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While the anthropogenic impact on ecosystems today is evident, it remains unclear if the detrimental effect of hominins on co‐occurring biodiversity is a recent phenomenon or has also been the pattern for earlier hominin species. We test this using the East African carnivore fossil record. We analyse the diversity of carnivores over the last four million years and investigate whether any decline is related to an increase in hominin cognitive capacity, vegetation changes or climatic changes. We find that extinction rates in large carnivores correlate with increased hominin brain size and with vegetation changes, but not with precipitation or temperature changes. While temporal analyses cannot distinguish between the effects of vegetation changes and hominins, we show through spatial analyses of contemporary carnivores in Africa that only hominin causation is plausible. Our results suggest that substantial anthropogenic influence on biodiversity started millions of years earlier than currently assumed., While the anthropogenic impact on ecosystems today is evident, it remains unclear if the detrimental effect of the human lineage on co‐occurring biodiversity is a recent phenomenon or has also been the pattern for earlier hominin species. Focusing on fossil data from East Africa, the cradle of mankind, we find support for a strong negative effect on co‐occurring biodiversity caused by our ancestors already in the Pliocene. Our results suggest that substantial anthropogenic influence on biodiversity started millions of years earlier than currently assumed.

Efficient ab initio computational methods for the calculation of thermoelectric transport properties of materials are of great avail for energy harvesting technologies. The BoltzTraP code has been largely used to efficiently calculate thermoelectric coefficients. However, its current version that is publicly available is based only on the constant relaxation time (RT) approximation, which usually does not hold for real materials. Here, we extended the implementation of the BoltzTraP code by incorporating realistic k-dependent RT models of the temperature dependence of the main scattering processes, namely, screened polar and nonpolar scattering by optical phonons, scattering by acoustic phonons, and scattering by ionized impurities with screening. Our RT models are based on a smooth Fourier interpolation of Kohn-Sham eigenvalues and its derivatives, taking into account non-parabolicity (beyond the parabolic or Kane models), degeneracy and multiplicity of the energy bands on the same footing, within very low computational cost. In order to test our methodology, we calculated the anisotropic thermoelectric transport properties of low temperature phase (Pnma) of intrinsic p-type and hole-doped tin selenide (SnSe). Our results are in quantitative agreement with experimental data, regarding the evolution of the anisotropic thermoelectric coefficients with both temperature and chemical potential. Hence, from this picture, we also obtained the evolution and understanding of the main scattering processes of the overall thermoelectric transport in p-type SnSe.

The coupling between electrons and phonons in solids plays a central role in describing many phenomena, including superconductivity and thermoelecric transport. Calculations of this coupling are exceedingly demanding as they necessitate integrations over both the electron and phonon momenta, both of which span the Brillouin zone of the crystal, independently. We present here an ab initio method for efficiently calculating electron-phonon mediated transport properties by dramatically accelerating the computation of the double integrals with a dual interpolation technique that combines maximally localized Wannier functions with symmetry-adapted plane waves. The performance gain in relation to the current state-of-the-art Wannier-Fourier interpolation is approximately 2n_s \times M, where n_s is the number of crystal symmetry operations and M, a number in the range 5 - 60, governs the expansion in star functions. We demonstrate with several examples how our method performs some ab initio calculations involving electron-phonon interactions., Comment: 9 pages, 5 figures

We present an ab initio study of the nature of the optical and quasiparticle gaps of SnSe. The density functional theory and the many-body perturbation theory under the GW approximation show that SnSe exhibits an indirect quasiparticle bandgap of 0.64 eV, which agrees with the experimental evidence. The optical gap, on the contrary, is direct and ranges between 0.9 and 1.1 eV for thin films. The optical absorption spectrum of SnSe evidences the formation of excitons, whose binding energies depend on the polarization of the incident electric field. Finally, we show evidences that the optical gap of single crystals arises from indirect transitions.

