Nearest-neighbour Atoms Research Articles

Investigation of the atomic structure of molten As–Se alloys using X-ray diffraction

Abstract An X-ray diffraction study on liquid As – Se alloys is reported. The average co-ordination number and the mean distance of nearest neighbour atoms obtained from the diffraction experiments and their relation to the composition of the alloy are in good agreement with earlier neutron diffraction results. After applying a model of micro-heterogeneous alloys it is suggested that As – Se alloys with less than 40 at.% As consist of structural units of As40Se60 stoichiometry in a matrix of Se-like structure. An assumption on the local arrangement of the nearest neighbours is deduced.

Protocol and quantum circuit for implementing the N-bit discrete quantum Fourier transform in cavity QED

We present a simple protocol and quantum circuit for efficient implementation of the N-bit discrete quantum Fourier transform by using two-qubit controlled-NOT gate and SWCZ gate that is a combination of -SWAP and controlled-Z gates in cavity quantum electrodynamics. In this protocol long-lived electronic states in circular Rydberg atoms are used as quantum bits and the one-bit and two-bit quantum gate operations required for implementing the discrete quantum Fourier transform in the quantum circuit can be easily achieved with atom–microwave resonant interaction and atom–cavity interaction occurring only between two nearest-neighbour atoms. We present the detailed experimental procedure and analyse the experimental feasibility.

Dynamic charge-transfer bond-order potential for gallium nitride

We present an analytical interatomic potential for gallium nitride which is based on a new environment-dependent dynamic charge-transfer model. The model consists of a short-ranged bond-order potential that accounts for covalent/metallic interactions and an ionic Coulomb potential with effective point charges that are dynamically adjusted. In contrast to established models, these point charges are distance-dependent and vary with the number and type of nearest neighbour atoms. The basic concepts stem from the idea of bond charges. We assume pairwise symmetric charge transfer between atoms of different type forming a bond. Charge contributions of all bonds to an atomic site are weighted and added, yielding the effective charge per atom. Mulliken charges, as obtained from density-functional theory calculations within the local-density approximation, are used for adjusting the parameters and functional form of the potential. The short-range contributions are chosen as angular-dependent many-body bond-order potentials, which can be understood as an extension of a Finnis–Sinclair type potential.

A computer simulation study of the accommodation and diffusion of He in uranium- and plutonium-doped zircon (ZrSiO 4)

We report the results of an atomistic computational study of He accommodation and diffusion in the Pu 4+- and U 4+-doped zircon (ZrSiO 4). The He–cation potentials derived for this work give results of comparable accuracy to DFT calculations. We have calculated the structural features of doped lattices as well as He solution energies in interstitial sites in the perfect and doped zircon and its diffusion in these lattices. The mode of He accommodation in the perfect zircon is influenced mainly by the topological features of the lattice, promoting site preference of He towards accommodation in the interstitial sites present in the middle of c cylinder channels, whereas the presence of Pu 4+ and U 4+ dopants in the zircon lattice significantly affects the energetics of He accommodation and diffusion in the lattice. Doping causes strong local structural distortions, extending to next nearest-neighbour atoms of the dopants to a radius of up to ∼4 Å, in agreement with experimental results. The presence of dopants in the vicinity of He enhances the solubility of He in the lattice compared to the perfect lattice. The mechanism of diffusion is also affected, where the dopants can create a He trap along the most energetically favourable pathway in the (0 0 1) direction, which may slow down the movement of He along the c direction. The dopants also lower the energy barriers by ∼50% in the octahedral sites.

CO Blocking of D2 Dissociative Adsorption on Ru(0001)

The influence of pre-adsorbed CO on the dissociative adsorption of D(2) on Ru(0001) is studied by molecular-beam techniques. We determine the initial dissociation probability of D(2) as a function of its kinetic energy for various CO pre-coverages between 0.00 and 0.67 monolayers (ML) at a surface temperature of 180 K. The results indicate that CO blocks D(2) dissociation and perturbs the local surface reactivity up to the nearest-neighbour Ru atoms. Non-activated sticking and dissociation become less important with increasing CO coverage, and vanish at theta(CO) approximately 0.33 ML. In addition, at high D(2) kinetic energy (>35 kJ mol(-1)) the site-blocking capability of CO decreases rapidly. These observations are attributed to a CO-induced activation barrier for D(2) dissociation in the vicinity of CO molecules.

