The Atomic Cluster Expansion (ACE) provides a formally complete basis for the local atomic environment. ACE is not limited to representing energies as a function of atomic positions and chemical species, but can be generalized to vectorial or tensorial properties and to incorporate further degrees of freedom (DOF). This is crucial for magnetic materials with potential energy surfaces that depend on atomic positions and atomic magnetic moments simultaneously. In this work, we employ the ACE formalism to develop a non-collinear magnetic ACE parametrization for the prototypical magnetic element Fe. The model is trained on a broad range of collinear and non-collinear magnetic structures calculated using spin density functional theory. We demonstrate that the non-collinear magnetic ACE is able to reproduce not only ground state properties of various magnetic phases of Fe but also the magnetic and lattice excitations that are essential for a correct description of finite temperature behavior and properties of crystal defects. © 2024, The Author(s).

Abstract: A set of ab initio calculations of the energy of the ground state as a function of volume, elastic properties, and phonon spectra of tin selenide in its different crystal modifications has been performed. Based on the data set we obtained, the SnSe interatomic interaction potential has been built by implementing the atomic cluster expansion method. The potential has been used to study the temperature dependences of the thermal and elastic characteristics of SnSe in the quasi-harmonic approximation. © 2022, Pleiades Publishing, Ltd.

The atomic cluster expansion (ACE) provides a general, local, and complete representation of atomic energies. Here we present an efficient framework for parametrization of ACE models for elements, alloys, and molecules. To this end, we first introduce general requirements for a physically meaningful description of the atomic interaction, in addition to the usual equivariance requirements. We then demonstrate that ACE can be converged systematically with respect to two fundamental characteristics—the number and complexity of basis functions and the choice of nonlinear representation. The construction of ACE parametrizations is illustrated for several representative examples with different bond chemistries, including metallic copper, covalent carbon, and several multicomponent molecular and alloy systems. We discuss the Pareto front of optimal force to energy matching contributions in the loss function, the influence of regularization, the importance of consistent and reliable reference data, and the necessity of unbiased validation. Our ACE parametrization strategy is implemented in the freely available software package pacemaker that enables largely automated and GPU accelerated training. The resulting ACE models are shown to be superior or comparable to the best currently available ML potentials and can be readily used in large-scale atomistic simulations. ©2022 American Physical Society

Traditionally, interatomic potentials assume local bond formation supplemented by long-range electrostatic interactions when necessary. This ignores intermediate-range multiatom interactions that arise from the relaxation of the electronic structure. Here, we present the multilayer atomic cluster expansion (ml-ACE) that includes collective, semi-local multiatom interactions naturally within its remit. We demonstrate that ml-ACE significantly improves fit accuracy and efficiency compared to a local expansion on selected examples and provide physical intuition to understand this improvement. © 2022 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

The development of classical interatomic potential for iron is a quite demanding task with a long history background. A new interatomic potential for simulation of iron was created with a focus on description of crystal defects properties. In contrast with previous studies, here the potential development was based on force-matching method that requires only ab initio data as reference values. To verify our model, we studied various features of body-centered-cubic iron including the properties of point defects (vacancy and self-interstitial atom), the Peierls energy barrier for dislocations (screw and mix types), and the formation energies of planar defects (surfaces, grain boundaries, and stacking fault). The verification also implies thorough comparison of a potential with 11 other interatomic potentials reported in literature. This potential correctly reproduces the largest number of iron characteristics which ensures its advantage and wider applicability range compared to the other considered classical potentials. Here application of the model is illustrated by estimation of self-diffusion coefficients and the calculation of fcc lattice properties at high temperature. © 2021 American Physical Society.

We devise automated workflows for the calculation of Helmholtz and Gibbs free energies and their temperature and pressure dependence and provide the corresponding computational tools. We employ nonequilibrium thermodynamics for evaluating the free energy of solid and liquid phases at a given temperature and reversible scaling for computing free energies over a wide range of temperatures, including the direct integration of P-T coexistence lines. By changing the chemistry and the interatomic potential, alchemical and upscaling free energy calculations are possible. Several examples illustrate the accuracy and efficiency of our implementation. ©2021 American Physical Society.