We used a semi-empirical method to extract carrier relaxation times at different temperatures (τ(T)) in thermoelectric materials from a combination of experimental results and first-principles calculations. The methodology is based on the Boltzmann transport equation formalism within the relaxation time approximation. It can be applied to single crystals and polycrystalline materials. We applied the method to investigate the electronic transport properties of the clathrate compound Ba8Ga16Ge30 type-I. The calculations indicate that the carrier relaxation process in single crystals is dominated by electron-phonon scattering (τ ∝ T-3/2), while in polycrystalline materials scattering at grain boundaries dominates (τ ∼ cte). The Seebeck coefficient, electrical conductivity, and electron heat conduction are in consistent agreement with experiment. Furthermore, the Slack relation for lattice heat conductivity was successfully applied to the material. The calculated figure of merit is in good agreement with experimental results.

We applied an efficient methodology to separate vibrational and configurational entropies in bulk metallic glasses by means of molecular dynamics simulation based on a combination of non-equilibrium adiabatic switching and reversible scaling methods. This approach involves calculating the vibrational free energy using the Einstein crystal as a reference for the solid phase and the recently proposed Uhlenbeck-Ford model for the fluid phase. This methodology has the advantage that it does not require a crystalline solid phase for separating the entropies. Therefore, in principle, it is applicable to any material, regardless of whether or not it has a crystalline phase. Using this methodology, we separate the vibrational and configurational entropies of two metallic glasses with different fragilities at zero external pressure, namely, Cu$_{50}$Zr$_{50}$ and Cu$_{46}$Zr$_{46}$Al$_{8}$. We find that the results for the former alloy are in quite reasonable agreement with recent experimental work by Smith \textit{et al.}[H. L. Smith \textit{et al.}, Nat. Phys. \textbf{13}, 900 (2017)]. We also find the configurational entropy of the glass containing Al to be 70\% larger than that of the other glass. Our results suggest that, although other factors may be at play, the configurational entropy can be used to investigate the effect of the addition of a minor-alloying element on the glass-forming ability of bulk metallic glasses., Comment: 8 pages, 9 figures

The conversion of heat to electricity by thermoelectric devices may play a key role in the future for energy harvesting. In order to meet that purpose, a variety of more efficient thermoelectric materials are needed. Intense research has been conducted over the past decade on type-I thermoelectric clathrates, and further developments may arise with successful introduction of trivalent rare-earth elements into the cages of this class of materials. This paper is dedicated to study the hypothetical charge-balanced compound La 2 Ga 6 Ge 40 , using first-principles calculations, in order to evaluate the direct effects of introducing La 3+ into Ge 46 without occupying all the other cages with other chemical species. Here we present first-principles calculations on the structural, electronic, vibrational and thermoelectric properties of type-I Ge clathrates hosting La 3+ guest ions. Our results indicate that the structures with the lowest formation energies are those in which the La 3+ guests are inside the Ge 46 dodecahedral cages ( 2a Wyckoff positions), with Ga substitution for Zintl charge balance. Furthermore, our calculations show that several features of the system, such as, the equilibrium position of the La 3+ guests inside the cages, the electronic structure near the band edges, the vibrational properties and anharmonic effects are significantly affected by the way the Ga atoms are distributed in the lattice. The influence of these features on the thermoelectric properties of these systems is discussed.

The water-solvated excess electron (EE) is a key chemical agent whose hallmark signature, its asymmetric optical absorption spectrum, continues to be a topic of debate. While nearly all investigation has focused on the liquid-water solvent, the fact that the crystalline-water solvated EE shows a very similar visible absorption pattern has remained largely unexplored. Here, we present spin-polarized density-functional theory calculations subject to periodic boundary conditions of the interplay between an EE and a number of intrinsic lattice defects in ice Ih. Our results show that the optical absorption signatures in the presence of three unsaturated hydrogen bonds (HB) are very similar to those observed experimentally. Its low-energy side can be attributed to transitions between the EE ground state and a single localized excited level, in a picture that is different from that for the liquid solvent, where this portion has been associated with hydrogen-like s → p excitations. The blue tail, on the other hand, relates to transitions between the EE ground state and delocalized excited states, which is in line with the bound-to-continuum transition interpretations for the EE in liquid water. Finally, we find that, depending on the number of dangling HBs participating in the EE trap, its charge density may spontaneously break the spin degeneracy through exchange interactions with the surrounding electrons, displaying the many-electron quantum nature of the EE problem in ice Ih.

