Pressure-induced isostructural phase transition of metal-doped silicon clathrates

We propose an atomistic model for the pressure-induced isostructural phase transition of metal-doped silicon clathrates, Ba8Si46 and K8Si46, that has been observed at 14 GPa and 23 GPa, respectively. The model explains successfully the equation of state, transition pressure, change of Raman spectra and dependence on the doped cations as well as the effects of substituting Si(6c) atoms with noble metals.Comment: 5 pages, two coumn, 5 figures. See http://www.iitaka.org/down.html for more informatio

Competing Phases, Strong Electron-Phonon Interaction and Superconductivity in Elemental Calcium under High Pressure

The observed "simple cubic" (sc) phase of elemental Ca at room temperature in the 32-109 GPa range is, from linear response calculations, dynamically unstable. By comparing first principle calculations of the enthalpy for five sc-related (non-close-packed) structures, we find that all five structures compete energetically at room temperature in the 40-90 GPa range, and three do so in the 100-130 GPa range. Some competing structures below 90 GPa are dynamically stable, i.e., no imaginary frequency, suggesting that these sc-derived short-range-order local structures exist locally and can account for the observed (average) "sc" diffraction pattern. In the dynamically stable phases below 90 GPa, some low frequency phonon modes are present, contributing to strong electron-phonon (EP) coupling as well as arising from the strong coupling. Linear response calculations for two of the structures over 120 GPa lead to critical temperatures in the 20-25 K range as is observed, and do so without unusually soft modes.Comment: 8 pages, 6 figures, 1 table, accepted for publication in Phys. Rev.

Нематеріальні активи та права на них, складові формування статутного капіталу підприємств та основа маркетингової системи

The world is now on the verge of large-scale transformations. This is happening under the influence of new digital technologies: multidimensional changes are taking place both at the global level and at the level of our state, industries, corporations. Companies change both externally and internally. External transformation introduces a new concept for working with clients, partners and contractors. At this stage, it is not enough to meet the client's expectations, they must be able to foresee his requirements, that are growing every day. A significant number of surveys of top managers of companies around the world show that time-to-market for a product or service is days, not months or years, as it used to be, and this is one of the most difficult challenges of our time. The article examines the system of intangible asset management as part of a system of measures to increase the authorized capital of companies. Accounting, the formation of property rights to intangible assets and their contribution to the authorized capital of the company increases it by 70-80 percent, which should be a consequence for the formation of a new concept of enterprise management. The new concept will not work on the existing management and operational models, since in order to work in the digital world, companies must completely rebuild themselves from the middle and it is necessary to start with a clear formation of the authorized capital, which was not paid attention to during corporatization, since at that time there was a task of rapid transformations without preparation. This is how the joint-stock companies Ukrtelecom, Ukrposhta, and Ukrzaliznytsia were created. For these companies and especially for Ukrtelecom, as for a company in which there is no state property, it is time to develop methodological support, teach specialists and carry out a qualified inventory of all balance sheet items and form its authorized capital, taking into account the requirements of the digitalization of the system and the inclusion of intangible assets and property rights to them in the authorized capital. This is necessary because the traditional model of building a business in our state, the management system and the operating model of the company, are mainly static, focused on slow gradual development.Наразі світ стоїть на порозі масштабних перетворень. Це відбувається  під впливом нових цифрових технологій: ідуть багатомірні зміни як на глобальному рівні, так і на рівні нашої держави, індустрій, галузей, корпорацій. Компанії  змінюються як зовнішньо так і в середині. Зовнішня  трансформація  впроваджує нову концепцію по роботі з клієнтами, партнерами та контрагентами. На даному етапі недостатньо відповідати чеканням клієнта, їх необхідно  вміти  передбачити, а його вимоги з кожним днем ростуть. Значна кількість опитувань  топ-менеджерів компаній по всьому світу показує, що термін виводу продукту або послуги на ринок (time-to-market) – дні, а не місяці або роки, як раніше, – і це є однією із самих важких задач сьогодення. У статті досліджено систему управління нематеріальними активами у складі системи заходів із підвищення розміру статутного капіталу компаній. Облік, формування прав власності на нематеріальні активи та внесення їх до статутного капіталу компанії підвищує його на 70-80 відсотків, що має стати наслідком для  формування нової концепції управління підприємствами. Нова концепція не запрацює  на діючих управлінських  і  операційних моделях тому що для роботи у цифровому світі компанії повинні перебудувати себе повністю з середини і почати необхідно з чіткого формування статутного капіталу, якому при  корпоратизації не була приділена увага, тому що на той час стояла задача  швидких перетворень, без підготовки. Саме так були створені акціонерні товариства «Укртелеком», «Укрпошта», «Укрзалізниця». Для цих компаній і, особливо, для «Укртелекома». як для компанії у якої нема державної власності, настав час розробити  методичне забезпечення, навчити спеціалістів та провести кваліфіковано інвентаризацію всіх статей балансу та сформувати свій статутний капітал з врахуванням вимог цифровізації системи та включення нематеріальних активів  та прав власності на них до статутного капіталу. Це необхідно тому що традиційна модель побудови бізнесу в нашій державі – система управління та  операційна модель компанії, в основному, статична, орієнтована на повільний поступовий розвиток

Linear response results for phonons and electron-phonon coupling in hcp Sc - spin fluctuations and implications for superconductivity