The discovery of new structural and functional materials is driven by phase identification, often using X-ray diffraction (XRD). Automation has accelerated the rate of XRD measurements, greatly outpacing XRD analysis techniques that remain manual, time-consuming, error-prone and impossible to scale. With the advent of autonomous robotic scientists or self-driving laboratories, contemporary techniques prohibit the integration of XRD. Here, we describe a computer program for the autonomous characterization of XRD data, driven by artificial intelligence (AI), for the discovery of new materials. Starting from structural databases, we train an ensemble model using a physically accurate synthetic dataset, which outputs probabilistic classifications—rather than absolutes—to overcome the overconfidence in traditional neural networks. This AI agent behaves as a companion to the researcher, improving accuracy and offering substantial time savings. It is demonstrated on a diverse set of organic and inorganic materials characterization challenges. This method is directly applicable to inverse design approaches and robotic discovery systems, and can be immediately considered for other forms of characterization such as spectroscopy and the pair distribution function. © 2021, This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.

We use an analytical model to propose candidate compositionally complex alloys of the Mo-Nb-Ta-W family with optimal yield stress. We then introduce a computationally tractable method based on first-principles calculations to model phase equilibria in complex alloys at arbitrary concentrations. We utilize this method to predict the phase diagram at the optimized compositions and observe a tendency towards ordering for some of the proposed alloys. By combining yield stress data and thermodynamic equilibria, we suggest two alloy compositions with optimal mechanical properties and a strong solid solution forming ability for further experimental validation. © 2021 American Physical Society.

The atomic cluster expansion is a general polynomial expansion of the atomic energy in multi-atom basis functions. Here we implement the atomic cluster expansion in the performant C++ code PACE that is suitable for use in large-scale atomistic simulations. We briefly review the atomic cluster expansion and give detailed expressions for energies and forces as well as efficient algorithms for their evaluation. We demonstrate that the atomic cluster expansion as implemented in PACE shifts a previously established Pareto front for machine learning interatomic potentials toward faster and more accurate calculations. Moreover, general purpose parameterizations are presented for copper and silicon and evaluated in detail. We show that the Cu and Si potentials significantly improve on the best available potentials for highly accurate large-scale atomistic simulations. © 2021, The Author(s).

We investigate the structural, electronic and magnetic properties of LaMnO3/BaTiO3 heterostructure by means of ab initio calculations within the GGA+U approach. We consider the heterostructure when ferroelectric polarization in the BaTiO3 film is oriented perpendicular to the LaMnO3 substrate. We present atom and spin-resolved density of states calculations for LaMnO3/BaTiO3 heterostructure with different number of BaTiO3 overlayers as well as layer-resolved spectra for the conducting heterostructure. We found that the LaMnO3/BaTiO3 heterostructure becomes conducting with a significant spin polarization indicating that the interface becomes ferromagnetically ordered. The propose concept of a ferroelectrically controlled interface ferromagnetism that offers the possibility to design novel electronic devices. © 2020 The Author(s). Published by IOP Publishing Ltd.

Abstract: The behavior of the sound spectrum of free vibrations of cylindrical ceramic samples for light yellow face bricks is interpreted depending on the firing conditions, as well as on reinforcing additives of finely milled fiberglass wastes. A correlation of the frequencies and relaxation times of vibrations with the presence of internal defects and the compressive and bending strengths is found. A testing methodology that reduces the costs of industrial scaling-up of such a reinforcing technology for construction ceramics is proposed. © 2020, Pleiades Publishing, Ltd.

Metal-semiconductor nanostructures are key objects for multifunctional electronics and optical design. We report a new interatomic potential for atomistic simulation of a ternary Si-Au-Al system. The development procedure was based on the force-matching method that allowed us to create the potential without use of experimental data at the fitting. Extensive validation including elastic, thermophysical and defect properties demonstrates a wide range of the potential applicability. Special attention was paid to the description of the silicon-metal alloys in liquid and amorphous states. We used the new potential for study of crystallization and glass transition in the undercooled melt. The simulation results revealed the beneficial conditions for the formation of the unique metal-semiconductor nanocrystalline structure, which is highly important for various applications in the field of nanophotonics. © 2020 Elsevier B.V.

Bond-order potentials (BOPs) provide a local and physically transparent description of the interatomic interaction. Here we describe the efficient implementation of analytic BOPs in the BOPfox program and library. We discuss the integration of the underlying non-magnetic, collinear-magnetic and noncollinear-magnetic tight-binding models that are evaluated by the analytic BOPs. We summarise the flow of an analytic BOP calculation including the determination of self-returning paths for computing the moments, the self-consistency cycle, the estimation of the band-width from the recursion coefficients, and the termination of the BOP expansion. We discuss the implementation of the calculations of forces, stresses and magnetic torques with analytic BOPs. We show the scaling of analytic BOP calculations with the number of atoms and moments, present options for speeding up the calculations and outline different concepts of parallelisation. In the appendix we compile the implemented equations of the analytic BOP methodology and comments on the implementation. This description should be relevant for other implementations and further developments of analytic bond-order potentials. © 2018 Elsevier B.V.