A methodology to sample the isothermal-isobaric ensemble using Langevin dynamics is proposed, which combines novel features of geometric integrators for the equations of motion. By employing the Trotter expansion, the methodology generates a robust, symmetric, and accurate numerical algorithm. In order to show that the proposed method correctly samples the phase-space, simulations in the isotropic NPT ensemble were carried out for two analytical examples. Also this method lets us study a solid-solid phase transition by conducting a fully flexible-cell molecular dynamics simulation. Additionally, we present an efficient method to determine the Gibbs free energy in a wide interval of pressure along an isothermal path, which allows us to determine the transition pressure in a driven by pressure solid-solid phase transition. Our calculations show that the methodology is highly suitable for the study of the phase diagram of crystalline solids.

An ultralow lattice thermal conductivity of 0.14 W m

In this paper, we review the physical concepts of the nonequilibrium techniques for the calculation of free energies applied to magnetic systems using Monte Carlo simulations of different nonequilibrium processes. The methodology allows the calculation of the free energy difference between two different system Hamiltonians, as well as the free energy dependence on temperature and magnetic field for a given Hamiltonian. As an illustration of the effectiveness of this approach, we apply the methodologies to determine the phase diagram of a simple microscopic model, the degenerate Ising model. Our results show very good agreement with those obtained from analytical (theoretical) methods.

An ultralow lattice thermal conductivity of 0.14 W$\cdot$ m$^{-1} \cdot$ K$^{-1}$ along the $\vec b$ axis of As$_2$Se$_3$ single crystals was obtained at 300 K by first-principles calculations involving the density functional theory and the resolution of the Boltzmann transport equation. This ultralow lattice thermal conductivity arises from the combination of two mechanisms: 1) a cascade-like fall of the low-lying optical modes, which results in avoided crossings of these with the acoustic modes, low sound velocities and increased scattering rates of the acoustic phonons; and 2) the repulsion between the lone-pair electrons of the As cations and the valence $p$ orbitals of the Se anions, which leads to an increase in the anharmonicity of the bonds. The physical origins of these mechanisms lie on the nature of the chemical bonding in the material and its strong anisotropy. These results, whose validity has been addressed by comparison with SnSe, for which excellent agreement between the theoretical predictions and the experiments is achieved, point out that As$_2$Se$_3$ could exhibit improved thermoelectric properties.

We have calculated energies of antiphase boundaries (APB) and stacking faults for the face-centered cubic L1 2 structured Ni 3 Al, Cu 3 Au, and Au 3 Cu compounds. The calculations were performed using widely employed semiempirical potentials, which are derived from the second-moment approximation of a tight-binding model, and adopting different parameterizations for these potentials found in the literature. Our calculations show that the energies of all the APB’s on the (0 0 1) plane are lower than that of the L1 2 ordered alloy. In addition, we performed metropolis Monte Carlo simulations allowing the interchange of atoms and verified that the system chooses spurious low energy APB configurations. These results indicate that the thermodynamically stable phase obtained using these potentials is not the experimentally verified L1 2 phase at low temperatures. This is a relevant result because in all situations where ordering is allowed to change, the physical description using these potentials is expected to fail.

We present a density-function theory (DFT) study of Bjerrum-defect trapping centers involving the molecular vacancy in ice Ih. As a first step, we compute the intrinsic migration barrier to D-defect motion using the nudged elastic band (NEB) method and find them to be of the same order of magnitude as the energy barriers involving intrinsic L-defect motion. This finding suggests that intrinsic mobility factors cannot explain the experimentally observed inactivity of D defects, supporting the idea that D defects are trapped at other lattice-defect sites. Next we study the defect complexes formed by the combination of isolated D and L defects with a molecular vacancy. The corresponding geometries show that the formation of these aggregates significantly reduces elastic distortions that are present in isolated Bjerrum defects. An analysis of the energetics involved in the formation of both defect complexes reveals a significant binding energy, indicating that the molecular vacancy represents a strong trapping center for Bjerrum defects. On the other hand, the fact that there is no difference between the absolute values of the binding energies for both D and L defects suggests that the vacancy affects both species of Bjerrum defects in a similar fashion, possibly ruling out the vacancy trapping centers as an explanation for the experimentally observed inactivity of D defects.