We present a study of the electronic structure, phonon frequencies and electron-phonon coupling in hcp Sc under pressure. The electron-phonon coupling constant is found to increase steadily with pressure in the hcp phase, until the pressure reaches a value where the hcp phase becomes unstable. Calculations for the normal pressure $c/a$ ratio predict a phase change somewhere between calculated pressures of 22 and 30 GPa. The calculated frequencies for the equilibrium hcp lattice parameters are in good agreement with the inelastic neutron scattering results. From the measured value of the electronic specific heat constant and the calculated values of the Fermi level density of states and electron-phonon coupling constant, we conclude that the electron-paramagnon coupling constant in hcp Sc should be comparable to the electron-phonon coupling constant. This indicates that the spin fluctuation effects are strong enough to suppress superconductivity completely in hcp Sc. Based on estimates of the electron-paramagnon coupling constants and the calculated or estimated electron-phonon coupling constants, we argue that the hcp phase may become superconducting with a very low transition temperature immediately prior to the transition to the Sc-II phase and that the Sc-II phase should indeed be superconducting.Comment: To appear in Journal of Physics: Condens. Matter. This is a substantially revised version of an earlier submission (arXiv:0706.3728v1 [cond-mat.supr-con] 25 Jun 2007) which was withdrawn due to some errors in the discussion. The revised version addresses those errors and includes additional result

Diamond and β\beta-tin structures of Si studied with quantum Monte Carlo calculations

We have used diffusion quantum Monte Carlo (DMC) calculations to study the pressure-induced phase transition from the diamond to $\beta$-tin structure in silicon. The calculations employ the pseudopotential technique and systematically improvable B-spline basis sets. We show that in order to achieve a precision of 1 GPa in the transition pressure the non-cancelling errors in the energies of the two structures must be reduced to 30 meV/atom. Extensive tests on system size errors, non-local pseudopotential errors, basis-set incompleteness errors, and other sources of error, performed on periodically repeated systems of up to 432 atoms, show that all these errors together can be reduced to well below 30 meV/atom. The calculated DMC transition pressure is about 3-4 GPa higher than the accepted experimental range of values, and we argue that the discrepancy may be due to the fixed-node error inherent in DMC techniques.Comment: 10 pages, 4 figure

Consistent Anisotropic Repulsions for Simple Molecules

We extract atom-atom potentials from the effective spherical potentials that suc cessfully model Hugoniot experiments on molecular fluids, e.g., $O_2$ and $N_2$. In the case of $O_2$ the resulting potentials compare very well with the atom-atom potentials used in studies of solid-state propertie s, while for $N_2$ they are considerably softer at short distances. Ground state (T=0K) and room temperatu re calculations performed with the new $N-N$ potential resolve the previous discrepancy between experimental and theoretical results.Comment: RevTeX, 5 figure

Raman spectrum and lattice parameters of MgB2 as a function of pressure

We report Raman spectra and synchrotron x-ray diffraction measurements of lattice parameters of polycrystalline MgB2 under hydrostatic pressure conditions up to 15 GPa. An anomalously broadened Raman band at 620 cm-1 is observed that exhibits a large linear pressure shift of its frequency. The large mode damping and Gruneisen parameter indicate a highly anharmonic nature of the mode, broadly consistent with theoretical predictions for the E2g in-plane boron stretching mode. The results obtained may provide additional constraints on the electron-phonon coupling in the system.Comment: 3 pages, 3 figure

Ab initio study of the beta$-tin->Imma->sh phase transitions in silicon and germanium

We have investigated the structural sequence of the high-pressure phases of silicon and germanium. We have focussed on the cd->beta-tin->Imma->sh phase transitions. We have used the plane-wave pseudopotential approach to the density-functional theory implemented within the Vienna ab-initio simulation package (VASP). We have determined the equilibrium properties of each structure and the values of the critical parameters including a hysteresis effect at the phase transitions. The order of the phase transitions has been obtained alternatively from the pressure dependence of the enthalpy and of the internal structure parameters. The commonly used tangent construction is shown to be very unreliable. Our calculations identify a first-order phase transition from the cd to the beta-tin and from the Imma to the sh phase, and they indicate the possibility of a second-order phase-transition from the beta-tin to the Imma phase. Finally, we have derived the enthalpy barriers between the phases.Comment: 12 pages, 16 figure

Uniaxial Phase Transition in Si : Ab initio Calculations

Based on a previously proposed thermodynamic analysis, we study the relative stabilities of five Si phases under uniaxial compression using ab initio methods. The five phases are diamond, beta-tin, sh, sc, and hcp structures. The possible phase-transition patterns were investigated by considering the phase transitions between any two chosen phases of the five phases. By analyzing the different conributions to the relative pahse stability, we identified the most important factors in reducing the phase-transition pressures at uniaxial compression. We also show that it is possible to have phase transitions occur only when the phases are under uniaxial compression, in spite of no phase transition when under hydrostatic commpression. Taking all five phases into consideration, the phase diagram at uniaxial compression was constructed for pressures under 20 GPa. The stable phases were found to be diamond, beta-tin and sh structures, i.e. the same as those when under hydrostatic condition. According to the phase diagram, direct phase transition from the diamond to the sh phase is possible if the applied uniaxial pressures, on increasing, satisfy the condition of Px>Pz. Simiilarly, the sh-to-beta-tin transition on increeasing pressures is also possible if the applied uniaxial pressures are varied from the condition of Px>Pz, on which the phase of sh is stable, to that of Px<Pz, on which the beta-tin is stable