Agreement Withexperiment Research Articles

Simulating Ultrasound Piezoelectric Power Transfer in the Near Field and Experimental Validations

Background: Acoustic power transfer is a method for wireless energy transfer to implanted medical devices that permits a greater range of separation between transmitter and receiver than is possible with inductive power transfer. In some cases, short-distance ultrasonic power transfer may be employed; consequently, their operation may be complicated by the near-field aspects of piezoelectric acoustic energy transfer. Methods: A piezoelectric energy transfer system consisting of two lead zirconate titanate (PZT) transducers was analyzed in this work using a combination of experimental measurements and computer simulations. Results: Simulations using the COMSOL Software package showed good agreement with measured output voltage as a function of the distance between and alignment of the transmitter and receiver with water as a medium. We also simulated how operating frequency affects power transfer efficiency at various distances between the transmitter and receiver and found reasonable agreement with experiments. We report model predictions for power transfer efficiency as a function of the thickness and diameter of the transmitter and receiver. Conclusion: The results show that with proper choice of parameters, piezoelectric systems can provide high power transfer efficiency in the near-field region.

Li4SnS4: Simulations of Its Structure and Electrolyte Properties

First principles simulations show that the ground state crystal structureof Li4SnS4 is isomorphous to Li4GeS4, in agreementwith experiment, and having interstitial sites in void channels along the c-axis. The simulations also show thata slightly meta-stable structure (Li4SnS4*) is formed by moving Li ions from their central sites to the interstitial positions, consistentwith X-ray patterns resulting from relatively low temperature synthesis methods. The simulations suggest that the Li4SnS4* material hasfavorable ion conducting properties.

Surface phonons on Pb(111)

We present an ab initio study of the lattice dynamics of the Pb(111) surface. Thecalculations were carried out within the density-functional theory using a linear responseapproach in the mixed-basis pseudopotential representation. We observe a rich spectrum ofsurface localized modes, and make a detailed assignment to measured modes in a recenthelium-atom scattering experiment. We find that the inclusion of spin–orbit couplingconsiderably softens the phonon spectrum of the surface, thereby improving the agreementwith experiment significantly.

Cu2O behavior under pressure: an ab initio study

We have performed density-functional theory calculations for three crystallographic phases (cuprite,CdI2, andCdCl2) of the cuprous oxide by using both the local-density approximation (LDA) and thePerdew–Burke–Ernzerhof generalized-gradient approximation. The latter gives a very gooddescription of the properties of the cuprite phase at room temperature. In particular, thebulk modulus and the elastic constants at zero pressure are in excellent agreement withexperiment. At 10 GPa (7 in LDA calculations), the transition from the cuprite to theCdI2 phase occurs, and the latter remains the phase having the smallest Gibbs energy up to themaximum pressure we have considered (20 GPa). We have also determined the elastic constants ofCu2O in the cuprite phase for various applied pressures. The results indicate that this structurebecomes unstable with respect to trigonal deformations before the transition to theCdI2 phase. On the other hand, no indication of instability with respect to tetragonaldeformations has been found. This kind of instability would occur at pressures greater thanthe phase transition pressure.

Electric-dipole transitions between group-III acceptor states in uniaxially compressedGe

We study the electric-dipole transitions between group-III acceptor states in Ge understress along the [001] crystallographic direction in the effective mass approximation. Wesystematically investigate the cases of zero stress, infinitesimal stress, and finite stressincluding the low-stress and the high-stress regions. Our results show quantitativeagreement with experimental data at zero stress and at infinitesimal stress. Therelative intensities of infinitesimal-stress-induced components of transitions from the1Γ8 + state tothe nΓ8 − states do not correlate significantly with the species of acceptors except for the transition to the1Γ8 − state. The oscillator strengths of some transitions are susceptible to the stress in the low-stress region(<0.3 kbar), and could be zero at a specific stress. The behaviours of the stress dependence ofoscillator strengths for different transition lines are explained in terms of the compositionsof the wavefunctions and the dipole matrix elements. In the high-stress region ( kbar), the ground state is s-like, and only the transitions to the p-like states can havenon-negligible oscillator strengths. The photon absorbed (emitted) and associated witheach electric-dipole transition between the s-like and the p-like states is polarized eitherparallel or perpendicular to the stress direction. We also calculate the absorption spectrafor Ge:Ga at liquid-helium temperature. The results are in good agreement withexperiment.