Divacancies are among the most important defects that alter the charge transport properties of single-walled carbon nanotubes (SWNT), and we here study, using ab initio calculations, their properties. Two structures were investigated, one that has two pentagons side by side with an octagon (585) and another composed of three pentagons and three heptagons (555777). We investigate their stability as a function of tube diameter, and calculate their charge transport properties. The 585 defect is less stable in graphene due to two broken bonds in the pentagons. We estimate that the 555777 becomes more stable than the 585 for a diameter of about 40 A (53 A) for an armchair (zigzag) SWNTs, indicating that they will prevail in large diameter multiwalled carbon nanotubes and graphene ribbons.

We have investigated the transitions between disordered phases in supercooled liquid silicon using computer simulations. The thermodynamic properties were directly obtained from the free energy, which was computed using the recently proposed reversible scaling method. The calculated free energies of the crystalline and liquid phases of silicon at zero pressure, obtained using the environment dependent interatomic potential, are in excellent agreement with the available experimental data. The results show that, at zero pressure, a weak first-order liquid-liquid transition occurs at 1135 K and a continuous liquid-amorphous transition takes place at 843 K. These results are consistent with the existence of a second critical point for the liquid-liquid transition at a negative pressure.

We studied the effects of arsenic on properties of dislocations in silicon. The theoretical investigation was carried out using ab initio total energy methods, based on the density functional theory. We find that the interaction of an arsenic impurity in the crystal with a dislocation results in a charge exchange, driving the dislocation core to a negative charge state. This interaction is essentially electrostatic and attractive, and leads to arsenic segregation. Although arsenic segregation to the core is energetically favourable, formation of arsenic pairs inside the core is energetically unfavourable. We also investigated the role of vacancies in arsenic diffusion inside the dislocation core.

We investigated through ab initio total energy calculations the interaction of arsenic impurities with the core of a 30° partial dislocation in silicon. It was found that when an arsenic atom sits in a crystalline position near the dislocation core, there is charge transfer from the arsenic towards the dislocation core. As a result, the arsenic becomes positively charged and the core negatively charged. The results indicate that the structural changes around the impurity are very small in both environments, namely, the crystal and the dislocation core. In this scenario, the interaction between arsenic and the core is essentially electrostatic, which eventually leads to arsenic segregation. The segregation energy was found to be as large as 0.5 eV/atom. Additionally, it was found that arsenic pairing inside the core is not energetically favorable.

We present a dynamic implementation of the Clausius–Clapeyron integration (CCI) method for mapping out phase-coexistence boundaries through a single atomistic simulation run. In contrast to previous implementations, where the reversible path of coexistence conditions is generated from a series of independent equilibrium simulations, dynamic Clausius–Clapeyron integration (d-CCI) explores an entire coexistence boundary in a single nonequilibrium simulation. The method gives accurately the melting curve for a system of particles interacting through the Lennard-Jones potential. Furthermore, we apply d-CCI to compute the melting curve of an ab initio pair potential for argon and verify earlier studies on the effects of many-body interactions and quantum effects in the melting of argon. The d-CCI method shows to be effective in both applications, giving converged coexistence curves spanning a wide range of thermodynamic states from relatively short nonequilibrium simulations.

We performed a theoretical investigation on the interaction of arsenic impurities with a 30° glide partial dislocation in silicon. Our calculations were performed by ab initio total energy methods, based on the density functional theory and the local density approximation. We find that an arsenic atom, in a crystalline position, gives away one electron to the dislocation, becoming positively charged while the dislocation core is negatively charged. The interaction between arsenic and the core is essentially electrostatic, which leads to arsenic segregation.

Using ab initio calculations, we computed the core reconstruction energies of {111} 30° partial dislocations in zinc-blende semiconductors. Our results show a direct correlation between core reconstruction energies and the experimental activation energies for the velocity of 60° dislocations. The electronic structure of unreconstructed dislocation cores comprises a half-filled band, which splits up in bonding and antibonding levels upon reconstruction. The levels in the electronic gap come from the core of β dislocations, while the levels related to α dislocations lie on the valence band.

We performed a theoretical investigation on the interaction of dopant atoms with extended defects in semiconductors using ab initio total energy calculations. Dopant atoms (donors and acceptors) were found to segregate in stacking faults in silicon and in gallium arsenide, with segregation energies as large as 0.2 eV. A general trend on the electronic properties of segregated dopants in stacking faults is that the donor (or acceptor) electronic energy levels become deeper in the electronic gap, as compared to those levels for the defect in a crystalline site.