Polarized ff-Emission of Terbium(III) by using the Stretched Polymer Film Technique

A Novel coordination system with ff-emission of Tb(III) was obtained on a polyvinyl alcohol (PVA) film surface. On this film the ff-emission is polarized, and is intensified by polymer stretching. XAFS spectra show that the mean bond distances between Tb(III) and the nearest neighbour atoms of the 1,10,-phenanthroline ligand on the film are close to those in [Tb(SA)3], Tb(NO3)H2O and [Tb(phen)2(NO3)3].

Elastic properties of Nd 2− xCe xCuO 4 ( x = 0.10–0.20) high-temperature superconductor

Finite deformation theory is used to obtain the strain energy density ϕ of a tetragonal K 2NiF 4 (2-1-4) type single crystal of the high-temperature superconductor Nd 2− x Ce x CuO 4. The complete set of second-order elastic constants and bulk modulus of the high-temperature superconductor Nd 2− x Ce x CuO 4 ( x = 0.10–0.20) is calculated by taking into account the interactions between nine nearest-neighbour atoms in the lattice and using the Mie–Gruneisen interatomic potential.

STRUCTURAL ANALYSIS OF LIQUID 3D TRANSITION METALS USING CHARGED HARD SPHERE REFERENCE SYSTEM

The Charged Hard Sphere (CHS) reference system is applied to study the structural
 analysis of liquid 3d transition metals. Here we report the structure factor S(q), pair
 distribution function g (r), interatomic distance r1 of nearest neighbour atoms and coordination
 number n$_1$ for liquid 3d transition metals viz: Ti, V, Cr, Mn, Fe, Co, Ni and Cu.
 To describe electron–ion interaction our own model potential is employed alongwith the local
 field correction due to Sarkar et al (SS). The present results of S(q) and g (r) are in
 good agreement with experimental findings. The maximum discrepancy obtained from the
 experimental data for the coordination number is 4.22% in the case of Ti while the lowest
 is 0.31% for Cu. Thus CHS is capable of explaining the structural information of a nearly
 empty d-shell, nearly filled d-shell and fully filled d-shell elements.

Scanning tunnelling microscopy and spectroscopy of MgB2

Experiments of the scanning tunnelling microscopy and spectroscopy (STM/STS) have been carried out on the layered superconductor MgB2 with Tc = 39 K. The measurements were done at 5 K using a Pt-Ir tip and the single-crystal grain. The STM images show characteristic hexagonal patterns with the distances of nearest neighbour atoms to be 0.3 - 0.35 nm and 0.15 - 0.2 nm, respectively, which are consistent with the Mg and B lattice sizes. The STS measurements clarify the superconducting gap 2Δ = 20 - 24 meV, which leads to the strong-coupling ratio 2Δ/kBTc = 5 - 6. The tunnelling spectra can be described by the correlated 2-gap model. These features are consistent with our previous break-junction measurements. The gap structure is homogeneous at least within the range of 200 nm × 200 nm.

Adsorption of methanol and methoxy on theα-Cr2O3(0001) surface

We present density functional theory calculations of methanol and methoxy adsorption at the Cr-terminatedα-Cr2O3(0001) surface. We report on the equilibrium geometries of the adsorbed methanol andmethoxy molecules, analyse the bonding to the surface, and discuss the dissociationenergetics of methanol into methoxy on the surface. We found that methanoladsorbs with its O atom situated above a threefold coordinated hollow site inthe surface O layer at a distance of 2.12 Å from the nearest-neighbour Cr atom,and with a calculated adsorption energy of 0.82 eV. For the methoxy moleculewe found the optimum adsorption geometry to be with the methoxy O on topof a Cr atom and with the CO-axis tilted away from the surface normal by∼55°. Methoxy is strongly bound to the surface with an estimated adsorption energy of3.3 eV.