A molecular dynamics simulation of the diffusion of the solute (Au) and the self-diffusion ofthe solvent (Cu) in a very dilute liquid Cu–Au solution

The identification of the manner in which a solute diffusion coefficient (D) might vary withtemperature (T) in a fused metal or semimetal has led to considerable experimental study and sometheoretical analysis. However, the conclusions of this work are inconsistent. In the presentwork, molecular dynamics studies of diffusion of a very dilute solute (Au) in liquid Cu arepresented. Using the simple Enskog theory of diffusion, it is shown that the ratio of thediffusion constant of the solute to the diffusion constant of the solvent for a very dilutesolution is approximately constant. This prediction is confirmed by molecular dynamicssimulations although the values of ratios agree only within 20%–25%. In agreement withexperiment, current simulations predict that within the usually investigated temperaturerange, the diffusion coefficient is linearly dependent on temperature. A very smallcontribution of parabolic behavior can only be observed for a temperature range muchwider than that available for physical experiments due to materials limitations.

Full potential results on the magneto-optical properties of the Heusler compoundsCo2FeX (X = Al, Ga, Si and Ge)

We have calculated the magneto-optical (MO) properties ofCo2FeX (X = Al, Ga, Si and Ge) Heusler compounds using the full potential linearized augmented planewave (FPLAPW) method as implemented in the WIEN2K code using the local spin densityapproximation (LSDA) and also by using the generalized gradient approximation (GGA)for the electronic exchange and correlation. In all the compounds, Kerr rotationθK has a strong minimum near 2 eV, the value of|θK| corresponding to this minimum being almost as large as in pure Co–Fecompounds. The calculated MO spectra help to identify the features of theexperimental spectra. A comparison of the results shows that the Kerr spectrum isquite similar from both LSDA and GGA but the latter gives better agreementwith experiment. Moreover, we find that inclusion of correlation effects usingGGA+U removes the discrepancy in magnetic moment ofCo2FeX (X = Si, Ge) though it has an insignificant effect on the Kerr spectra.

Quantum theory of the anisotropy of the magnetic properties of ferrimagnetic holmiumiron garnet single crystals

The pronounced anisotropy of the magnetization caused by theHo3+ ions in the ferrimagnetic holmium iron garnet has been investigated basedon quantum theory. The strong anisotropy of the magnetization of theHo3+ ions originates mainly from the effect of the crystal field upon theHo3+ ions and theanisotropic Ho3+–Fe3+ superexchange interaction. Following the expression of theYb3+–Fe3+ exchange interaction used by Alben, the anisotropy of theHo3+–Fe3+ exchange interaction is defined by three principal values of the exchange tensorG. Becausethe six Ho3+ sublattices are magnetically non-equivalent, we calculate the magnetic quantities of theHo3+ at the six sublattices and compare the average value of the so-obtained six quantitieswith the measured values. The calculated results are in good agreement withexperiments. An interpretation on the anisotropy of the magnetic properties of HoIG isgiven.

X-ray absorption near edge structure/electron energy loss near edge structurecalculation using the supercell orthogonalized linear combination of atomic orbitalsmethod

Over the last eight years, a large number of x-ray absorption near edge structure (XANES)and/or electron energy loss near edge structure (ELNES) spectroscopic calculations forcomplex oxides and nitrides have been performed using the supercell-OLCAO (orthogonalizedlinear combination of atomic orbitals) method, obtaining results in very good agreementwith experiments. The method takes into account the core-hole effect and includes thedipole matrix elements calculated from ab initio wavefunctions. In this paper, we describethe method in considerable detail, emphasizing the special advantages of this method forlarge complex systems. Selected results are reviewed and several hitherto unpublishedresults are also presented. These include the Y K edge of Y ions segregated to the core of aΣ31 grain boundary in alumina, O K edges of water molecules, C K edges in different types ofsingle walled carbon nanotubes, and the Co K edge in the cyanocobalamin (vitaminB12) molecule. On the basis of these results, it is argued that the interpretation of specificfeatures of the calculated XANES/ELNES edges is not simple for complex material systemsbecause of the delocalized nature of the conduction band states. The long-standing notionof the ‘fingerprinting’ technique for spectral interpretation of experimental data is nottenable. A better approach is to fully characterize the structure under study, using eithercrystalline data or accurate ab initio modeling. Comparison between calculatedXANES/ELNES spectra and available measurements enables us to ascertain the validity ofthe modeled structure. For complex crystals or structures, it is necessary to use theweighted sum of the spectra from structurally nonequivalent sites for comparison with themeasured data. Future application of the supercell-OLCAO method to complexbiomolecular systems is also discussed.

Vibrational properties of crystallineSb2Te3 from first principles

Phonon dispersion relations and infrared and Raman spectra of crystallineSb2Te3 were computed within density functional perturbation theory. Overall good agreement withexperiments is obtained, which allows us to assign the Raman and IR peaks to specificphonons.

