Aleksandr V. Chernatynskiy

Kapitza resistance of Si/SiO2 interface

A phonon wave packet dynamics method is used to characterize the Kapitza resistance of a Si/SiO2 interface in a Si/SiO2/Si heterostructure. By varying the thickness of SiO2 layer sandwiched between two Si layers, we determine the Kapitza resistance for the Si/SiO2 interface from both wave packet dynamics and a direct, non-equilibrium molecular dynamics approach. The good agreement between the two methods indicates that they have each captured the anharmonic phonon scatterings at the interface. Moreover, detailed analysis provides insights as to how individual phonon mode scatters at the interface and their contribution to the Kapitza resistance.

Crossover in thermal transport properties of natural, perovskite-structured superlattices

Atomic-level simulations are used to analyze the thermal-transport properties of a naturally layered material: the Ruddlesden–Popper phase, formed by interleaving perovskite layers of strontium titanate with strontium oxide rocksalt layers. The thermal conductivity parallel to the plane of structural layering is found to be systematically greater than that perpendicular to the layering. With decreasing number of perovskite blocks in the structure, a transition is seen from the thermal-transport properties of a bulk solid containing interfaces to that of an anisotropic monolithic material. The exact transition point should be temperature dependent and might enable tuning of the thermal conductance properties of the material.

Phonon Transport Simulator (PhonTS)

Abstract Thermal conductivity prediction remains an important subject in many scientific and engineering areas. Only recently has such prediction become possible on the basis of the first principles calculations, thus ensuring high quality results. Implementation of the methodology, however, is technically challenging and requires a lengthy development process. We thus introduce the Phonon Transport Simulator (PhonTS), a Fortran90, fully parallel code to perform such calculations. PhonTS possesses a large array of options and returns the thermal conductivity tensor together with related quantities, such as spectral thermal conductivity, phonon lifetimes, mean free paths and Gruneisen parameters. First principles calculations are implemented via convenient interfaces to widely-used third-party codes, while many classical potentials are included in PhonTS itself. The code is carefully validated against data published in the literature from various thermal conductivity computational techniques and against experimental data. Program summary Program title: PhonTS Catalogue identifier: AEVO_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEVO_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: GNU General Public License, version 3 No. of lines in distributed program, including test data, etc.: 7060788 No. of bytes in distributed program, including test data, etc.: 58119383 Distribution format: tar.gz Programming language: Fortran90. Computer: Linux cluster. Operating system: Linux. Has the code been vectorised or parallelized?: Yes, via the MPI-libraries RAM: 200 MB Classification: 7.8, 7.9. External routines: LAPACK, MPI-libraries Nature of problem: Computes thermal conductivity in crystal solids from the level of the interatomic interactions. Solution method: Iterative or variational solution of the Boltzmann Transport Equation for phonons, by treating anharmonicity as a perturbation to the harmonic problem. Additional comments: Due to the large file size, PhonTS is not delivered directly when download or Email is requested. Instead an html file giving details of how the program can be obtained is sent. Running time: Problem dependent. For solid argon (4 atoms in the conventional cell, k -space mesh of 9×9×9, classical potential, fully convergent results) simulations take ∼ 10 4 seconds on a single processor.

Phonon Density of States and Anharmonicity of UO 2

Phonon density of states (PDOS) measurements have been performed on polycrystalline UO2 at 295 and 1200 K using time-of-flight inelastic neutron scattering to investigate the impact of anharmonicity on the vibrational spectra and to benchmark ab initio PDOS simulations performed on this strongly correlated Mott-insulator. Time-of-flight PDOS measurements include anharmonic linewidth broadening inherently and the factor of ~ 7 enhancement of the oxygen spectrum relative to the uranium component by the neutron weighting increases sensitivity to the oxygen-dominated optical phonon modes. The first-principles simulations of quasi-harmonic PDOS spectra were neutron-weighted and anharmonicity was introduced in an approximate way by convolution with wavevector-weighted averages over our previously measured phonon linewidths for UO2 that are provided in numerical form. Comparisons between the PDOS measurements and the simulations show reasonable agreement overall, but they also reveal important areas of disagreement for both high and low temperatures. The discrepancies stem largely from an ~ 10 meV compression in the overall bandwidth (energy range) of the oxygen-dominated optical phonons in the simulations. A similar linewidth-convoluted comparison performed with the PDOS spectrum of Dolling et al. obtained by shell-model fitting to their historical phonon dispersion measurements shows excellent agreement with the time-of-flight PDOS measurements reported here. In contrast, we show by comparisons of spectra in linewidth-convoluted form that recent first-principles simulations for UO2 fail to account for the PDOS spectrum determined from the measurements of Dolling et al. These results demonstrate PDOS measurements to be stringent tests for ab initio simulations of phonon physics in UO2 and they indicate further the need for advances in theory to address lattice dynamics of UO2.

