Elastic constants of solids at high pressures

The isothermal and adiabatic nth-order (n ?? 2) elastic constants of a loaded crystal are defined. These constants fully determine the behavior of solids at an arbitrary load and are controlled by both an interatomic interaction and an applied load. Expressions that relate these constants (of the second, third, and fourth order) to Brugger elastic constants of the corresponding order, which are only determined by an inter-atomic interaction, are found for cubic symmetry crystals under hydrostatic pressure. These expressions are used to calculate the equation of state and the second- and third-order elastic constants of bcc tantalum at T = 0 K over a wide pressure range (0?C600 GPa) using an electron density functional method. The results of calculating the equation of state and the second-order elastic constants agree with available experimental data and the calculation results obtained in other works.     

Significant correlation between macroscopic and microscopic parameters for the description of localized plastic flow auto-waves in deforming alloys

Understanding of mechanical properties of materials and a possibility to predicting them from ab initio calculations have fundamental importance for solid state theory. In this work we establish a significant correlation between the product of the macroscopic parameters of localized plastic flow auto-waves in deforming alloys, their length and propagation rate and the product of the microscopic (lattice) parameters of these materials, the spacing between close-packed planes of the lattice and the rate of transverse elastic waves. Thus, these products can be regard as invariants of plastic and elastic deformation processes, respectively. Moreover, the established regularity suggests that the elastic and the plastic processes simultaneously involved in the deformation are closely related. Our work also demonstrates that ab initio simulations can be used for the prediction of parameters of localized plastic flow auto-waves in deforming alloys.     

Variable range hopping conductivity in manganites

The nature of variable range hopping (VRH) conductivity which is observed in the insulating state of doped rare-earth manganites with perovskite structure is considered in the two component model of metallic-like droplets embedded in dielectric matrix. When the density of the metallic droplets is less than the percolation limit, the system falls into the insulating state with VRH conductivity defined by inter granular tunneling and electrostatic barriers. With temperature increasing the VRH regime is transforming into the hopping regime of small radius polarons.     

Application of the Monte Carlo method to the problem of surface segregation simulation

A generalization of the Monte Carlo method to the case of grand canonical ensemble allowing the elimination of the problem of determination of the chemical potential of alloy components was proposed. The method is particularly convenient for the calculations of surface segregations because it excludes time-consuming calculation of the temperature-dependent bulk chemical potential μ(T). The new method was used for calculating segregations at the (100), (110), and (111) surfaces of the Ni₅₀Pd₅₀ alloy using the Ising model with ab initio effective interatomic interaction potentials.     

Electronic topological transition in metastable Al-Ge solid solutions

Al-rich Al-Ge solid solutions synthesized under high pressure demonstrate physical properties strikingly different from those of pure Al. In particular, enhanced superconductivity temperature (T_c), anomalies in the phonon spectra and decrease of the Debye temperature have been observed. We show from first-principles, based on calculations of the electronic spectra and Fermi surfaces of Al-Ge substitutional solid solutions, that an electronic topological transition (ETT) takes place in the system. We predict anomalies in transport properties to be revealed experimentally for Al-Ge solid solutions with the Ge concentration ≈10 at.%. The influence of the ETT on the thermodynamic properties of the system is discussed and, in particular, concentration dependence of the Debye temperature is reproduced in good agreement with experiment.     

Electron spectrum and wave functions of icosahedral quasicrystals

Electron spectra and wave functions of icosahedral quasicrystals have been investigated in the tight-binding approximation using the two-fragment structural model (the Amman-MacKay network) with “central” decoration. A quasicrystal has been considered as a limiting structure in a set of optimal cubic approximants with increasing lattice constants. The method of level statistics indicates that the energy spectrum of an icosahedral quasicrystal contains a singular (nonsmooth) component. The density of electron states has been calculated for the first four optimal cubic approximants of the icosahedral quasicrystal, and the respective Lebesgue measures of energy spectra of these approximants have been obtained. Unlike the case of a one-dimensional quasiperiodic structure, the energy spectrum of an icosahedral quasicrystal does not contain a hierarchical gap structure typical of the Cantor set of measure zero in a one-dimensional quasicrystal. Localization of wave functions in an icosahedral quasicrystal has been studied, and their “critical” behavior has been detected. The effect of disorder due to substitutional impurities on electron properties of icosahedral quasicrystals has been investigated. This disorder makes the electron spectrum “smoother” and leads to a tendency to localization of wave functions. Zh. éksp. Teor. Fiz. 113, 1009–1025 (March 1998)     

Theoretical analysis of impurity distributions in crystalline silicon

Calculations of the total energy of oxygen and carbon impurities in silicon at T=0 K are presented. The equilibrium position of point defects is determined for low (10⁻³–10⁻² at. %) concentrations. Fiz. Tverd. Tela (St. Petersburg) 39, 2001–2002 (November 1997)     

Smooth transition from metastability to instability in phase separating protein solutions

For insight into the structure and dynamics of phases emerging upon crossing the metastability/instability boundary we monitor with optical microscopy, in real time and in real space, the generation of a dense liquid phase in high-concentration solutions of the protein lysozyme after temperature quenches into thermodynamically defined metastable and unstable regions. We show with this system, which is a poor fit to mean-field assumptions, that the evolution of the structure factor during nucleation is similar to that during spinodal decomposition and reveals no singularity predicted upon crossing the metastability boundary. We introduce two kinetic definitions of the metastability/instability boundary that yield values within approximately 1.5 K, i.e., the boundary appears as an area rather than a line, which is near and above the thermodynamic prediction. Delay times for the appearance of the new phase in the unstable regime are significant, i.e., new-phase growth is hindered by kinetic barriers. While our results agree with predictions of the non-mean-field theories of phase transformations, the experimentally observed behavior is richer than the one envisioned by theory.