gain without inversion. We explain these results in terms of the coherence produced by the opening of an extra pathway.     

Energy calculation of (0 1 1) twist grain boundary in noble metals

With modified analytical embedded atom method (MAEAM), the energy of (0 1 1) twist grain boundary (GB) has been calculated for three noble metals Cu, Ag and Au. The results show that the unrelaxed energy keeps almost constant with twist angle θ except several cusps at low Σ boundaries. The GB energies drop significantly after expansion perpendicular to the boundary. In-boundary translation results in a periodic energy variation and the rectangular period is 1/Σ of their own CSL smallest unit cell. Three specific positions, the corners or centre of the periodic rectangle, or the midpoints of the sides, are preferable in GB translation.     

Structural properties and diffusion processes of the Cu3Au (0 0 1) surface

The surface relaxation and surface energy of both the mixed AuCu and pure Cu terminated Cu₃Au (0 0 1) surfaces are simulated and calculated by using the modified analytical embedded-atom method. We find that the mixed AuCu termination is energetically preferred over the pure Cu termination thereby the mono-vacancy diffusion is also investigated in the topmost few layers of the mixed AuCu terminated Cu₃Au (0 0 1) surface. In the mixed AuCu terminated surface the relaxed Au atoms are raised above Cu atoms for 0.13 Å in the topmost layer. All the surface atoms displace outwards, this effect occurs in the first three layers and changes the first two inter-layer spacing. For mono-vacancy migration in the first layer, the migration energies of Au and Cu mono-vacancy via two-type in-plane displace: the nearest neighbor jump (NNJ) and the second nearest neighbor jump (2NNJ), are calculated and the results show that the NNJ requires a much lower energy than 2NNJ. For the evolution of the energy requirements for successive nearest neighbor jumps (SNNJ) along three different paths: circularity, zigzag and beeline, we find that the circularity path is preferred over the other two paths due to its minimum energy barriers and final energies. In the second layer, the NN jumps in intra- and inter-layer of the Cu mono-vacancy are investigated. The calculated energy barriers and final energies show that the vacancy prefer jump up to a proximate Cu site. This replacement between the Cu vacancy in the second layer and Cu atom in the first layer is remunerative for the Au atoms enrichment in the topmost layer.     

The periodicity in translation of Ag (0 0 1) and (1 1 0) twist grain boundary

The energies of Ag (0 0 1) and (1 1 0) twist grain boundary (GB) in translation have been calculated with the modified analytical embedded atom method (MAEAM). The energy period corresponds exactly to the DSC lattice unit cell and the area of the energy period referred to the CSL unit cell is 1/Σ². The ‘energy grooves’ are parallel to the sides of the CSL or DSC lattice unit cell. The most preferable sliding direction is parallel to identical sides of the square CSL unit cell for (0 0 1) boundaries and to the short side of the rectangular CSL unit cell for (1 1 0) boundaries. From energy minimization, the stable configuration appears when two adjacent grains are translated relatively to the corners, centre or sides’ midpoint of the DSC lattice unit cell.     

The energy and structure of (0 0 1) twist grain boundary in noble metals

The relaxed energy and structure of (0 0 1) twist grain boundary (GB) in noble metals Au, Ag and Cu are simulated by the MAEAM. In-boundary translation between two adjacent grains results in a periodic energy variation and the period is a square with the side length L_Σ/Σ. The lowest energy appears when the two grains are translated relatively to either corner or center of the periodic square. The relaxed GB energy increases smoothly for low-angle boundaries and levels off for larger-angle boundaries except a cusp appeared at θ = 36.87° (Σ = 5). After relaxation, the symmetry of the GB structure is not changed but the displacement of the atoms parallel to the GB plane decreases with increasing the distance of the atoms from the GB plane.     

Lineshape analysis of the J = 3 ← 2 and J = 5 ← 4 rotational transitions of room temperature CO broadened by N2, O2, CO2 and noble gases

Collisional relaxation has been considered for millimeter lines of carbon monoxide at room temperature. Accurate measurements of carbon dioxide- and rare gases-broadened widths have been performed on the J = 3 ← 2 rotational line of ¹²CO by using a video-type spectrometer. Measurements of nitrogen-, oxygen-, and xenon-broadened widths of the J = 5 ← 4 rotational line of ¹³CO were also carried by using a frequency-modulated spectrometer. A lineshape study performed on all the investigated binary systems provide confirmation that Voigt profile is not a suitable model to analyse experimental lines in the millimeter-waves region. On one hand, using this profile in the low pressure range, i.e. in the Doppler regime, the retrieved collisional linewidths do not follow a linear variation with the perturbing gas pressure. On the other hand, regardless of the pressure, lineshapes exhibit a narrowed profile. An accurate analysis of the pressure dependence of relaxation rates show that the Galatry profile is not appropriate and that experimental lineshapes are actually Speed Dependent Voigt profiles. Accurate broadening parameters were retrieved from this profile and compared to previous reported values and predictions calculated from the Robert-Bonamy formalism. Finally a variation of the ratio of relaxation speed dependence to broadening parameters versus relative masses of the collision partners is presented.     