Charge optimized many-body (COMB) potential for Al 2 O 3 materials, interfaces, and nanostructures

This work presents the development and applications of a new empirical, variable charge potential for Al2O3 systems within the charge optimized many-body (COMB) potential framework. The potential can describe the fundamental physical properties of Al2O3, including cohesive energy, elastic constants, defect formation energies, surface energies and phonon properties of α-Al2O3 comparable to that obtained from experiments and first-principles calculations. The potential is further employed in classical molecular dynamics (MD) simulations to validate and predict the properties of the Al (1 1 1)-Al2O3 (0 0 0 1) interface, tensile properties of Al nanowires, Al2O3 nanowires, Al2O3-covered Al nanowires, and defective Al2O3 nanowires. The results demonstrate that the potential is well-suited to model heterogeneous material systems involving Al and Al2O3. Most importantly, the parameters can be seamlessly coupled with COMB3 parameters for other materials to enable MD simulations of a wide range of heterogeneous material systems.

Charge optimized many-body potential for aluminum.

An interatomic potential for Al is developed within the third generation of the charge optimized many-body (COMB3) formalism. The database used for the parameterization of the potential consists of experimental data and the results of first-principles and quantum chemical calculations. The potential exhibits reasonable agreement with cohesive energy, lattice parameters, elastic constants, bulk and shear modulus, surface energies, stacking fault energies, point defect formation energies, and the phase order of metallic Al from experiments and density functional theory. In addition, the predicted phonon dispersion is in good agreement with the experimental data and first-principles calculations. Importantly for the prediction of the mechanical behavior, the unstable stacking fault energetics along the [Formula: see text] direction on the (1 1 1) plane are similar to those obtained from first-principles calculations. The polycrsytal when strained shows responses that are physical and the overall behavior is consistent with experimental observations.

Phonon thermal transport through tilt grain boundaries in strontium titanate

In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO3. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO3 contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scatteri...

A Coherent Phonon Pulse Model for Transient Phonon Thermal Transport

Abstract In this work, we present a novel heat source model, the coherent phonon pulse (CPP), composed of spatiotemporal Gaussian wave packets to mimic the coherent excitation of a non-equilibrium phonon population by ultrashort laser techniques, for the study of transient phonon thermal transport. Through molecular dynamic simulations of phonon transport in bicrystalline silicon-nanowires containing Σ 3 and Σ 19 grain-boundaries (GBs), we demonstrate that the new model facilitates not only a quantitative measurement of phonon-interface scattering, but also a mechanistic understanding of the highly non-equilibrium process of phonon transport with the coherent wave nature being preserved.

A molecular dynamics study of tilt grain boundary resistance to slip and heat transfer in nanocrystalline silicon

We present a molecular dynamics study of grain boundary (GB) resistance to dislocation-mediated slip transfer and phonon-mediated heat transfer in nanocrystalline silicon bicrystal. Three most stable ⟨110⟩ tilt GBs in silicon are investigated. Under mechanical loading, the nucleation and growth of hexagonal-shaped shuffle dislocation loops are reproduced. The resistances of different GBs to slip transfer are quantified through their constitutive responses. Results show that the Σ3 coherent twin boundary (CTB) in silicon exhibits significantly higher resistance to dislocation motion than the Σ9 GB in glide symmetry and the Σ19 GB in mirror symmetry. The distinct GB strengths are explained by the atomistic details of the dislocation-GB interaction. Under thermal loading, based on a thermostat-induced heat pulse model, the resistances of the GBs to transient heat conduction in ballistic-diffusive regime are characterized. In contrast to the trend found in the dislocation-GB interaction in bicrystal models with different GBs, the resistances of the same three GBs to heat transfer are strikingly different. The strongest dislocation barrier Σ3 CTB is almost transparent to heat conduction, while the dislocation-permeable Σ9 and Σ19 GBs exhibit larger resistance to heat transfer. In addition, simulation results suggest that the GB thermal resistance not only depends on the GB energy but also on the detailed atomic structure along the GBs.

Computational discovery of lanthanide doped and Co-doped Y3Al5O12 for optoelectronic applications

We systematically elucidate the optoelectronic properties of rare-earth doped and Ce co-doped yttrium aluminum garnet (YAG) using hybrid exchange-correlation functional based density functional theory. The predicted optical transitions agree with the experimental observations for single doped Ce:YAG, Pr:YAG, and co-doped Er,Ce:YAG. We find that co-doping of Ce-doped YAG with any lanthanide except Eu and Lu lowers the transition energies; we attribute this behavior to the lanthanide-induced change in bonding environment of the dopant atoms. Furthermore, we find infrared transitions only in case of the Er, Tb, and Tm co-doped Ce:YAG and suggest Tm,Ce:YAG and Tb,Ce:YAG as possible functional materials for efficient spectral up-conversion devices.