Theory of Umklapp-assisted recombination of bound excitons in Si:P

We present calculations for the oscillator strength of the recombination of excitonsbound to phosphorus donors in silicon. We show that the direct recombination ofthe bound exciton cannot account for the experimentally measured oscillatorstrength of the no-phonon line. Instead, the recombination process is assisted byan Umklapp process of the donor electron state. We make use of the empiricalpseudopotential method to evaluate the Umklapp-assisted recombination matrixelement in second-order perturbation theory. Our result is in good agreementwith experiment. Being potentially useful for quantum computing, the process ofUmklapp-assisted recombination can be used to detect optically the spin state of thenucleus of a phosphorus donor, which requires that the energy levels of the nuclearspin are optically resolvable. We therefore present two methods to improve theoptical resolution of the optical detection of the spin state of a single nucleus inSi:P.

Optical Anisotropy of Quantum Disks in the External Static Magnetic Field

We show how to compute the optical functions (the complex magneto-susceptibility, dielectric function, magneto{re°ection and ellipsometric spec-tra) for semiconductor quantum disks exposed to a uniform magnetic fleldin the growth direction, including the excitonic efiects. The optical responseis calculated for an oblique incidence of the propagating electromagneticwave and for input waves with difierent polarization. The method uses themicroscopic calculation of nanostructure excitonic wave functions and en-ergy levels, and the macroscopic real density matrix approach to computethe electromagnetic flelds and susceptibilities. The electron{hole screenedCoulomb potential is adapted and the valence band structure is taken intoaccount in the cylindrical approximation, thus separating light- and heavy--hole motions. The novelty of our approach is that the solution is obtainedin terms of known one-particle electron and hole eigenfunctions, since, in theconsidered nanostructure due to conflnement efiects accompanied by the e{hCoulomb interaction, the separation of the relative- and center-of-mass mo-tion is not possible. We obtain both the eigenvalues and the eigenfunctions.The convergence of the proposed method is examined. We calculate themagnetooptical functions, including the optical Stokes parameters and ellip-sometric functions for the case of oblique incidence. Numerical calculationswere performed for InAs (disk)/ GaAs (barrier) disks. A good agreementwith experiments was obtained.PACS numbers: 71.35.Cc, 71.35.Ji, 73.21.La, 78.20.Ls, 78.67.Hc

Electrical resistivity of liquid Mg–Al, Na–Cs, Ni–Ce, Li–Cd and Mg–Cd alloys within the‘2kF’ scattering model

The resistivities of liquid Mg–Al, Na–Cs, Ni–Ce, Li–Cd and Mg–Cd alloys werecalculated. The Faber–Ziman diffraction model and the Morgan–Howson–Saub‘2kF’ scattering model were used in calculations. The results obtained within these two modelsfor the same alloy and with use of the same set of parameters differ substantially, even foralloys that are not highly resistive. It is possible in principle to achieve good agreementwith experiment by means of each of these two models; however we suggest that the‘2kF’ scattering model is the appropriate method of calculation of the electrical resistivity forliquid alloys because the quantum interference effects should always be taken intoaccount.

Nanoindentation and nanoscratching of rutile and anataseTiO2 studied using molecular dynamics simulations

The results of molecular dynamics (MD) simulations of nanoindentationand nanoscratching of titanium dioxide lattices are presented. TheTiO2 surfaces investigated are the (001) and {110} rutile surfaces and the anatase {100} and (001)surfaces. A spring model consisting of a cube cornered indenter connected tosupports, via a set of springs, was used to model the cantilever in an atomic forcemicroscope. Indentations were conducted to depths of 12 and 18 Å on each surface andscratching simulations were conducted for a length of 80 Å and at a depth of 6 Å.Pile-up patterns were found to be dependent on the crystal surface orientation. Noevidence of the activation of slip systems was discovered. The relative values ofthe contact pressure and Young’s modulus were in reasonable agreement withexperiment and the coefficient of friction was found to be independent of theTiO2 polymorph.

Bulk and surface structure of the clean and adsorbate-covered decagonal Al–Co–Niquasicrystal

We review our Al adsorption experiments on the tenfold-symmetry surface of the decagonalAl–Co–Ni quasicrystal and present computational simulations of adsorption on a structuralmodel based on a fundamental Al–Co cluster with 20 Å diameter, symmetry , and 8 Å periodicity. This cluster is the building unit ofτ2-Al13Co4, from which, by a sequence of minor changes, the structures of the phases in the stabilityregion of decagonal Al–Co–Ni can be derived. The model used for the decagonalAl70Co15Ni15 is an idealized model with a two-layer periodicity (4 Å) and no chemical or structuraldisorder. We find that the bulk and surface properties of this model are in good agreementwith experiments. Our molecular-dynamics simulations of Al adsorption reproduce theexperimental results and show that by varying the thermal relaxation rates of the adsorbedlayer, a variety of different surface morphologies can be achieved. We also present ourrecent experiments on dissociative adsorption of oxygen on the decagonal surface.

