Search

We propose a Frenkel-Kontorova model for a 1D chain of electrons forming a Wigner solid over 4He. It is a highly idealized picture, but with the model at hand we can study the movement of the chain. We find out that the energetically most preferable movement is the successive sliding of a kink or an antikink through the chain. Then the force for a movement does not depend on the length of the chain. The force uniformly applied to all electrons must be larger than a force exciting only a kink or an antikink. We calculate two cases, one with stiff ‘springs’ between the electrons and one with weak ‘springs’. The side potential of the ‘dimples’ is additionally damped at the periphery. We study the cases with 33, 66, and 101 particles. [ABSTRACT FROM AUTHOR]

Shear-induced coupled evolution behavior of intragranular crack and grain boundary (GB) in nanoscale bicrystal system is studied by using phase-field-crystal method. Evolution process of system with different crack sizes and grain boundary misorientations (GBMs) is analyzed. Instead of normally extending under applied strain, intragranular crack can partially heal and even disappear under shear strain. This healing process is realized by compression stress field provided by GB migration. Increasing GBM and decreasing crack size can both facilitate crack healing. Besides, with the existence of intragranular crack, centripetal GB migration can be accelerated. The acceleration effect will be decreased by large GBM and small crack size. Low GBM is a prerequisite, but decreasing crack size can only make this effect not obvious. Increasing GBM or crack size has opposite effects on crack healing and centripetal GB migration. [ABSTRACT FROM AUTHOR]

The aim purpose of this work is to investigate the various physical properties of the new synthesized alkali-metal based Heusler alloy Li2CoSb using the density functional theory. The results showed that the compound has ferromagnetic ground state and is mechanically stable against any small deformations. We performed calculations for different exchange-correlation potentials to obtain the magneto-electronic properties. The generalized gradient approximation (GGA) findings showed a metallic electronic nature with a weak magnetic moment. However, calculations by the modified Becke–Johnson exchange potential combined with GGA correlation (mBJ-GGA) indicated an increase in the magnetic moment but the alloy remains with a metallic character. To respect the strong electron correlation effect for the localized d states in the calculations, the mBJ-GGA + U scheme was used. The Half-metallic ferromagnetism behavior with integer total magnetic moment was found. The half-metallic property is preserved at a range 2–5 eV of Hubbard U parameter. The thermoelectric properties were calculated and divulged that the compound studied exhibit a high figure of merit, large Seebeck coefficient, and low thermal conductivity. Hence, the Li2CoSb is a good thermoelectric compound. The thermal and pressure effect on the isochoric and isobaric heat capacity and Debye temperature were studied using the quasiharmonic approximation. It is important to emphasis that all the investigated properties have not been studied yet for the present Li-based Heusler compound. Therefore, the present work can be considered as a first report. [ABSTRACT FROM AUTHOR]

Following the recent research enthusiasm on the effect of geometry on near-field heat transfer (NFHT) enhancement, we present an analysis based on simplified yet highly efficient graphene and nanotube models. Two geometries are considered: that of two parallel infinite "graphene" surfaces and that of a one-dimensional infinite "nanotube" line in parallel with an infinite surface. Due to its symmetry, the former is in principal simpler to analyze and even so, earlier works suggested that the application of a full model in this problem still demands heavy computations. Among other findings, our simplified computation – having successfully replicated the results of relevant earlier works – suggests a sharper NFHT enhancement dependence on distance for the line-surface system, namely J ~ d−5.1 as compared to J ~ d−2.2 for the parallel surface. Such comparisons together with applications of our efficient approach would be the important first steps in the attempt to find a general rule describing geometric dependence of NFHT. [ABSTRACT FROM AUTHOR]

The distribution of sodium and diffusion mechanism in sodium-silicate melt with various compositions are investigated via molecular dynamics simulation. The microstructure and dynamical characteristic have been studied with the help of Voronoi polyhedron, simplex and Si-O subnet, oxygen-cluster. The simulation results reveal that Na atoms tend to be in the O polyhedrons and not in Si ones. Moreover, the Na atoms are mainly located in non-bridging oxygen (NBO) and free oxygen (FO) polyhedrons. The Voronoi volume of bridging oxygen (BO) or NBO weakly depends on the number of Na located in polyhedron which indicates the strong Si-O bond compared to Na-O bond. The structure of sodium silicate melt consists of two separate regions: the Na-poor regions of Si-BO subnets and a Na-rich region of NBO-FO cluster. The density of sodium in the NBO-FO cluster is by 3–6 times larger than the one of Si-O subnets. This largest NBO-FO cluster represents a diffusion channel for sodium in sodium-silicate. Furthermore, we find that each BO polyhedron contains 0 or 1 Na atom meanwhile each NBO polyhedron contains 1 or 2 Na atoms. Thereby, the BO and NBO polyhedron contains 1 and 2 sites, respectively. The energy for Na atom located in BO site is larger than that in NBO site. The transition energy for Na atom moving from present site to neighboring NBO site is smaller than that to neighboring BO site. So, Na atoms move frequently to neighboring polyhedrons of NBO, and rarely to the BO ones. [ABSTRACT FROM AUTHOR]