We investigated core properties of dislocations in zinc-blende semiconductors using ab initio total energy calculations. The core reconstruction energy of partial dislocations was found to scale almost linearly with the experimental dislocation activation energy. The electronic band structure related to dislocation cores was also determined. In an unreconstructed core, the gap states comprise a half-filled one-dimensional band, which splits up in bonding and antibonding states upon reconstruction. The energy states which lie in the electronic gap come from the cores of β partials, while those related to α partials remain resonant in the valence band.

Using ab initio total-energy calculations, we computed core reconstruction energies of partial dislocations in zinc-blende semiconductors. The reconstruction energy of 30° partials was found to scale almost linearly with the experimental activation energy of 60° dislocations. The electronic structure of a dislocation shows that in an unreconstructed core, the gap states comprise a half-filled one-dimensional band, which splits up into bonding and antibonding states upon reconstruction. The energy states which lie in the electronic gap come from the core of β-partials, while those related to α-partials remain resonant in the valence band.

Discusses a particular application of computational physics: the calculation of thermodynamic properties by computer simulation methods. More specifically, we focus on one of the most difficult tasks in this context: the estimation of free energies in problems such as the study of phase transformations in solids and liquids, the influence of lattice defects on the mechanical properties of technological materials, the kinetics of chemical reactions, and protein-folding mechanisms in biological processes. Because the free energy plays a fundamental role, the development of efficient and accurate techniques for its calculation has attracted considerable attention during the past 15 years and is still an active field of research. We discuss some general aspects of equilibrium and nonequilibrium approaches to free-energy measurement and present the "reversible scaling" technique which we have recently developed.

We present a method, for highly efficient free-energy calculations by means of molecular dynamics and Monte Carlo simulations, which is an optimized combination of coupling parameter and adiabatic switching formalisms. This approach involves dynamical reversible scaling of the potential energy function of a system of interest, and allows accurate determination of its free energy over a wide temperature interval from a single simulation. The method is demonstrated in two applications: crystalline Si at zero pressure and a fcc nearest-neighbor antiferromagnetic Ising model.

We present a new simulation technique that allows accurate and very efficient determination of free energies as a function of temperature using a single constant temperature molecular dynamics or Monte Carlo simulation. The method is based on the dynamical reversible scaling of the potential energy function of the system of interest and is implemented using the adiabatic switching method. Application to the calculation of the free energy of crystalline silicon using a semi-empirical interatomic potential demonstrates that the reversible-scaling method provides an accurate and very efficient tool for the calculation of free energies over a wide temperature interval.

The structure of the single vacancy in silicon, one of the most common point defects and an important mediator of atomic diffusion, is examined through extensive first principles calculations. We find a hitherto unexpected result, namely that there exist two distinct distortions associated with the vacancy with essentially identical formation energies at zero pressure. The two distortions are distinguished by their different relaxations, volumes of formation, and formation enthalpies. We discuss how, at finite pressure, one of the two distortions should become dominant, and suggest experimental tests of this effect.

Agradecimentos: The MD simulations were performed on a Digital Alpha 3000/900 of the CENAPAD-SP Computer Center. The authors gratefully acknowledge the financial support granted by the Brazilian funding agencies CAPES, FAPESP, and CNPq Abstract: We study the application of the adiabatic switching molecular dynamics method to determine bulk and vacancy-formation Gibbs free energies as a function of temperature at zero pressure for copper. The bulk free energy has been determined through isochoric isothermal switching procedures in which a system consisting of 500 copper atoms interacting through a semiempirical tight-binding potential is turned into a system of 500 independent identical three-dimensional harmonic oscillators. The equilibrium volumes of these-simulations were determined from equilibrium isobaric isothermal molecular dynamics simulations. The frequency of the oscillators is chosen to be of the order of a principal phonon frequency of copper in order to achieve competitive convergence. The resulting bulk free energy and entropy are in excellent agreement with experimental values. The vacancy-formation Gibbs free energy has been computed from isobaric isothermal switching procedures in which the interactions of a single copper atom are switched off. Considering the limited accuracy of the interatomic potential and the numerical noise present in the small energy differences measured, the estimated formation enthalpies and entropies agree remarkably well with experimental data. The method has shown to be computationally efficient. Typically, 6 h of CPU time on a Digital Alpha 3000/900 were required per data point for the bulk as well as the vacancy-formation parameters COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Fechado