Vibrational spectra of vitreous SiO2 and vitreous GeO2 from first principles

Using a density-functional approach, we calculate the principal vibrational spectra of vitreousSiO2 andvitreous GeO2 and discuss their analogies and differences. For both glasses, we generate model structuresconsisting of a random network of corner-sharing tetrahedra and differing only by theirpacking density. The comparison between calculated and measured neutron structurefactors supports the validity of our model structures. Our investigation then extends to thevibrational properties, including the inelastic-neutron, infrared, and Raman spectra. Forthese spectra, good agreement with experiment is also found. Our results supportthe picture that silica and germania are constituted by a continuous randomnetwork of corner-sharing tetrahedra. In particular, the good agreement withexperiment for the Raman spectra supports the average intertetrahedral angles of148° and135° found in ourmodels of vitreous SiO2 and vitreous GeO2, respectively. The concentration of small ring structures in these glasses is also discussed.

First-principles study of ω-phase formation in the Ti3Al2V system

Using first-principles methods, the phase stability of the underlying body-centered-cubic (bcc) structureof Ti3Al2V and slightly rearranged atomic structures are investigated. Thecalculated ground-state energies show an instability in the ternaryTi3Al2V alloy withrespect to the ω structure-type atomic displacement. A Mulliken population analysis shows strong bondingbetween the transition metals and Al. It is shown that Ti–Al is the strongest bond and thatω-type displacements increase the population overlap for this bond and reduce the energy ofthe system. The first-principles calculations are extended to finite temperature andvarious contributions to the free energy are calculated within the quasiharmonicapproximation. It is shown that, at high temperatures, the bcc structure is stabilized bythe contribution of the low-energy modes to lattice entropy. In agreement withexperiment and in contrast to the Ti–Al–Nb system, we find that the metastableB82 structure cannot form in this alloy.

Time-Dependent Density-Functional Calculations forOptical Spectra of Na2 and Na4 Clusters

With the frame of the time-dependent local density approximation,an efficient description of the optical response of clusters hasbeen used to study the photo-absorption cross section of Na2and Na4 clusters. It is shown that our calculated results arein good agreement with the experiment. In addition, our calculatedspectrum for the Na4 cluster is in better agreement withexperiment than the GW absorption spectrum.

The Role of Simulations in Nanoscience, a Case Study: Gold Nanowires

Suspended gold nanowires can be made atomically thin with as many as five atoms, showing extremely large Au-Au bond distances. Using tools derived from Density Functional Theory (DFT) we study many questions posed by the experiments. First we use realistic molecular dynamics simulation to study the mechanisms of formation, evolution and breaking of these atomically thin Au nanowires under stress. We show how defects induce the formation one-atom chains that can grow as long as five-atoms before breaking. Results are in excellent agreement with experiments, except for the resulting shorter bond distances. In order to address this question, we use ab initio electronic structure calculations to show that the exceedingly large Au-Au interatomic distances experimentally obtained could be the effect of impurities. We studied the effect of single impurities H, B, C, N, O, S and small molecules as H2 on the nanowire's electronic and structural properties, in particular how they affect the maximum Au-Au bond length.

Numerical Evaluation of Ultimate Strengths of Composites Considering Both In-Plane Damage and Delamination

Traditional laminate strength analysis only considers face failure under in-plane loads. In fact, owing to the mismatch of the mechanical properties of the adjacent layers, a three-dimensional interlaminar singular stress fields develop in a small boundary region in the vicinity of the free edges of the laminate under mechanical load, which may lead to interlaminar delamination failure. Neglecting this interlaminar failure mode, the failure strength of laminate will be overestimated. In this paper, face failure and interlaminar failure are both considered. So for a lamina, three major failure modes are considered: matrix failure, fiber breakage and delamination. Finite element method is used to obtain the stresses in a laminate under mechanical loads. Stress-based criterions are adopted to predict the failure mode of laminas. When a lamina is failed, the lamina stiffness is reduced according to the corresponding failure mode, and the stresses of the laminate are re-analyzed. This procedure is repeatedly performed until the whole laminate fails and thus the ultimate strength is determined. The predicted ultimate strengths are in good agreement with experiment results in the open literature.