Herein, we report our density functional theory calculations for structure, electronic, thermodynamic and mechanical properties of MBiO3 (M = Rb, Cs, Tl), compounds of perovskites oxide family. Exchange-correlation potential was treated with generalized gradient and local density approximations. From available ionic data, the tolerance factors and lattice constants were calculated. The values of tolerance factors provided the stability guarantee of MBiO3 compounds in cubic phase while the obtained lattice constants are in accordance to the available data. The cubic phase stability was further confirmed from elastic constants. The electronic structure results disclosed these materials as metallic. Mechanically, all the three perovskites were found as brittle from Cauchy's pressure, Pugh ratio and Poisson's ratio. The thermodynamic calculations have been performed using quasi-harmonic Debye model. Thermal properties like Debye temperature, specific heat capacity and thermal expansion have been presented with the variation of temperature and pressure. [ABSTRACT FROM AUTHOR]

The effects of spin-orbit coupling on the atomic and electronic structures of clean W(110) surface and Sn deposited W(110) surface are studied using the Density Functional Theory. While the relaxed atomic geometry remains unaffected, there are changes in the surface electronic states with the inclusion of spin-orbit coupling. For the clean W(110) surface, we have identified a total of three surface and one resonance states, and discussed the anisotropic behaviour of Dirac-like states. With increasing Sn monolayer coverages on the W(110)-(1×4) slab, the Dirac-like cone behaviour is found to progressively distort, and Sn-derived states develop. For a (1×1) prototypical model of monolayer Sn coverage, we have calculated the maximum splittings of 0.10 eV and 0.52 eV for the two surface states characterised mainly by the W 3d5∕2 and W 3d3∕2 orbitals. [ABSTRACT FROM AUTHOR]

Structural, elastic and electronic band structure properties of A-15 type Nb-based intermetallic compounds Nb3B (B = Os, Pt, Au) have been revisited using first principles calculations based on the density functional theory (DFT). All these show excellent agreement with previous reports. More importantly, electronic bonding, charge density distribution and Fermi surface features have been studied in detail for the first time. Vickers hardness of these compounds is also calculated. The Fermi surfaces of Nb3B contain both hole- and electron-like sheets, the features of which change systematically as one move from Os to Au. The electronic charge density distribution implies that Nb3Os, Nb3Pt and Nb3Au have a mixture of ionic and covalent bondings with a substantial metallic contribution. The charge transfer between the atomic species in these compounds has been explained via the Mulliken bond population analysis and the Hirshfeld population analysis. The bonding properties show a good correspondence to the electronic band structure derived electronic density of states (DOS) near the Fermi level. Debye temperature of Nb3B (B = Os, Pt, Au) has been estimated from the elastic constants and shows a systematic behavior as a function of the B atomic species. A good correspondence among the elastic, electronic and charge density distribution properties are found. The superconducting transition temperature is found to be dominated by the electronic density of states at the Fermi level. We have discussed possible implications of the results obtained in this study in details in this paper. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations were performed to investigate stabilities and properties for uracil (U)-functionalized carbon nanotubes (CNTs). To this aim, the optimized molecular properties were evaluated for ( n, 0) models of CNTs ( n = 3-16) in the original and U-functionalized forms. The results indicated that the dipole moments and energy gaps were independent of tubular diameters whereas the binding energies showed that the U-functionalization could be better achieved for n = 8-11 curvatures of ( n, 0) CNTs. Further studies based on the evaluated atomic-scale properties, including quadrupole coupling constants ( C), indicated that the electronic properties of atoms could detect the effects of diameters variations of ( n, 0) CNTs, in which the effects were very much significant for the atoms around the U-functionalization regions. Finally, the achieved results of singular U, original CNTs, and CNT-U hybrids were compared to each other to demonstrate the stabilities and properties for the U-functionalized ( n, 0) CNTs. [ABSTRACT FROM AUTHOR]

We study qualitatively ultra-fast proton transfer (PT) in the first singlet (S) state of liquid water (absorption onset) through excited-state dynamics by means of time-dependent density functional theory and ab initio Born-Oppenheimer molecular dynamics. We find that after the initial excitation, a PT occurs in S in form of a rapid jump to a neighboring water molecule, on which the proton either may rest for a relatively long period of time (as a consequence of possible defect in the hydrogen bond network) followed by back and forth hops to its neighboring water molecule or from which it further moves to the next water molecule accompanied by back and forth movements. In this way, the proton may become delocalized over a long water wire branch, followed again by back and forth jumps or short localization on a water molecule for some femtoseconds. As a result, the mechanism of PT in S is in most cases highly non-Grotthuss-like, delayed and discrete. Furthermore, upon PT an excess charge is ejected to the solvent trap, the so-called solvated electron. The spatial extent of the ejected solvated electron is mainly localized within one solvent shell with overlappings on the nearest neighbor water molecules and delocalizing (diffuse) tails extending beyond the first solvent sphere. During the entire ultra-short excited-state dynamics the remaining OH radical from the initially excited water molecule exhibits an extremely low mobility and is non-reactive. [ABSTRACT FROM AUTHOR]

A computational method for studying molecular vibrations and spectra for symmetrical systems with many degrees of freedom was developed. The algorithm allows overcoming difficulties on the automation of calculus related to the symmetry determination of such oscillations in complex systems with many degrees of freedom. One can find symmetrized displacements and, consequently, obtain and classify normal oscillations and their frequencies. The problem is therefore reduced to the determination of eigenvectors by common numerical methods, and the algorithm simplifies the procedure of symmetry determination for normal oscillations. The proposed method was applied to studying molecular vibrations and spectra of the fullerene molecule C, and the comparison of theoretical results with experimental data is drawn. The computational method can be further extended to other problems of group theory in physics with applications in clusters and nanostructured materials. [ABSTRACT FROM AUTHOR]