Agradecimentos: We gratefully acknowledge support from the Brazilian agencies CNPq, Fapesp, Capes and the Center for Computational Engineering and Sciences-Fapesp/Cepid #2013/08293-7. Part of the calculations were performed at CCJDR-IFGW-UNICAMP and CENAPAD-SP. We would like to thank the anonymous reviewers for their helpful suggestions Abstract: Using molecular dynamics simulations we analyze the dynamics of two atomic liquids that display a liquid-liquid phase transition (LLPT): Si described by the Stillinger-Weber potential and Ga as modeled by the modified embedded-atom model. In particular, our objective is to investigate the extent to which the presence of a dip in the self-intermediate scattering function is a manifestation of an excess of vibrational states at low frequencies and may be associated with a fragile-to-strong transition (FTST) across the LLPT, as suggested recently. Our results suggest a somewhat different picture. First, in the case of Ga we observe the appearance of an excess of vibrational states at low frequencies, even in the absence of the appearance of a dip in the self-intermediate scattering function across the LLPT. Second, studying the behavior of the shear viscosities traversing the LLPTs we find that both substances are fragile in character above and below their respective LLPT temperatures. Instead of a FTST in an absolute sense these findings are more in line with a view in which the LLPTs are accompanied by a transition from a more fragile to a less fragile liquid. Furthermore, we do not find this transition to correlate with the presence of a dip in the intermediate scattering function CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado

In this paper we investigate the behavior of an Einstein crystal as a reference system in adiabatic switching procedures. We study the canonical massive Nos\'e-Hoover chain (MNHC) dynamics [G. J. Martyna, M. L. Klein, and M. Tuckerman, J. Chem. Phys. 97, 2635 (1992)] and show it can be used to determine Helmholtz free energies within an adiabatic switching procedure. We calculate the Helmotz free energy difference between two different Einstein crystals, each consisting of 100 identical independent harmonic oscillators with different characteristic frequencies by a MNHC molecular dynamics adiabatic switching procedure. The simulations were performed using two different switching functions. Applying the quantitative error analysis of Tsao, Sheu, and Mou [J. Chem. Phys. 101, 2302 (1994)], it is found that systematic errors during the switching process can be estimated quantitatively, allowing a correction of the converged results. The corrected results obtained by adiabatic switching deviate less than 1% from the analytical value. It is observed that quantitative correction of converged results can be avoided by choosing a proper switching function. \textcopyright{} 1996 The American Physical Society.

The structural and electronic properties of vacancies in a fully tetrahedral model of amorphous silicon have been investigated by ab initio calculations. A very rich behavior was observed after the atomic relaxation, from the complete rearrangement of the atoms (self-healing) to the creation of stable vacancies. We have observed that the vacancy internal relaxation volume is negative. After relaxation, the vacancy formation energies were found to be smaller than that of the crystal, and the deep gap levels either disappear or move towards the band edges.

We study a hypothetical form of solid carbon ${\mathrm{C}}_{28}$C, with a unit cell which is composed of the ${\mathrm{C}}_{28}$ fullerene cluster and an additional single carbon atom arranged in the zinc-blende structure. Using ab initio calculations, we show that this form of solid carbon has lower energy than hyperdiamond, the recently proposed form composed of ${\mathrm{C}}_{28}$ units in the diamond structure. To understand the bonding character of these cluster-based solids, we analyze the electronic structure of ${\mathrm{C}}_{28}$C and of hyperdiamond and compare them to the electronic states of crystalline cubic diamond.

Using ab initio electronic structure calculations, we study a cluster-assembled form of carbon which is based on ${\mathrm{C}}_{28}$ units arranged in a diamond crystal. We investigate how the charge density and electronic spectrum of an isolated ${\mathrm{C}}_{28}$ unit is altered in the infinite lattice. We analyze the nature of electronic states near the Fermi level of both an isolated ${\mathrm{C}}_{28}$ cluster and its solid form. A tight binding model, with parameters fitted to reproduce energy eigenvalues calculated from first principles, provides insight on how the states of thin structure are derived from those of ${\mathrm{C}}_{28}$.