A public data-analytics competition was organized by the Novel Materials Discovery (NOMAD) Centre of Excellence and hosted by the online platform Kaggle by using a dataset of 3,000 (AlxGayIn1–x–y)2O3 compounds. Its aim was to identify the best machine-learning (ML) model for the prediction of two key physical properties that are relevant for optoelectronic applications: the electronic bandgap energy and the crystalline formation energy. Here, we present a summary of the top-three ranked ML approaches. The first-place solution was based on a crystal-graph representation that is novel for the ML of properties of materials. The second-place model combined many candidate descriptors from a set of compositional, atomic-environment-based, and average structural properties with the light gradient-boosting machine regression model. The third-place model employed the smooth overlap of atomic position representation with a neural network. The Pearson correlation among the prediction errors of nine ML models (obtained by combining the top-three ranked representations with all three employed regression models) was examined by using the Pearson correlation to gain insight into whether the representation or the regression model determines the overall model performance. Ensembling relatively decorrelated models (based on the Pearson correlation) leads to an even higher prediction accuracy. © 2019, The Author(s).

We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in a LaAlO3 slab can suppress the surface conductivity. In contrast, in insulating SrTiO3 slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating LaAlO3/SrTiO3 heterostructure slabs with three LaAlO3 overlayers lead to the emergence of interface conductivity. Calculated defect formation energies reveal strong preference of hydrogen dopant atoms for surface sites for all structures and concentrations considered. Strong decrease of the defect formation energy of hydrogen adatoms with increasing thickness of the LaAlO3 overlayer and crossover from positive to negative values, taken together with the metallic conductivity induced by hydrogen adatoms, seamlessly explains the semiconductor-metal transition observed for these heterostructures as a function of the overlayer thickness. Moreover, we show that the potential drop and concomitant shift of (layer resolved) band edges is suppressed for the metallic configuration. Finally, magnetism with stable local moments, which form atomically thin magnetic layers at the interface, is generated by oxygen vacancies either at the surface or the interface, or by hydrogen atoms buried at the interface. In particular, oxygen vacancies in the TiO2 interface layer cause drastic downshift of the 3d eg states of the Ti atoms neighboring the vacancies, giving rise to strongly localized magnetic moments, which add to the two-dimensional background magnetization. © 2019 IOP Publishing Ltd Printed in the UK.

Titanium is the base material for a number of technologically important alloys for energy conversion and structural applications. Atomic-scale studies of Ti-based metals employing first-principles methods, such as density functional theory, are limited to ensembles of a few hundred atoms. To perform large-scale and/or finite temperature simulations, computationally more efficient interatomic potentials are required. In this work, we coarse grain the tight-binding (TB) approximation to the electronic structure and develop an analytic bond-order potential (BOP) for Ti by fitting to the energies and forces of elementary deformations of simple structures. The BOP predicts the structural properties of the stable and defective phases of Ti with a quality comparable to previous TB parameterizations at a much lower computational cost. The predictive power of the model is demonstrated for simulations of martensitic transformations. © 2019 IOP Publishing Ltd.

To support and accelerate the development of simulation protocols in atomistic modelling, we introduce an integrated development environment (IDE) for computational materials science called pyiron (http://pyiron.org). The pyiron IDE combines a web based source code editor, a job management system for build automation, and a hierarchical data management solution. The core components of the pyiron IDE are pyiron objects based on an abstract class, which links application structures such as atomistic structures, projects, jobs, simulation protocols and computing resources with persistent storage and an interactive user environment. The simulation protocols within the pyiron IDE are constructed using the Python programming language. To highlight key concepts of this tool as well as to demonstrate its ability to simplify the implementation and testing of simulation protocols we discuss two applications. In these examples we show how pyiron supports the whole life cycle of a typical simulation, seamlessly combines ab initio with empirical potential calculations, and how complex feedback loops can be implemented. While originally developed with focus on ab initio thermodynamics simulations, the concepts and implementation of pyiron are general thus allowing to employ it for a wide range of simulation topics. © 2019 The Authors

The structural characterization of the various phases that occur in Ti–Ta-based high-temperature shape-memory alloys is complicated by the presence of many competing phases as a function of composition. In this study, we resolve apparent inconsistencies between experimental data and theoretical calculations by suggesting that phase separation and segregation of undesired phases are not negligible in these alloys, and that finite temperature effects should be taken into account in the modeling of these materials. Specifically, we propose that the formation of the ω phase at low Ta content and of the σ phase at high Ta content implies a difference between the nominal alloy composition and the actual composition of the martensitic and austenitic phases. In addition, we show that temperature affects strongly the calculated values of the order parameters of the martensitic transformation occurring in Ti–Ta. © 2018, ASM International.