Quartz has found a wide range of applications over the past years. In the present work, the temperature dependence of microcrystalline quartz is investigated with Raman spectroscopy and DFT-based molecular dynamics simulations. We aimed to determine the structure at short and medium range distances as a function of the increasing temperature. The dynamics and the structural changes are analysed in terms of time-dependent properties, and the vibrational analysis obtained from calculated dipole trajectory and vibrational density of states (VDOS). The computed data is compared to Raman and infrared spectroscopic measurements. The approach is of a particularly great interest when we focus on the structural behaviour, and the dynamical disorder observed and characterised through geometric and thermodynamic data. The calculations confirm that the infrared and Raman signature as a function of temperature provide a sensitive analysis of the structural behaviour of quartz. [ABSTRACT FROM AUTHOR]

This paper investigates, using first principle calculations, the charge transition levels Fe and FeB, and the FeB binding energy in SiGe alloy with composition x = 3 − 25%. The alloys were generated using an efficient code for the stochastic generation of special quasirandom structures. We found that the separation between Fe and FeB donor levels was ~0.24 eV (experimental value =0.28 eV) and was independent on x, in an agreement with the experiment. The pattern of the variation of the levels and band gap energies with x agreed very well with the experiment especially for x< 25 %. The formation of FeB-pairs was found to be favorable over individual Fe formation with average binding energy ~0.2 eV, agreeing with the first-principle calculation report using finite supercell size. In particular, the reliability of our method to reproduce the experimental results associated with the composition controlled FeB donor levels has successfully been demonstrated in the industrially interesting SiGe alloy material. [ABSTRACT FROM AUTHOR]

The surface diffusion of Al dimmers on Mg clusters with hexahedral structure was studied using the combination of quenched molecular dynamics and the embedded atom method. The system energy barriers of typical minimum energy diffusion paths for Al dimers on the Mg clusters were calculated using the Nudged Elastic Band method. In our study range (153-4061 atoms), the binding energies on the (0001) facets and the (1 $$\hbox{$\overline 1 $}$$ 01) facets differed, the binding energy on the former was lower than that on the latter. Moreover, cluster size only slightly influenced the binding energy values. Two possible diffusion paths were studied. Results showed that the diffusion of the dimer on the (0001) facet easily occurred at low temperatures. Furthermore, the interaction between the two atoms of the dimer facilitated the dimer crossing of the step edge between the (1 $$\hbox{$\overline 1 $}$$ 01) facets by hopping mechanism. [ABSTRACT FROM AUTHOR]

Previous studies of digital magnetic heterostructures have mainly focused on 3 d transition metal δ-doped semiconductors. In this work, sp-electron digital magnetic heterostructures without magnetic ions are proposed. Based on a theoretical density functional investigation, electronic structures and magnetic properties of MgO and CaO δ-doped with K were reported. The results show that these heterostructures are half-metallic ferromagnetic materials having semiconducting up spins and metallic down spins, with an exchange interaction much stronger than that of a random alloy with similar K concentration. Our first-principles calculations show that the carriers at the Fermi level are strongly confined within a few monolayers around the KO plane. This strong confinement is responsible for the large exchange coupling and the two-dimensional half-metallic behavior. The thickness of the host semiconducting spacer does not significantly change the global electric and magnetic features. [ABSTRACT FROM AUTHOR]

Complex systems from science, technology or mathematics usually appear to be very different in their specific dynamical evolution. However, the concept of an energy landscape with its basins corresponding to locally ergodic regions separated by energy barriers provides a unifying approach to the description of complex systems dynamics. In such systems one is often confronted with the task to control the dynamics such that a certain basin is reached with the highest possible probability. Typically one aims for the global minimum, e.g. when dealing with global optimization problems, but frequently other local minima such as the metastable compounds in materials science are of primary interest. Here we show how this task can be solved by applying control theory using magnesium fluoride as an example system, where different modifications of MgF are considered as targets. In particular, we generalize previous work restricted to temperature controls only and present controls which simultaneously adjust temperature and pressure in an optimal fashion. [ABSTRACT FROM AUTHOR]

By monitoring the sampling of states with different magnetizations in transition matrix procedures a family of accurate and easily implemented techniques are constructed that automatically control the variation of the temperature or energy as the calculation proceeds. The accuracy of these methods for a single Markov chain exceeds that of standard transition matrix procedures that accumulate elements from multiple chains. [ABSTRACT FROM AUTHOR]

How fast can a laser pulse ionize an atom? We address this question by considering pulses that carry a fixed time-integrated energy per-area, and finding those that achieve the double requirement of maximizing the ionization that they induce, while having the shortest duration. We formulate this double-objective quantum optimal control problem by making use of the Pareto approach to multi-objective optimization, and the differential evolution genetic algorithm. The goal is to find out how a precise time-profiling of ultra-fast, large-bandwidth pulses may speed up the ionization process. We work on a simple one-dimensional model of hydrogen-like atoms (the Pöschl-Teller potential) that allows to tune the number of bound states that play a role in the ionization dynamics. We show how the detailed shape of the pulse accelerates the ionization, and how the presence or absence of bound states influences the velocity of the process. [ABSTRACT FROM AUTHOR]

Individuals building populations are subject to variability. This variability affects progress of epidemic outbreaks, because individuals tend to be more or less resistant. Individuals also differ with respect to their recovery rate. Here, properties of the SIR model in inhomogeneous populations are studied. It is shown that a small change in model's parameters, e.g. recovery or infection rate, can substantially change properties of final states which is especially well-visible in distributions of the epidemic size. In addition to the epidemic size and radii distributions, the paper explores first passage time properties of epidemic outbreaks. [ABSTRACT FROM AUTHOR]