Higher-order elastic constants of La1.84Sr0.16CuO4 high-temperature superconductor

Finite deformation theory is used to obtain the strain energy density ϕ of a tetragonal 2–1–4-type single crystal of the high-temperature superconductor La2−xSrxCuO4. The complete set of second and third-order elastic constants of the high-temperature superconductor La2−xSrxCuO4 (x=0.16) is calculated by taking into account the interactions between nine nearest-neighbour atoms in the lattice and using Mie–Grüneisen interatomic potential. For the sake of comparison we have also computed the values of these constants for x=0.13–0.20. The values of third-order elastic constants of La2−xSrxCuO4 (x=0.13–0.20) are negative and their absolute magnitudes are one order higher than those of the second-order elastic constants.

Hydrodynamics for a system of harmonic oscillators perturbed by a conservative noise

We consider a heat conduction model for solids. Nearest neighbour atoms interact as coupled oscillators exchanging velocities in such a way that the total energy is conserved. The system is considered under periodic boundary conditions. We will show that the system has a hydrodynamic limit given by the solution of the heat equation and we discuss some aspects of the model.

Simulation of dislocation glide in dilute Fe–Cu alloys

The effects on dislocation glide of the substitutional element copper in solution in α-iron are being investigated by computer simulation. In the first phase, the critical stress for a 1 / 2 〈1 1 1〉 {1 1 0} edge dislocation to overcome configurations of either a single or two nearest-neighbour solute atoms is simulated. Molecular statics and dynamics methods are used to simulate effects at temperature equal to and greater than 0 K, respectively. Single copper atoms and nearest-neighbour pairs in the first atomic plane below the glide plane give the strongest barrier to dislocation glide, in partial agreement with elasticity theory. In addition to temperature, obstacle-spacing effects are considered.

Comparative Ab Initio Study of Point Defect Energies and Atom Migration Profiles in the L1<sub>2</sub>-Ordered Intermetallic Compounds Ni<sub>3</sub>Al, Ni<sub>3</sub>Ga, Pt<sub>3</sub>Ga, Pt<sub>3</sub>In

aFormation energies of antisite defects and vacancies were derived for the L12-ordered intermetallic Ni3Al, Ni3Ga, Pt3Ga, and Pt3In by a supercell ab initio approach. A thermodynamic treatment of point-like defects was then used for the calculation of temperature-dependent defect properties. For all compounds antisite formation requires less energy than vacancy formation, the difference being larger (by 1.2 - 2 eV) for the Ni compounds than for the Pt compounds (by ~0.7-1.1 eV). Some of the tendencies observed can be made plausible by arguments of atom size and relative rigidity of the lattice. Energy profiles for atom jumps were calculated by statically displacing the jumping atom and relaxing the surrounding neighbours. The influence of variable atomic neighbourhoods on the migration barrier and the stability of the initial and final states were studied by systematically exchanging nearest and next nearest neighbour atoms.

Electronic structure of the Ga 1 − xCr xN studied by high-energy photoemission spectroscopy

Valence band spectra of Ga 1 − x Cr x N have been investigated by high-energy photoemission spectroscopy at the photon energy of 5.95 keV. Cr doping does introduce a novel electronic structure in the bandgap and causes some change in valence band structure. Based on the first-principle calculation, Cr-associated electronic levels in the bandgap are assigned to nonbonding and antibonding d states while the change of the valence band suggests that the Ga 4s originated states are significantly modified through hybridization with the Cr 3d orbital. The present result evidences that the Ga valence electrons are considerably modified through the interaction with the second nearest-neighbour Cr atoms.