A computer simulation of a quasi-one-dimensional Coulomb crystal in a parabolic confinement has been carried out using molecular dynamics. The melting transition has been investigated and critical temperatures have been determined from the behavior of the modified Lindemann parameter, translational correlation function, and structure factor. The values obtained for the critical temperature are in good agreement with the earlier experiment. © 2019, Pleiades Publishing, Inc.

The results of the investigations of high conducting and superconducting areas at the interfaces between ferroelectric oxide and insulating oxide are presented. The numerical simulations of BaTiO3/LaMnO3 and BaTiO3/La2CuO4 heterostructures have been performed. It is found that in the samples of the Ba0.8Sr0.2TiO3/LaMnO3 heterostructure the electrical resistance decreases significantly and exhibits metallic behaviour at low temperatures for the case when the c axis of ferroelectric film is directed along the normal to the surface of the single crystal. The superconducting behaviour with transition temperature Tc about 30K has been found at the interface of the Ba0.8Sr0.2TiO3/La2CuO4 heterostructure. The proposed concept promises the ferroelectrically controlled interface conductivity and superconductivity. © Published under licence by IOP Publishing Ltd.

Interatomic potentials are widely used in computational materials science, in particular for simulations that are too computationally expensive for density functional theory (DFT). Most interatomic potentials have a limited application range and often there is very limited information available regarding their performance for specific simulations. We carried out high-throughput calculations for molybdenum and silicon with DFT and a number of interatomic potentials. We compare the DFT reference calculations and experimental data to the predictions of the interatomic potentials. We focus on a large number of basic materials properties, including the cohesive energy, atomic volume, elastic coefficients, vibrational properties, thermodynamic properties, surface energies and vacancy formation energies, which enables a detailed discussion of the performance of the different potentials. We further analyze correlations between properties as obtained from DFT calculations and how interatomic potentials reproduce these correlations, and suggest a general measure for quantifying the accuracy and transferability of an interatomic potential. From our analysis we do not establish a clearcut ranking of the potentials as each potential has its strengths and weaknesses. It is therefore essential to assess the properties of a potential carefully before application of the potential in a specific simulation. The data presented here will be useful for selecting a potential for simulations of Mo or Si. © 2019 IOP Publishing Ltd.

The dynamical behavior of the currency exchange rate after its large-scale catastrophe is discussed through a case study of the rate of Russian rubles to US dollars after its crash in 2014. It is shown that, similarly to the case of the stock market crash, the relaxation is characterized by a power law, which is in analogy with the Omori-Utsu law for earthquake aftershocks. The waiting-time distribution is found to also obey a power law. Furthermore, the event-event correlation is discussed, and the aging phenomenon and scaling property are observed. Comments are made on (non-)Markovianity of the aftershock process and on a possible relevance of glassy dynamics to the market system after the crash. © CopyrightEPLA, 2018.

The structural and electronic properties of heterostructures based on transition metal oxides containing strongly correlated electrons are compared. The investigated structures are LaAlO3/SrTiO3 (LAO/STO), LaAlO3/BaTiO3 (LAO/BTO), and BaTiO3/SrTiO3 (BTO/STO). The role of structural relaxation in the formation of a two-dimensional electron gas at the interface of two dielectrics is revealed. The contribution from different orbitals and atoms to conductivity is analyzed, along with the correlation between structural distortions induced by the dipole moment in an LAO layer and conductivity. © 2018, Allerton Press, Inc.

In the present study, we have performed ab initio calculations of vibrational properties of RbFeS2 compound utilizing density functional theory. Total and element specific phonon densities of states (PDOS) were calculated within a direct approach of harmonic approximation. We used phonon density of states to calculate lattice contribution to the specific heat. The calculated phonon density of states shows a large number of high-frequency vibrational modes of Fe and S atoms, which strongly restricts application of the Debye model for analysis of thermodynamical properties of RbFeS2. The results of the present paper could be used in a further estimation of magnetic specific heat of RbFeS2. © 2018 Author(s).