Probing the diffusion of vacuum ultraviolet (λ = 172 nm) induced oxidants by nanoparticles immobilization

Vacuum ultraviolet (VUV, λ = 172 nm) patterning of alkyl monolayer on silicon surface has been demonstrated with emphasis on the diffusion of VUV induced oxygen-derived active species, which are accountable for the pattern broadening. The VUV photons photo-dissociates the atmospheric oxygen and water molecules into the oxygen-derived active species (oxidants). These oxidants photo-oxidize the hexadecyl (HD) monolayer in VUV irradiated regions (Khatri et al., Langmuir. 24 (2008) 12077), as well as the little concentration of oxidants diffuses towards the masked areas. In this study, we performed VUV patterning at a vacuum pressure of 10 Pa to track the diffusion pathways for the oxidants with help of gold nanoparticles (AuNPs; ? = 10 nm) immobilization. At VUV irradiated sites AuNPs are found as uniformly distributed, but adjacent to the pattern boundary we observed quasi-linear arrays of AuNPs, which are determined by diffusion pathways of the oxidants. The diffusion of oxidants plays vital role in pattern broadening. The site selective anchoring of AuNPs demonstrates the utility of VUV photons for the construction of functional materials with microstructural architecture.     

High-resolution infrared and subterahertz spectroscopy of the v2 = 1, v5 = 1, and v3 = 2 levels of CH3Cl

High-resolution Fourier-transform infrared spectra between 1235 and 1680 cm⁻¹ and subterahertz spectra between 250 and 630 GHz of monoisotopic ¹³CH₃³⁵Cl have been recorded and analyzed simultaneously, with all Coriolis, α-resonance, and l-type interactions in the polyad of the v₂ = 1, v₅ = 1, and v₃ = 2 levels taken into account. Several α-resonances (Δk = ±2, Δl = ?1) generating perturbation-allowed transitions have been assigned in the rovibrational spectra. These resonances enabled us to determine accurately and independently the ground state rotational and centrifugal distortion parameters A₀ = 5.205 746 9 (55) cm⁻¹ and . Even , which is, however, correlated to higher-order α-resonance terms, was determined. With 51 upper state parameters varied, about 5800 rovibrational wavenumbers and more than 550 rotational frequencies pertaining to the excited vibrational states were fitted within their experimental accuracy.     

First-principles calculations of C diffusion through the surface and subsurface of Ag/Ni(1 0 0) and reconstructed Ag/Ni(1 0 0)

Density functional theory calculations are performed to investigate the C diffusion through the surface and subsurface of Ag/Ni(1 0 0) and reconstructed Ag/Ni(1 0 0). The calculated geometric parameters indicate the center of doped Ag is located above the Ni(1 0 0) surface owing to the size mismatch. The C binding on the alloy surface is substantially weakened, arising from the less attractive interaction between C and Ag atoms, while in the subsurface, the C adsorption is promoted as the Ag coverage is increased. The effect of substitutional Ag on the adsorption property of Ni(1 0 0) is rather short-range, which agrees well with the analysis of the projected density of states. Seven pathways are constructed to explore the C diffusion behavior on the bimetallic surface. Along the most kinetically favorable pathway, a C atom hops between two fourfold hollow sites via an adjacent octahedral site in the subsurface of reconstructed Ag/Ni(1 0 0). The “clock” reconstruction which tends to improve the surface mobility, is more favorable on the alloy surface because the c(2 × 2) symmetry is inherently broken by the Ag impurity. As a consequence, the local lattice strain induced by the C transport is effectively relieved by the Ag-enhanced surface mobility and the C diffusion barrier is lowered from 1.16 to 0.76 eV.     