We report on the first principle density functional calculation of the charge transition levels of native defects (vacancies and interstitials) in CaF structure. The transition level was defined as the Fermi level where two charge states of given defect have the same formation energy. The common error in the band gap inherited to semiclocal density functional has been accounted for by incorporating the hybrid density functional method, leading to correct placement of the transition levels within the band gap. The band gap size from hybrid calculation has been validated using the full potential, Linearized Augmented Planewave method with the Modified-Becke-Johnson exchange potential. Prior to level calculations, we ensured that an agreement between the formation energies from small (95-97 atoms) and large (323-325 atoms) supercells was achieved after applying the Makov-Payne correction method. Our calculated transition level for the anion vacancy was 2.97 eV below the conduction band, agreeing with the experimental optical absorption band at 3.3 eV associated with the electron transition from the ground state F-center to the conduction band in CaF. [ABSTRACT FROM AUTHOR]

We discuss an approach to accurate numerical computations of slowly convergent properties in two-electron atoms/ions which include the negatively charged Ps ( e e e ) and H ions, He atom and positively charged, helium-like ions from Li to Ni. All these ions are considered in their ground 1S-state(s). The slowly convergent properties selected in this study include the electron-nulceus ( r ) and electron-electron ( r ) expectation values for k = 2, 3, 4 and 5. [ABSTRACT FROM AUTHOR]

The electronic and optical properties of α-graphyne sheet are investigated by using density functional theory. The results confirm that α-graphyne sheet is a zero-gap semimetal. The optical properties of the α-graphyne sheet such as dielectric function, refraction index, electron energy loss function, reflectivity, absorption coefficient and extinction index are calculated for both parallel and perpendicular electric field polarizations. The optical spectra are strongly anisotropic along these two polarizations. For ( E ∥ x), absorption edge is at 0 eV, while there is no absorption below 8 eV for ( E ∥ z). [ABSTRACT FROM AUTHOR]

We employ state-of-the-art ab initio density functional theory techniques to investigate the structural, dynamical, mechanical stability and electronic properties of the ternary AgInS compounds under pressure. Using cohesive energy and enthalpy, we found that from the six potential phases explored, the chalcopyrite and the orthorhombic structures were very competitive as zero pressure phases. A pressure-induced phase transition occurs around 1.78 GPa from the low pressure chalcopyrite phase to a rhombohedral RH-AgInS phase. The pressure phase transition around 1.78 GPa is accompanied by notable changes in the volume and bulk modulus. The calculations of the phonon dispersions and elastic constants at different pressures showed that the chalcopyrite and the orthorhombic structures remained stable at all the selected pressure (0, 1.78 and 2.5 GPa), where detailed calculations were performed, while the rhombohedral structure is only stable from the transition pressure 1.78 GPa. Pressure effect on the bandgap is minimal due to the small range of pressure considered in this study. The meta-GGA MBJ functional predicts bandgaps which are in good agreement with available experimental values. [ABSTRACT FROM AUTHOR]

The electron tunneling current through nanostructures is considered in the presence of the electron-phonon interactions. In the Keldysh nonequilibrium formalism, the lesser, greater, advanced and retarded self-energies components are expressed by means of appropriate Langreth rules. We discuss the key role played by the entailed Hilbert transforms, and provide an analytic way for their evaluation. Particular attention is given to the current-conserving lowest-order-expansion for the treament of the electron-phonon interaction; by means of an appropriate elaboration of the analytic properties and pole structure of the Green's functions and of the Fermi functions, we arrive at a surprising simple, elegant, fully analytic and easy-to-use expression of the Hilbert transforms and involved integrals in the energy domain. [ABSTRACT FROM AUTHOR]

We present an efficient impurity solver for the dynamical mean-field theory (DMFT). It is based on the separation of bath degrees of freedom into the low energy and the high energy parts. The former is solved exactly using exact diagonalization and the latter is treated approximately using Green's function equation of motion decoupling approximation. The two parts are combined coherently under the standard basis operator formalism. The impurity solver is applied to the Anderson impurity model and, combined with DMFT, to the one-band Hubbard model. Qualitative agreement is found with other well established methods. Some promising features and possible improvements of the present solver are discussed. [ABSTRACT FROM AUTHOR]

Fractional Brownian motion (fBm) is a nonstationary self-similar continuous stochastic process used to model many natural phenomena. A realization of the fBm can be numerically approximated by discrete paths which do not entirely preserve the self-similarity. We investigate the self-similarity at different time scales by decomposing the discrete paths of fBm into intrinsic components. The decomposition is realized by an automatic numerical algorithm based on successive smoothings stopped when the maximum monotonic variation of the averaged time series is reached. The spectral properties of the intrinsic components are analyzed through the monotony spectrum defined as the graph of the amplitudes of the monotonic segments with respect to their lengths (characteristic times). We show that, at intermediate time scales, the mean amplitude of the intrinsic components of discrete fBms scales with the mean characteristic time as a power law identical to that of the corresponding continuous fBm. As an application we consider hydrological time series of the transverse component of the transport process generated as a superposition of diffusive movements on advective transport in random velocity fields. We found that the transverse component has a rich structure of scales, which is not revealed by the analysis of the global variance, and that its intrinsic components may be self-similar only in particular cases. [ABSTRACT FROM AUTHOR]