The magnetic specific heat of RbFeSe2 and the spin state of Fe3+ ions in the compound have been studied. Phonon dispersion and phonon density of states (PDOS), element specific and total, were evaluated from first-principles calculations. It is shown that iron atoms in quasi-one-dimensional chains have dramatically different vibrational properties against Rb and Se atoms: the Fe PDOS is mostly concentrated within two Einstein-like optical phonon peaks at high frequencies. Analysis of our Mössbauer data for RbFeSe2, utilizing the calculated Fe PDOS as well as our optical absorption measurements, have shown full agreement with the location of the high-frequency optical-type lattice vibrations within the FeSe4 tetrahedra. The calculated PDOS was utilized to evaluate the lattice contribution to the specific heat. The phonon heat capacity has been used to evaluate the magnetic specific heat of the quasi-one-dimensional antiferromagnetically correlated Fe3+ ion chains in RbFeSe2. An intermediate spin state S=3/2 has been found most closely relevant to our magnetic entropy analysis for Fe3+ ions in RbFeSe2. © 2018 American Physical Society.

LaAlO3/SrTiO3 heterostructures as covered by the on-site Coulomb repulsion within the GGA U approach is investigated. Performing a systematic variation of the values of the Coulomb parameters applied to the Ti 3d and La 4f orbitals we put previous suggestions to include a large value for the La 4f states into perspective. Furthermore, our calculations provide deeper insight into the band gap landscape in the space spanned by these Coulomb parameters and the resulting complex interference effects. In addition, we identify important correlations between the local Coulomb interaction within the La 4f shell, the band gap, and the atomic displacements at the interface. In particular, these on-site Coulomb interactions influence buckling within the LaO interface layer, which via its strong coupling to the electrostatic potential in the LAO overlayer causes considerable shifts of the electronic states at the surface and eventually controls the band gap. © 2017 IOP Publishing Ltd. Printed in the UK.

The FeTe parent compound for iron-superconductor chalcogenides was studied applying Mössbauer spectroscopy accompanied by ab initio calculations of electric field gradients at the iron nuclei. Room-temperature (RT) Mössbauer spectra of single crystals have shown asymmetric doublet structure commonly ascribed to contributions of over-stoichiometric iron or impurity phases. Low-temperature Mössbauer spectra of the magnetically ordered compound could be well described by four hyperfine-split sextets, although no other foreign phases different from Fe1.05Te were detected by XRD and microanalysis within the sensitivity limits of the equipment. Density functional ab initio calculations have shown that over-stoichiometric iron atoms significantly affect electron charge and spin density up to the second coordination sphere of the iron sub-lattice, and, as a result, four non-equivalent groups of iron atoms are formed by their local environment. The resulting four-group model consistently describes the angular dependence of the single crystals Mössbauer spectra as well as intensity asymmetry of the doublet absorption lines in powdered samples at RT. We suppose that our approach could be extended to the entire class of Fe1+ySe1-xTex compounds, which contain excess iron atoms. (Figure presented.). © 2016 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The band structures of single and bilayer graphene with one-dimensional topological defects were calculated along the defect line, and appearance of the flat band near the Fermi level was observed. In addition, the influence of deformation (compression/expansion) on the flat band was studied. It was shown that compression across the grain boundary leads to disappearance of the flat band near the Fermi level, while the stretching along this direction does not significantly change the band structure. However, neither compression nor stretching along the grain boundary destroys the flat band. © 2016, Springer Science+Business Media New York.

The study of the adsorption phenomenon of helium began many decades ago with the discovery of graphite as a homogeneous substrate for the investigation of physically adsorbed monolayer films. In particular, helium monoatomic layers on graphite were found to exhibit a very rich phase diagram. In the present work we have investigated the adsorption phenomenon of helium atoms on graphene and silicene substrates by means of density functional theory with Born–Oppenheimer approximation. Helium–substrate and helium–helium interactions were considered from first principles. Vibrational properties of adsorbed monolayers have been used to explore the stability of the system. This approach reproduces results describing the stability of a helium monolayer on graphene calculated by quantum Monte Carlo (QMC) simulations for low and high coverage cases. However, for the moderate coverage value there is a discrepancy with QMC results due to the lack of helium zero point motion. © 2016, Springer Science+Business Media New York.