Electronic applications require the ability to dope diamond p- and n-type. Boron is well known to dope both natural and synthetic diamond p-type. N-type doping, however, has proven exceedingly difficult. In this work, the suitability of several impurities for n-type doping is investigated theoretically. We also examine the well-known nitrogen deep impurity as well as the effect of simultaneous doping with N and B on the thermodynamic equilibrium between diamond and graphite. The calculations were carried out using local density theory, the pseudopotential formalism, and the Car- Parrinello method. The impurities were embedded in a large supercell and atomic relaxations were computed using ab initio forces. The impurities Li, Na and P are shown to be shallow donors, but they have very low solubilities. This makes their incorporation via in-diffusion difficult and leaves ion implantation and possibly incorporation during growth as the only alternatives. Once incorporated, Li is found to be a fast diffuser whereas Na will be stable up to moderate temperatures. The most suitable shallow donor is Na, which occupies an interstitial site. It is particularly appropriate for ion implantation, since no self-implantation step to create vacant sites is necessary.

We have developed a methodology to study the thermodynamics of order-disorder transformations in $n$-component substitutional alloys that combines nonequilibrium methods, which can efficiently compute free energies, with Monte Carlo simulations, in which configurational and vibrational degrees of freedom are simultaneously considered on an equal footing basis. Furthermore, with this methodology one can easily perform simulations in the canonical and in the isobaric-isothermal ensembles, which allow the investigation of the bulk volume effect. We have applied this methodology to calculate configurational and vibrational contributions to the entropy of the ${\text{Ni}}_{3}\text{Al}$ alloy as functions of temperature. The simulations show that when the volume of the system is kept constant, the vibrational entropy does not change upon transition while constant-pressure calculations indicate that the volume increase at the order-disorder transition causes a vibrational entropy increase of $0.08{k}_{B}/\text{atom}$. This is significant when compared to the configurational entropy increase of $0.27{k}_{B}/\text{atom}$. Our calculations also indicate that the inclusion of vibrations reduces in about 30% the order-disorder transition temperature determined solely considering the configurational degrees of freedom.

The properties of native defects and impurities in diamond and SiC are investigated via large-scale band structure and Car-Parrinello calculations. In diamond, the activation energy for self-diffusion is very high in the intrinsic material (9 eV) but decreases by up to 3 eV in either p- or n-type material. Phosphorus, lithium and sodium are shallow donors, but their solubilities are very low, which makes them unsuitable for incorporation into diamond via in-diffusion. Instead, kinetic trapping during growth or ion implantation must be used. Considering the stability at the dopant site, substitutional phosphorus is expected to diffuse by the vacancy mechanism and to have a high activation energy by analogy to self-diffusion. Both lithium and sodium diffuse through the interstitial channel. Lithium is a relatively fast diffuser while sodium should be stable up to moderately high temperatures. For nitrogen in diamond, the well known (111) distortion is found to be due to the interaction of the fully occupied nitrogen lone pair with the dangling bond of the C(111) atom. The single electron associated with the center resides in an antibonding orbital formed from the dangling hybrid and the nitrogen lone pair. This orbital has most of its amplitude on the carbon atom. In SiC, the lowest energy defect in n-type and intrinsic material is the electrically inactive silicon antisite, while the lowest energy defect in p-type SiC is the doubly positive carbon vacancy. The electrons released by the vacancies compensate acceptor dopants, leading to strong self-compensation effects when doping occurs during crystal growth. In carbon-rich SiC, the dominant defect for all Fermi level positions is the electrically inactive carbon antisite. In boron-doped SiC, BC is preferred for silicon-rich material while, in carbon-rich SiC, BC and BSi have similar formation energies.