High harmonic generation (HHG) provides a flexible framework for the development of coherent light sources in the extreme-ultraviolet and soft X-ray regimes. However it suffers from low conversion efficiencies as the control of the HHG spectral and temporal characteristics requires manipulating electron trajectories on attosecond time scale. The phase matching mechanism has been employed to selectively enhance specific quantum paths leading to HHG. A few important fundamental questions remain open, among those how much of the enhancement can be achieved by the single-emitter and what is the role of correlations (or the electronic structure) in the selectivity and control of HHG generation. Here we address those questions by examining computationally the possibility of optimizing the HHG spectrum of isolated hydrogen and helium atoms by shaping the slowly varying envelope of a 800 nm, 200-cycles long laser pulse. The spectra are computed with a fully quantum mechanical description, by explicitly computing the time-dependent dipole moment of the systems using a time-dependent density-functional approach (or the single-electron Schrödinger equation for the case of H), on top of a one-dimensional model. The sought optimization corresponds to the selective enhancement of single harmonics, which we find to be significant. This selectivity is entirely due to the single atom response, and not to any propagation or phase-matching effect. Moreover, we see that the electronic correlation plays a role in the determining the degree of optimization that can be obtained. [ABSTRACT FROM AUTHOR]

First principles density functional theory (DFT) calculations for bulk structural, electronic and optical properties of ternary compounds AgAlX (X = S, Se, Te) were performed with two flavours of generalised gradient approximations (PBE and PBEsol) and the hybrid functional HSE06. Using cohesive energy as a stability criterion, we found that the chalcopyrite structure is the favoured phase for these materials. PBEsol gives structural properties closer to the experimental values when compared to the results of PBE. Tetragonal distortion and anion displacement were calculated and we found them to be the cause of the crystal field splitting. Reduction of the bandgap and band splitting around the Γ in the Brillouin zone was noted when spin-orbit coupling was included in our study especially in the case of AgAlTe. The HSE06 bandgap and frequency dependent dielectric function were in very good agreement with experimental results. We have also shown that the maximum absorption peak lies in the ultraviolet range irrespective of the functional used. The refractive index is also discussed. [ABSTRACT FROM AUTHOR]

In this work we perform a numerical study of a rotating, harmonically trapped, Bose-Einstein condensate of microcavity polaritons. An efficient numerical method (toolbox) to solve the complex Gross-Pitaevskii equation is developed. Using this method, we investigate how the behavior of the number of vortices formed inside the condensate changes as the various system parameters are varied. In contrast to the atomic condensates, we show, there exists an (experimentally realizable) range of parameter values in which all the vortices can be made to vanish even when there is a high rotation. We further explore how this region can be tuned through other free parameters and also discuss how this study can help to realize the synthetic magnetic field for polaritons and hence paving the way for the realization of the quantum Hall physics and many other exotic phenomena. [ABSTRACT FROM AUTHOR]

Absorbing boundaries are frequently employed in real-time propagation of the Schrödinger equation to remove spurious reflections and efficiently emulate outgoing boundary conditions. These conditions are a fundamental ingredient for the calculation of observables involving infinitely extended continuum states in finite volumes. In the literature, several boundary absorbers have been proposed. They mostly fall into three main families: mask function absorbers, complex absorbing potentials, and exterior complex-scaled potentials. To date none of the proposed absorbers is perfect, and all present a certain degree of reflections. Characterization of such reflections is thus a critical task with strong implications for time-dependent simulations of atoms and molecules. We introduce a method to evaluate the reflection properties of a given absorber and present a comparison of selected samples for each family of absorbers. Further, we discuss the connections between members of each family and show how the same reflection curves can be obtained with very different absorption schemes. [ABSTRACT FROM AUTHOR]

The effect of atomic antisite and swap defects on the electronic and magnetic properties of TiNiAl inverse Heusler alloy is investigated by the first-principles calculations within density functional theory. In the ordered TiNiAl alloy, there are eight antisites and five swaps which are established by the replacement of an atom by another and the exchange in positions of atoms, respectively. The Ni antisite is found to be the most probable defect due to the lowest formation energy, whereas the least probable defects are the Al and Ni antisites as well as Ni-Ti(A) and Al-Ti(B) swaps due to the higher formation energies compared with Ni antisite. The Ti and Al antisites as well as Al-Ti(A)/Ni and Ni-Ti(B) swaps are highly unlikely to be formed due to the positive values of formation energy. Moreover, we deduce from the relative binding energy of the swap with respect to their antisites that the Ni prefers atomic antisite to site swap, while Al prefers site swap to atomic antisite. The spin polarization is markedly reduced in Ni/Al antisite as well as Ni-Ti(A) and Al-Ti(B) swaps due to the occurrence of defect states at the Fermi level, while a very high spin polarization is obtained for Ni/Al antisites and only the Ni antisite retains the half-metallicity with a perfect spin polarization. The magnetic moments of all the likely defected structures decrease in comparison to the ordered TiNiAl mainly due to the decrease of local magnetic moments of the defect atom and its near neighbors. [ABSTRACT FROM AUTHOR]

It is virtually impossible to evaluate the magnetic properties of large anisotropic magnetic molecules numerically exactly due to the huge Hilbert space dimensions as well as due to the absence of symmetries. Here we propose to advance the finite-temperature Lanczos method (FTLM) to the case of single-ion anisotropy. The main obstacle, namely the loss of the spin rotational symmetry about the field axis, can be overcome by choosing symmetry related random vectors for the approximate evaluation of the partition function. We demonstrate that now thermodynamic functions for anisotropic magnetic molecules of unprecedented size can be evaluated. [ABSTRACT FROM AUTHOR]