Recently, it was established that a two-dimensional electron system can arise at the interface between two oxide insulators LaAlO3 and SrTiO3. This paradigmatic example exhibits metallic behaviors and magnetic properties between non-magnetic and insulating oxides. Despite a huge amount of theoretical and experimental work a thorough understanding is yet to be achieved. We analyzed the structural deformations of a LaAlO3 (001) slab induced by hydrogen adatoms and oxygen vacancies at its surface by means of density functional theory. Moreover, we investigated the influence of surface reconstruction on the density of states and determined the change of the local density of states at the Fermi level with increasing distance from the surface for bare LaAlO3 and for a conducting LaAlO3/SrTiO3 interface. In addition, the Al-atom displacements and distortions of the TiO6-octahedra were estimated. © 2016, Springer Science+Business Media New York.

This chapter reviews the experimental evidence obtained for the existence of embedded interfaces between crystalline regions with Bernal and/or rhombohedral stacking order in usual graphite samples, and their relationship with the observed metallic and superconducting behavior. © Springer International Publishing Switzerland 2016.

Aquathermolysis is often proposed as a method to reduce the viscosity of heavy oils. In the present work we have investigated the aquathermolysis reaction of cyclohexyl phenyl sulfide in water medium by means of density functional theory. The water molecule was considered as a reagent and as a catalyst. We have shown that ab initio quantum chemistry methods could be applicable for comparative analysis of chemical reaction pathways in aquathermolysis processes. The obtained results could be useful for the systems like tert-alkyl (secondary-alkyl) thiophenyl ethers, which could be formed in heavy oils at harsh conditions. The scheme of different reaction directions with corresponding calculated values of reaction barriers, which are correlated with experimental data, is presented and could be used for comparison with other reaction mechanisms. © 2016 Elsevier B.V.

Recently, an unusual metallic state with a substantially nonuniform distribution of the charge and magnetic density in CoO2 planes was found experimentally in the NaxCoO2 compound with x>0.6. We have investigated the origin of such an electron disproportionation in the lamellar sodium cobaltates by calculating the ion states as a function of the strength of the electron correlations in the d(Co) shells within the GGA+U approximation for a system with a realistic crystal structure. It was found that the nonuniformity of spin and charge densities are induced by an ordering of the sodium cations and enhanced correlations. Two important magnetic states of cobalt lattice competing with each other at realistic values of the correlation parameter were found - low-spin hexagons lattice (LS) and higher-spin kagome lattice (HS-KSL). In the heterogeneous metallic HS-KSL phase, magnetic Co ions form a kagome structure. In LS phase, the kagome pattern is decomposed into hexagons and the Co ions possess the minimal values of their spin. Coexistence of these states could explain the emergence of the disproportionation with the peculiar kagome structure experimentally revealed in previous studies of the cobaltates. © 2016 American Physical Society.

We present a detailed study of structural transitions of an electron system on liquid helium in quasi-1D confinement geometry. The structural transitions are experimentally observed as current oscillations in transport measurements with changing electrostatic confinement parameters. Finite element modelling and Monte Carlo simulations were used to investigate the electron configuration. With increasing electron density, the single chain of electrons splits into a two-, three- and so on row configuration. A proliferation of defects accompanies each structural transition. We find a good agreement between the observed current modulation and the evolution of the electron row configuration predicted by our calculations. © 2015, Springer Science+Business Media New York.

The production of heavy oil is increasing in coming years due to short fall of conventional light crude. However, extremely high viscosity and abundant amount of heteroatoms (S, O and N) in the structure of heavy oil molecules are one of the main challenges in their exploitation, transportation and processing. Aquathermolysis are often proposed as a method to reduce the viscosity and improve the rheological properties of heavy oils. Aquathermolysis is a reaction of heated water with hydrocarbons molecules in the absence of oxygen. In the present work we have considered different reactions of cyclohexyl phenyl sulfide molecule with water as a very particular model for aquathermolysis process by means of density functional methods. Obtained tendencies in reaction pathways are coherent with previous experimental results. Thus, ab initio methods demonstrated applicability for comparative studies of chemical reaction pathways in aquathermolysis and could be used for the further screening of possible catalysts for this process. © 2015 AIP Publishing LLC.

In the present work we have performed an ab initio calculation of vibrational properties of CuTe2O5 by means of density functional theory (DFT) method. One has compared calculated values with known experimental data on Raman and infrared spectroscopy in order to verify the obtained results. Lattice contribution to the heat capacity obtained from the ab initio simulations was added to magnetic contribution calculated from the simple spin Hamiltonian model in order to obtain total heat capacity. Obtained results are in good agreement with the experimental data. Thus, the DFT methods could complement the experimental and theoretical studying of low-dimensional magnetic systems such as CuTe2O5. © 2014, Pleiades Publishing, Inc.