Ferromagnetic thin films of binary alloys

We consider the ferromagnetic- and lattice-order–disorder phase transitions in thin films of the binary, AB 3 type, alloys. The theory is based on the model consisting of both, magnetic and atomic interactions between nearest neighbour atoms belonging to the sublattices in the form of the monoatomic layers parallel to the surface. The main idea of the present description was applied at first by Valenta and Sukiennicki in the case of permalloy films for their order–disorder characterisation. The ferromagnetism together with the lattice long-range order was considered by Moran-Lopez and Falicov in bulk binary alloys. The present paper extends the above reference works to the case of the treatment for ferromagnetic and lattice disordering in thin films.

Pair correlations and interaction energies in FePd single crystal

Diffuse neutron scattering measurements were carried out in a FePd single crystal at 1020 K in the (100) and (110) reciprocal planes. The diffuse intensity is nearly symmetric, showing that the static displacements are small. A set of displacement and short-range order parameters have been fitted to the measured data using a least square procedure. The short-range order parameters have been used in conjunction with an inverse cluster variation method to deduce the pair interaction energies. A strong attractive interaction between second nearest-neighbour like atoms is found. PACS. 61.12.Ex Neutron scattering - 64.60.Cn Order-disorder transformations - 61.66.Dk Alloys

THE STRUCTURE OF LIQUID Fe-C ALLOY NEAR THE MELTING POINT

The structure of liquid Fe-C alloy near the melting point has been studied at three different temperatures (1540, 1560 and 1580 °C). It is shown that the structural correlation length (or the cluster size), coordination number, distance between the nearest neighbour atoms and number of atoms in a cluster all decrease with increasing temperature. Small prepeaks before the first main peak were found in the structure factors of liquid alloy and the prepeaks became slightly obscure as the temperature increased. In general, the presence of the prepeaks corresponds to a medium range order structure. The prepeaks fade away with increasing temperature implying that the degree of superheat contributes to the disappearance of the medium range order.On a étudié la structure de l’alliage liquide Fe–C près du point de fusion, à trois températures différentes (1540, 1560 et 1580 °C). On montre que la longueur de corrélation de la structure (ou taille de la grappe), le nombre de coordination, la distance des atomes les plus proches voisins et le nombre d’atomes dans une grappe diminuent tous avec une augmentation de la température. On a trouvé de petits pré–pics avant le premier pic principal dans les facteurs de structure de l’alliage liquide et ceux-ci s’obscurcissaient légèrement à mesure que la température s’élevait. En général, la présence des pré–pics correspond à une structure d’ordre à moyenne portée. Les pré–pics s’estompent avec une augmentation de la température, impliquant que le degré de surchauffe contribue à la disparition de l’ordre à moyenne portée.

Order–disorder phenomena in binary alloy thin films

We consider the order–disorder phase transition in thin films of the binary, AB 3 type, alloys. The theory is based on the model consisting of pairwise interactions between nearest neighbour atoms belonging to the sublattices in the form of the monoatomic layers parallel to the surface. This model, the so-called Valenta's model is well-known in literature. It is essential to take it into account for the description of thin films while the model of continuum can be successfully used for semi-infinite samples. The numerical results only slightly differ for both the models in the case when the film thickness tends to infinity. In this context, the present paper brings the new, original results for the influence of the surface on the properties of long-range ordering in alloy thin films. The long-range order parameter and the concentration distribution, mutually dependent, depend on the film thickness and the boundary conditions at the surfaces. First of all, the interesting phenomenon, surface disordering, is discussed in analogy to the surface melting. Moreover, an interpretation of the disordering kinetics type is proposed in connection with the behaviour of disorder which appears as layer by layer process starting from the surface planes.

The influence of Al content on the Zener relaxation of Fe–Al alloys

The effect of Al content on the Zener relaxation peak of annealed Fe–Al alloys has beeninvestigated using a computer-controlled automatic inverted torsion pendulumthrough the method of free decay and forced vibration. It has been shown thatalloys with medium Al contents show strong relaxation while lower or higher Alcontents lead to relatively weak Zener relaxation peaks. The Zener relaxation inFe–Al alloys originates from the next-nearest-neighbour atom pairs and theirinteraction rather than the nearest-neighbour atom pairs. The latter may notproduce Zener relaxation because of the ordered structure, which suppresses Zenerrelaxation.