Going beyond the limitations of our earlier works [X. Zheng, F. Wang, C.Y. Yam, Y. Mo, G.H. Chen, Phys. Rev. B  75, 195127 (2007); X. Zheng, G.H. Chen, Y. Mo, S.K. Koo, H. Tian, C.Y. Yam, Y.J. Yan, J. Chem. Phys. 133, 114101 (2010)], we propose, in this manuscript, a new alternative approach to simulate time-dependent quantum transport phenomenon from first-principles. This new practical approach, still retaining the formal exactness of HEOM framework, does not rely on any intractable parametrization scheme and the pole structure of Fermi distribution function, thus, can seamlessly incorporated into first-principles simulation and treat transient response of an open electronic systems to an external bias voltage at both zero and finite temperatures on the equal footing. The salient feature of this approach is surveyed, and its time complexity is analysed. As a proof-of-principle of this approach, simulation of the transient current of one dimensional tight-binding chain, driven by some direct external voltages, is demonstrated. [ABSTRACT FROM AUTHOR]

We study the effect of multiple interpenetrating pinning arrays on the vortex motion in the presence of an ac driving force, f( t), by using extensive molecular dynamics (MD) simulations. Firstly, the response to a square ac wave f( t) has been explored for the vortices interacting with a periodic square pinning array which has different pinning strengths and sizes. The effect of the type of an ac drive and its amplitude on the oscillatory dynamics of vortices have been investigated in detail. For very low displacements of the vortices, we have found that the single-particle model can produce results analytically similar to the ones obtained by the MD simulations. It is shown that the collective motion of vortices can be controlled easily by varying the number of multiple interpenetrating square pinning lattices ( N). A regular sequence of peaks has been observed for N = 3 in the time evolution of the average velocity of the vortices (i.e., V̅ curves). The number of peaks ( N) strongly depends on the magnitude of f( t), and increases with increasing the magnitude of f. The close relation between N and f is considered as an indication of controlling vortex motion in a multiple periodic pinning structure. Finally, the variation of the power spectrum of noise S( ν) with N has been investigated. For N = 3, it has been found that the plastic motion of the row of vortices evolves at low frequencies, i.e., 1/ ν behavior, whereas, at high frequencies, S( ν) shows a typical behavior of Gaussian white noise. [ABSTRACT FROM AUTHOR]

Self-interstitial interactions causing volume expansion in bcc Fe are studied through an idealized microstructure evolution model in which only self-interstial atoms (SIAs) are inserted. Using a combination of non-equilibrium molecular dynamics simulations and a metadynamics algorithm, meta-stable SIA clusters are observed to nucleate and grow into dislocation loops or localized amorphous phases, both contributing to swelling behavior persisting well beyond the atomistic time scale. A non-monotonic local density variation with dose rate is found and attributed to competing evolutions of different defective structures. [ABSTRACT FROM AUTHOR]

We report here ab initio density functional theory (DFT) calculations of structural, elastic, Peierls stress, thermodynamic and optical properties of RRhB ( R = Y, Zr and Nb) using the plane wave pseudopotential method. The materials possess better ductile behavior in comparison with a selection of layered MAX phases but the anisotropy is strong, particularly in NbRhB. The Peierls stress, approximately 3-4 times larger than in MAX phases, show that dislocation movement may follow but with much reduced occurrences compared to MAX phases. The temperature and pressure dependence of bulk modulus, specific heats, thermal expansion coefficient, and Debye temperature are calculated for the first time for two of the three compounds using the quasi-harmonic Debye model with phononic effects for elevated temperature and pressure. The obtained results are discussed in comparison to the behavior of other related compounds. Further the features of optical functions obtained for the first time are discussed. The study reveals that the reflectivity is high in the IR-UV regions up to ~17.5 eV (YRhB, ZrRhB) and 20 eV (NbRhB), thus showing promise as good coating materials. [ABSTRACT FROM AUTHOR]

The reverse Monte Carlo (RMC) simulation is applied in the study of an aqueous electrolyte LiCl6HO. On the basis of the available experimental neutron scattering data, RMC computes pair radial distribution functions in order to explore the structural features of the system. The obtained results include some unrealistic features. To overcome this problem, we use the hybrid reverse Monte Carlo (HRMC), incorporating an additional energy constraint in addition to the usual constraints of the pair correlation functions and average coordination. Our results show a good agreement between experimental and computed partial distribution functions (PDFs) as well as a significant improvement in pair partial distribution curves. This kind of study can be considered as a useful test for a defined interaction model for conventional simulation techniques [ABSTRACT FROM AUTHOR]

Site preference of transition metal elements habitually determined by the number of their valence electrons has been extensively used in the design of the full-Heusler alloys X2YZ. In this work, the full-Heusler materials Sc2CoZ (Z = Al, Ga, In) were investigated and the comparison between the L21 and XA types structures of those alloys were studied based on first-principles calculations. The ferromagnetic L21 structure is found to be energetically more favorable than the XA structure. The phonon dispersion curve predicts that all alloys are dynamically stable in L21 structure. The influence of atomic ordering with respect to the Wyckoff sites on the phase stability, electronic structure and magnetism were also analyzed. Nearly half-metallic behavior is observed for the L21-type. However, XA-type is nonmagnetic semiconductor. For L21-type, all alloys have a weak magnetic moment at equilibrium lattice parameter, mainly attributed to the Co atom. Thermodynamic properties of both types of structure are studied using quasi-harmonic Debye model. [ABSTRACT FROM AUTHOR]