We propose an explanation for the appearance of superconductivity at the interfaces of graphite with Bernal stacking order. A network of line defects with flat bands appears at the interfaces between two slightly twisted graphite structures. Due to the flat band the probability to find high temperature superconductivity at these quasi one-dimensional corridors is strongly enhanced. When the network of superconducting lines is dense it becomes effectively two-dimensional. The model provides an explanation for several reports on the observation of superconductivity up to room temperature in different oriented graphite samples, graphite powders as well as graphite-composite samples published in the past. © 2014, Pleiades Publishing, Inc.

The distribution of liquid 4He in different types of confinements-adsorbing and nonadsorbing aerogel on the basis of silicon dioxide SiO2 and an absorbing homogeneous strand-has been studied using the density functional theory. It has been demonstrated that the helium atoms tend to be adsorbed on the concave aerogel surface. It has been shown that, in the confinement with fractional mass dimension within certain scales, liquid helium also has a fractional mass dimension within these scales. The dependence of the energy of liquid helium on the number of atoms has been studied for different types of adsorbing surfaces. It has been established that the specific energy of liquid helium behaves differently in the cases of attractive and unattractive potentials with decreasing number of particles. This indicates that the system under consideration is nonextensive. Thus, the necessity of taking into account the surface effects and the fractional mass dimension in the studies of the properties of liquid helium in the restricted space geometry has been demonstrated. © 2013 Pleiades Publishing, Inc.

A topological insulator is an unusual state of quantum matter which, while being an insulator in the bulk, has topologically protected electronic states at the surface. These states could be used in different applications, such as spintronics and quantum computing. However, it is difficult to distinguish the surface and bulk contributions into transport properties, such as conductivity. In order to distinguish surface and bulk contributions an external pressure could be applied. In the present work we have performed ab initio calculations of topological insulator Bi2Se3 under the stress for bulk and surface models. Calculations have been made by means of density functional theory within generalized gradient approximation, the spin-orbit interaction was taken into account as well. It was found that topologically protected surface states remains robust under the stress. Moreover, pressure tends to increase the Fermi velocity of surface electrons, as well as increase electronic density of states at the bottom of the conduction band of the bulk of Bi2Se 3. Thus, the results of ab initio calculations could complement the experimental investigations of high pressure transport properties of topological insulators. The experimentally detected increase of carrier density could be related to the effects of the bulk.

Resent experiments in the lamellar system NaxCoO2 detected a transition of Co planes into a puzzling metallic state at x ≥ 2/3, which co-exists with a robust arrangement of the 3d cobalt electrons: The triangular Co lattices are disproportionated in the spinless Co3+ sites (Co1), and Co3.44+ sites (Co2) with enhanced magnetism forming conducting sublattices. This textures concur with a tightening of the ferromagnetic (FM) interaction in planes, and emerge when the sodium ions become arranged in layers in between the CoO2 slabs. In the present research we have investigated ab-initio the appearance of such state in Na2/3CoO2. Towards this end in view we studied an interplay between the electronic coupling to the superstructure of the Na+ ions and local correlations of the itinerant d electrons treated within the GGA+U approximation. Employing the exact crystallographic supercell, the electronic organization has been analyzed upon increasing the energy U of the Coulomb repulsion within the 3d shells at T = 0. The metallic ground state, being a spin density wave with the inplane FM and antiferromagnetic interplane correlations, has been obtained and established to posses two regimes. When U > 2 eV, a crossover develops from a uniform state of the d-lattice to the regular phase with the spin/charge disproportionation between the sites. In particular at the representative value U = 5 eV, the Co13+ sites with suppressed magnetism appears, while the spin-active Co4+ holes are accumulated by the Co2 sites. A related formation of an isolated, narrow conduction band at the Fermi level implies a considerable enhancement of the electron correlations in the crystal field imposed by the Na+ patterns.

A simple model is presented to illustrate the equilibrium thermostatistics of a nonentensive finite system. Interaction between the finite system and the reservoir is taken into account as a nonextensive term λ H1H2 in the expression of total energy ( H1 and H2 are the energy of the finite system and the reservoir respectively, λ is nonadditivity parameter). In the present paper, a case with harmonic reservoir potential is considered. Energy probability distribution, average energy, heat capacity and entropy function for energy distribution are derived in different finite systems including those with constant density of state in energy, the ideal gas and the phonon gas. © 2012 Elsevier B.V. All rights reserved.