Iron isotope effect on Tc in optimally-doped (Ba,K)Fe2As2 (Tc = 38 K) and SmFeAsO1−y (Tc = 54 K) superconductors

We report the iron isotope effect on a transition temperature (T_c) in an optimally-doped (Ba,K)Fe₂As₂ (T_c = 38 K) and SmFeAsO_1−y (T_c = 54 K) superconductors. In order to obtain the reliable isotope shift in T_c, twin samples with different iron isotope mass are synthesized in the same conditions (simultaneously) under high-pressure. We have found that (Ba,K)Fe₂As₂ shows an inverse iron isotope effect α_Fe = −0.18 ± 0.03 while SmFeAsO_1−y shows a small iron isotope effect α_Fe = −0.02 ± 0.01, where the isotope exponent α is defined by T_c ∼ M^−α (M is the isotopic mass). The results show that α_Fe changes in the iron-based superconductors depending on the system. The distinct iron isotope effects imply the exotic coupling mechanism in the iron-based superconductors.     

Probing Rb MOT in 5P3/2(F = 4) → 5D5/2(F′) transitions. In search for effective Rabi frequency

The aim of this work was to provide a simple justification and applicability limits for the concept of effective Rabi frequency being related to an average atom-field interaction in MOT. We sampled ⁸⁵Rb MOT with a weak probe beam tuned across the 5P_3/2 (F′ = 4) → 5D_5/2(F″ = 3, 4, 5) hyperfine transitions, while the 5S_1/2(F = 3) → 5P_3/2(F′ = 4) transition was driven by the red-detuned trapping beam. The probe absorption spectra were registered for a number of detunings Δ and intensities P of the trapping beam. The Autler-Townes splitting δ of the clearly dominating F′ = 4 → F″ = 5 line was the subject of analysis. The character of the space-dependent interactions of atoms with MOT fields is of a complex nature, which brings the notion of the effective Rabi frequency for MOT into challenge. However, we argue that for the range of the typical values of P and Δ, it is justified to characterize MOT with an effective Rabi frequency Ω_0eff, by using the intuitive formula , where is a mean scaling factor experimentally determined, basing on predictions of a straightforward 3-level model. We postulate that our simple procedure, providing both the value and the applicability limits of the approach, should be repeated with each new implementation of MOT (e.g., with trap beams realignment), which may change conditions experienced by cold atoms.     

Spin-engineering in the Co75Fe25/Cu(1 1 0) system

We present results on the growth and magnetic anisotropies of Co₇₅Fe₂₅ films grown on a Cu(1 1 0) single crystal. Angular dependent MOKE measurements show a thickness dependent, in-plane rotation of the easy axis of magnetisation of up to 60° from the [0 0 1] direction (towards [−1 1 0]). For a film thickness of 5 ML, just greater than that required for the onset of ferromagnetism, uniaxial anisotropy is observed with the easy axis along the [0 0 1] direction. As the film thickness increases this is seen to rotate in-plane towards the [−1 1 0] direction as the contribution from the cubic anisotropy constant grows. At a film thickness of 9 ML there is predominantly cubic anisotropy and at 10 ML the easy axis is rotated to 150° with respect to the [1 −1 0] axis, where it is stabilised.     

The Lamb-dip spectrum of the J = 1 ← 0 transition of DF: Crossing resonances and hyperfine components

The Lamb-dip technique has been applied to the observation of the J = 1 ← 0 transition of DF: for the first time, the hyperfine structure due to D and F have been resolved by using microwave spectroscopy. The high accuracy of this technique allows us to provide hyperfine parameters that are in very good agreement with those obtained from molecular beam experiment. In addition, our frequencies together with the unresolved ones up to J″ value of 47 allow us to provide the most accurate ground state rotational constants of DF known at the moment. Furthermore, due to the presence of a relevant number of strong crossing resonances, the J = 1 ← 0 transition of DF can be considered an illustrative case to show how they modify the shape of Lamb-dip spectra.     

Facetting and (n × 1) reconstructions of SrTiO3(1 1 0) surfaces

(n × 1) reconstructions and facetting of the (1 1 0) polar surface of SrTiO₃ are studied by means of a combination of shell model and density functional calculations. The polarity compensation can be achieved through the formation of {1 0 0} nano-facets, which play a crucial role in the reconstruction process. The behaviors of various possible terminations (Sr, Ti, and O) are analyzed, as well as their atomic structure and energetics. Their stability in different chemical environments is discussed, with respect to previous formulations and experimental results. The Sr-terminated surface tends to expose large facets, while the TiO and O terminations are marginally stabilized or even destabilized by (n × 1) reconstructions, respectively. Trend to facetting results from a subtle competition between the thermodynamic stability of the ideal non stoichiometric (n × 1) surfaces, and huge atomic relaxations that contribute to the lowering of the surface energy differently for each termination.