The physical properties of AHfO3 (A = Ca, Ba) are revealed using the density functional theory (DFT) based FP-LAPW+lo approach. The existing work investigates the pressure dependence of mechanical, electronic characteristics for specifying the optical and thermoelectric device applications of alkaline rare-earth hafnate perovskites. The PBEsol-GGA functional has been applied for the dealing of the exchange-correlation energy. The lattice constants of the stable cubic phases are extracted by structural optimization, which is similar to the existing experimental and theoretical literature. The bulk moduli (B) and cubic elastic constants are computed for evaluating the mechanical strength against external pressure up to 15 GPa. The electronic properties reveal that Hf-3d states primarily construct conduction band minima, while O–2p states construct valence band maxima at 0 GPa, exhibiting an indirect bandgap (Γ–M), which has been transformed to direct bandgap (Γ–Γ) at 15 GPa. Investigations of the optical properties illustrate that change in pressure can tune the optical parameters of these materials within ultraviolet (UV) energies suggesting commercial optoelectronic utilities. Our analysis shows that BAHfO3 exhibits better thermoelectric properties than CAHfO3 at room temperature whereas, thermoelectric performance both the compounds become comparable at a higher temperature. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated some physical properties of Co3In2−xSnxS2 (x = 0, 1, and 2) compounds. The doping in Co3In2S2, through chemical substitution of indium by tin as a low-cost neighboring element, affects their structural, electronic, magnetic, thermodynamic, and thermoelectric properties. The density functional theory (DFT) calculations show that indium substitution leads to a transition from weak-ferromagnetic metal (x = 0), to nonmagnetic semiconductor with low band gap energy at x = 1, and to a ferromagnetic half-metal at x = 2. The thermal properties, obtained by using the Gibbs code, were evaluated with temperature at various pressures from 0 to 20 GPa. The results demonstrated that chemical substitution in the studied materials affects their physical properties leading to an interest candidate for thermoelectric uses at ambient or at low temperature. [ABSTRACT FROM AUTHOR]

The crystal structure, mechanical, and electronic properties of W0.71Fe0.15B3 under pressure were studied by first principles. Our results show that the structural parameters obtained by geometry optimization are in agreement with other experimental and theoretical results; the main effect of pressure on the structure is compression along the c-axis. The independent elastic constants, mechanical modules, and the Debye temperature increase under pressure, whereas the hardness decreases. Born's structural stability criteria shows that the structure with space group P63/mmc is mechanically stable up to 50 GPa; while, Pugh's and Poisson criteria suggest a transition from brittle to ductile between 30 and 35 GPa. Finally, the density of states at the Fermi energy decreases and a charge transfer from W/Fe to B under pressure is determined. [ABSTRACT FROM AUTHOR]

A two-orbital two-electron diatomic model resembling LiH is used to investigate the differences between the exact Löwdin–Shull and approximate Hartree–Fock–Bogoliubov and Baerends–Buijse density matrix functionals in the medium- to long-distance dissociation region. In case of homolytic dissociation (one electron on each atom), the approximate functionals fail to generate the correct energy due to a compromise between the Hartree–Fock component (which favors partial charge transfer) and the strong correlation component (which hampers charge transfer). The exact functional is able to generate the physically correct answer by enforcing the equi-charge distribution of the bonding and antibonding orbitals. Besides, the approximate functionals also have issues in correctly describing heterolytic dissociation (two electrons on one atom) due to the strong correlation component hampering charge transfer. In this work, we propose a new scheme in which the homolytic dissociation problem for approximate functionals is avoided by adding a Lagrange multiplier that enforces equi-charge distribution of the bonding and antibonding orbitals. The symmetry-based nature of the findings implies that they are most likely transferable to other cases in which one uses an approximate one-particle method in conjunction with a symmetrical particle-hole correction factor. [ABSTRACT FROM AUTHOR]

The full potential linear augmented plane wave method including Hubbard potential and spin-orbit coupling are performed to study the structural, electronic, magneto-optical and magnetic anisotropy properties of tetragonal BiFeO3. Using the exchange correlations potentials generalized gradient plus Hubbard parameter (GGA + U) approximations are used for the description of electron-electron interactions. We studied first the structural properties which present a tetragonal distortion results from the stereochemical 6s2 lone pair of Bi+2 and the Jahn-Teller (JT) distortion effect of Fe+3 and the value of c∕a = 1.28. The calculated gap is 2.0 eV at Ueff = 4 eV. The magnetic moment of Fe in phase is 3.65 μB. Kerr and ellipticity are calculated by using a spin-orbit coupling and Hubbard potential which present a high angles values −1.0° and 1.5° respectivly. In plane uniaxial and fourfold anisotropy constants are determined from the fit curves of DFT calculation. We observed a predominance of uniaxial anisotopy on the fourdfold anisotropy. [ABSTRACT FROM AUTHOR]