The processes of sample fractionation by two-step atomization with the intermediate condensation of the analyte on a cold surface in graphite furnaces were theoretically studied. The transfer equation was solved for the atoms, molecules, and condensed particles of the sample from a flow of argon directed along this surface. The spatial distributions of vapor and the condensate formed were calculated depending on the composition and flow rate. It was found that a cold surface section with a length of 6 mm is sufficient for the complete trapping of atomic analyte vapor from an argon layer having a velocity of about 1 m/sec and a thickness of 5 mm. In this case, the molecules and clusters condensation coefficients smaller than unity were deposited insignificantly; that is, they were fractionally separated. The results of the shadow spectral visualization of the process of sample fractionation on a cold probe surface of in commercial HGA and THGA atomizers were interpreted. The advantages of analytical signals upon the evaporation of a sample condensate from the probe in these atomizers and inductively coupled plasma were demonstrated. © 2012 Pleiades Publishing, Inc.

In this paper an applications of Monte-Carlo simulation in natural gas production is presented. We have investigated model of natural gas of the Bavlinskoye deposit located in the southeast of the Republic of Tatarstan. For this natural gas and for pure methane and ethane gases we have obtained thermal expansivity, isothermal compressibility, compressibility factor, heat capacity, Joule-Thompson coefficient and density at pressures up to 110 MPa at deposit temperature (463 K). Also we have obtained vapor pressures and liquid-vapor phase diagrams. Simulated properties for methane are in a good agreement with available experimental data.

This work is devoted to the modeling of energy fluctuation effect on the behavior of small classical thermodynamic systems. It is known that when an equilibrium system gets smaller and smaller, one of the major quantities that becomes more and more uncertain is its internal energy. These increasing fluctuations can considerably modify the original statistics. The present model considers the effect of such energy fluctuations and is based on an overlapping between the Boltzmann-Gibbs statistics and the statistics of the fluctuation. Within this "overlap statistics", we studied the effects of several types of energy fluctuations on the probability distribution, internal energy and heat capacity. It was shown that the fluctuations can considerably change the temperature dependence of internal energy and heat capacity in the low energy range and at low temperatures. Particularly, it was found that, due to the lower energy limit of the systems, the fluctuations reduce the probability for the low energy states close to the lowest energy and increase the total average energy. This energy increasing is larger for lower temperatures, making negative heat capacity possible for this case.

The complex behavior of such quantum fluids like liquid 4He and liquid 3He in nanoporous media is determined by influence of randomly distributed geometrical confinement as well as by significant contribution from the surface atoms. In the present paper Fractional Schrodinger equation has been used for deriving two-fluid hydrodynamical equations for describing the motion of superfluid helium in the fractal dimension space. Nonlinear equations for oscillations of pressure and temperature are obtained and coupling of pressure and temperature oscillations is observed. Moreover coupling should disappear at very low temperatures which provide an experimental test for this theory.

In the present work simulation of Van der Waals potential between helium atom and part of silica aerogel strand by means of ab initio methods was performed. Cell with alpha quartz structure was used as building block of aerogel strand, because it is the most stable structure at low temperature, and only the surface layer of aerogel has been considered. For modeling absorption potential field in plane, summation of potential from individual building blocks has been provided. Two dimensional Van der Waals energy field was calculated for different geometries of aerogel strands. A rather deep potential well has been found in the corner formed due to aerogel strand crossing.

The complex behavior of such quantum fluids like liquid 4He and liquid 3He in nanoporous media is determined by spatial quantization because of geometrical confinement as well as by significant contribution from the surface atoms. In the present report we will review the procedure, results and discuss the issues for fractionalized nonextensive hydrodynamical approach to describe the properties of quantum fluids inside nanopores and propose consideration of strong correlated quantum liquid by means of fractionalized Schrödinger equation. © 2011 Science China Press and Springer-Verlag Berlin Heidelberg.

There are several approaches to describe the behavior of superfluid helium-4. For example, two-fluid model, the microscopic description based on the Gross-Pitaevskii equation and one-fluid theory in the framework of extended thermodynamics. Recently the observable peculiarities of quantum liquids behavior in the confined geometries (nanopores, aerogels, etc.) have caused the interest to the correct description of quantum liquids at nanoscale. The fractal geometry and the effects of huge inner surface area should be taken into account to describe dynamics and thermodynamics of liquid helium-4 inside nanoporous media. In the present paper we propose a two-fluid hydrodynamic model in fractal dimension space on the basis of a nonextensive entropy and energy approach. In the framework of this model the coupling between temperature and pressure oscillations ("sound modes conversion") due to fractal geometry is found. © 2009 Springer Science+Business Media, LLC.