Agradecimentos: The work was supported by the Brazilian agencies FAPESP, CNPq, and CAPES. Part of the computations was carried out at CENAPAD-SP Abstract: We report on theoretical results that lend support to recent experimental observations suggesting the existence of a first-order liquid-liquid phase transformation (LLPT) in gallium. Using molecular dynamics simulation based on a modified embedded-atom model, we observe a transition from a high-density to a low-density liquid in the supercooled regime. The first-order character of the transition is established through the detection of the release of latent heat and our findings suggest that the LLPT terminates in a critical point that is located in the tensile-strained domain of the metastable phase diagram FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Aberto

We investigated the interaction of intrinsic and extrinsic point defects with stacking faults in silicon. The calculations were carried out using ab initio total energy methods. The results show that the formation energies of intrinsic defects and impurities (P, As, and Al) are lower at the stacking fault as compared to the respective defects in crystalline environment. Therefore, stacking faults should have a large concentration of defects, and they should play an important role on the mechanisms of dislocation motion.

The path integral formulation has been combined with several methods to determine free energies of quantum many-body systems, such as adiabatic switching and reversible scaling. These techniques are alternatives to the standard thermodynamic integration method. A quantum Einstein crystal is used as a model to demonstrate the accuracy and reliability of these free energy methods in quantum simulations. Our main interest focuses on the calculation of the melting temperature of Ne at ambient pressure, taking into account quantum effects in the atomic dynamics. The free energy of the solid was calculated by considering a quantum Einstein crystal as reference state, while for the liquid, the reference state was defined by the classical limit of the fluid. Our findings indicate that, while quantum effects in the melting temperature of this system are small, they still amount to about 6% of the melting temperature, and are therefore not negligible. The particle density as well as the melting enthalpy and entropy of the solid and liquid phases at coexistence is compared to results obtained in the classical limit and also to available experimental data.

We present a model for the determination of the thermal equilibrium concentrations of Bjerrum defects, molecular point defects, and their aggregates in ice Ih. First, using a procedure which minimizes the free energy of an ice crystal with respect to the numbers of defect species, we derive a set of equations for the equilibrium concentrations of free Bjerrum and point defects, as well their complexes. Using density-functional-theory calculations, we then evaluate the binding energies of Bjerrum-defect/vacancy and Bjerrum-defect/interstitial complexes. In contrast to the complexes which involve the molecular vacancy, the results suggest that the molecular interstitial binds preferentially to the D-type Bjerrum defect. Using both theoretical binding and formation free energies as well as the available experimental data, we find that the preferential binding and the substantial presence of the interstitial as the predominant point defect in ice Ih may lead to conditions in which the number of free D defects becomes considerably smaller than that of free L defects. Such a scenario could possibly be involved in the experimentally observed inactivity of D-type Bjerrum defects in the electrical properties of ice Ih FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado

We investigate, through ab initio density-functional theory calculations, the electronic and structural properties of neutral Mn impurities at tetrahedral interstitial and substitutional sites in both Si and Ge layers of a $\mathrm{Si}∕\mathrm{Ge}$ heterostructure. We conclude that substitutional Mn at the Ge layers is more stable than interstitial Mn at the Si layers by approximately $0.45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and we estimate an energy barrier of at least $1.12\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to diffuse away from these most stable substitutional sites. Mn has a magnetic moment in the heterostructure that is similar to that in the bulk, and for the compressed Ge layer the Mn-Mn exchange interaction is always weakly antiferromagnetic. Varying the lattice constant of the substrate, the Mn-Mn ground state becomes ferromagnetic. This result opens up the possibility of manipulating the interaction between Mn impurities at Ge layers grown over a ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ substrate by changing $x$.

Tetrahedral substances, such as silicon, water, germanium, and silica, share various unusual phase behaviors. Among them, the so-called polyamorphism, i.e., the existence of more than one amorphous form, has been intensively investigated in the last three decades. In this work, we study the metastable relations between amorphous states of silicon in a wide range of pressures, using Monte Carlo simulations. Our results indicate that the two amorphous forms of silicon at high pressures, the high density amorphous (HDA) and the very high density amorphous (VHDA), can be decompressed from high pressure (∼20 GPa) down to the tensile regime, where both convert into the same low density amorphous. Such behavior is also observed in ice. While at high pressure (∼20 GPa), HDA is less stable than VHDA, at the pressure of 10 GPa both forms exhibit similar stability. On the other hand, at much lower pressure (∼5 GPa), HDA and VHDA are no longer the most stable forms, and, upon isobaric annealing, an even less dense form of amorphous silicon emerges, the expanded high density amorphous, again in close similarity to what occurs in ice.