The physical properties of alkaline-earth zirconates AZrO3 (A = Ca, Ba, Sr) are revealed using density functional theory (DFT) based FP-LAPW+lo approach. The present study investigates the structural, optoelectronic, and thermoelectric features, which are elucidated using GGA-PBEsol functional. The changing A cations from Ba to Sr to Ca result in increasing lattice constant comparable with experimental data and reducing bulk modulus. The CaZrO3 exhibits comparatively higher stiffness or hardness than that of the SrZrO3 and BaZrO3. The applied pressure improves mechanical stability by increasing ductility. Moreover, electronic structures are computed under varying pressures 0–30 GPa. All three compounds show indirect bandgap (Γ–M) up to 20 GPa, and the transition to direct bandgap (Γ–Γ) is illustrated at 30 GPa. Consequently, the significance of optoelectronic applications is revealed. The pressure-dependent various optical parameters are also explored and validation of Penn's model, transparency, and maximum reflectivity at specific energy ranges expose their possible commercial candidature. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated some physical properties of Co3In2−xSnxS2 (x = 0, 1, and 2) compounds. The doping in Co3In2S2, through chemical substitution of indium by tin as a low-cost neighboring element, affects their structural, electronic, magnetic, thermodynamic, and thermoelectric properties. The density functional theory (DFT) calculations show that indium substitution leads to a transition from weak-ferromagnetic metal (x = 0), to nonmagnetic semiconductor with low band gap energy at x = 1, and to a ferromagnetic half-metal at x = 2. The thermal properties, obtained by using the Gibbs code, were evaluated with temperature at various pressures from 0 to 20 GPa. The results demonstrated that chemical substitution in the studied materials affects their physical properties leading to an interest candidate for thermoelectric uses at ambient or at low temperature. [ABSTRACT FROM AUTHOR]

The full potential linear augmented plane wave method including Hubbard potential and spin-orbit coupling are performed to study the structural, electronic, magneto-optical and magnetic anisotropy properties of tetragonal BiFeO3. Using the exchange correlations potentials generalized gradient plus Hubbard parameter (GGA + U) approximations are used for the description of electron-electron interactions. We studied first the structural properties which present a tetragonal distortion results from the stereochemical 6s2 lone pair of Bi+2 and the Jahn-Teller (JT) distortion effect of Fe+3 and the value of c∕a = 1.28. The calculated gap is 2.0 eV at Ueff = 4 eV. The magnetic moment of Fe in phase is 3.65 μB. Kerr and ellipticity are calculated by using a spin-orbit coupling and Hubbard potential which present a high angles values −1.0° and 1.5° respectivly. In plane uniaxial and fourfold anisotropy constants are determined from the fit curves of DFT calculation. We observed a predominance of uniaxial anisotopy on the fourdfold anisotropy. [ABSTRACT FROM AUTHOR]

In the present work, a systematic analysis of the microscopic plastic deformation mechanism and mechanical properties of interstitial impurities (H, O, N, S, and P) and Ni doped, or not doped, with Cr, was conducted based on generalized stacking fault energy curves generated via first-principles calculations. Focus has been put on the effects of Cr on plastic deformation for Ni doped interstitial impurities, upon the GPFE curve, with the aim to investigate the effects of Cr on the deformation mode and mechanical properties for doped Ni systems. It is found that a solid solution of Cr caused the tendency of partial dislocation in Ni. The evaluation of the Rice criterion reveals that Cr tends to decrease the ductility in Ni, and it cannot reverse interstitial H promoting the probability of cleavage fracture in Ni, while increases the ductility of O, P and S doped Ni, particular in O doped Ni, due to increasing the value of ductility D remarkably, so possibly changes the tendency of cleavage fracture. Besides, the solid solution of Cr is beneficial in promoting the dissociation of dislocation into fragments more easily in Ni, and enhances dislocation nucleation, while O, N, and S impurities have a slower rate of partial dislocation emission in Ni when interacting with Cr. Furthermore, Cr promotes the probability of twinning in Ni, and probably switches the deformation mechanism of H doped Ni from dislocation mediated slipping to twinning. Our study provides important insights toward the understanding and control of dislocation dynamics in doped Ni. [ABSTRACT FROM AUTHOR]

Recent theoretical works on NiO have not agreed upon the nature of the first optical transition. By altering band filling – with highly concentrated O vacancies and Fe impurities – here, the orbital density of states is changed near the Fermi energy. The variation in optical properties, relative to the changes in orbital character, along with group theory analysis of hybridized orbitals, provides new insight when evaluating the first optical transition of NiO. Here, based on density functional theory, the first optical transition is found to have two possibilities – either superexchange site-hopping or a transition from the hybridized eg state to the hybridized a1u state, rather than the intra-atomic transitions which are causing disagreement in the recent literature. [ABSTRACT FROM AUTHOR]

Based on the density functional theory, we have calculated an atomic and electronic structures of the Pd2Sn/Pd(111)-(√3 × √3)R30° surface alloy including spin orbit coupling (SOC) effects. In the single layer Pd2Sn surface alloy, Sn segregated structure in which Pd occupies the second layer while Sn floats to the surface is found to be energetically favourable by 0.14 eV. This structure allows alloy formation at the surface in the form of Pd2Sn as suggested by recent experimental study. Considering second internal Pd2Sn alloy layer together with outmost Pd2Sn alloy layer, we have determined out of plane corrugation which has a corrugation value of 0.26 Å. In the single layer Pd2Sn surface alloy, the related electronic band structure and characteristic of the surface states have been calculated. An examination of origin of these surface states is mainly contributed by hybridization between s- and p-like orbitals of Sn atom and s-, p-, and d-like orbitals of Pd atom. This surface exhibits smaller Rashba spin splitting, i.e. the amount of split of the surface bands is about 0.05 eV in PBE functional and 0.03 eV in LDA functional within SOC effects. [ABSTRACT FROM AUTHOR]

Finite element calculations have been performed in Cartesian coordinates using the density functional approach for a number of small molecules. In order to aid convergence of the orbitals and total energies a suitable cusp factor was employed, such that the resulting effective potential is non-singular at all nuclei. The resulting total energies and densities were compared with those obtained using the Gaussian basis set package NWChem [M. Valiev et al., Comput. Phys. Commun. 181, 1477 (2010)] and excellent agreement was found. [ABSTRACT FROM AUTHOR]