Lattice Dynamics and the High Pressure Equation of State of Au

Elastic constants and zone-boundary phonon frequencies of gold are calculated by total energy electronic structure methods to twofold compression. A generalized force constant model is used to interpolate throughout the Brillouin zone and evaluate moments of the phonon distribution. The moments are used to calculate the volume dependence of the Gruneisen parameter in the fcc solid. Using these results with ultrasonic and shock data, we formulate the complete free energy for solid Au. This free energy is given as a set of closed form expressions, which are valid to compressions of at least V/V_0 = 0.65 and temperatures up to melting. Beyond this density, the Hugoniot enters the solid-liquid mixed phase region. Effects of shock melting on the Hugoniot are discussed within an approximate model. We compare with proposed standards for the equation of state to pressures of ~200 GPa. Our result for the room temperature isotherm is in very good agreement with an earlier standard of Heinz and Jeanloz.Comment: 13 pages, 8 figures. Accepted by Phys. Rev.

A non trivial extension of the two-dimensional Ising model: the d-dimensional "molecular" model

A recently proposed molecular model is discussed as a non-trivial extension of the Ising model. For d=2 the two models are shown to be equivalent, while for d>2 the molecular model describes a peculiar second order transition from an isotropic high temperature phase to a low-dimensional anisotropic low temperature state. The general mean field analysis is compared with the results achieved by a variational Migdal-Kadanoff real space renormalization group method and by standard Monte Carlo sampling for d=3. By finite size scaling the critical exponent has been found to be 0.44\pm 0.02 thus establishing that the molecular model does not belong to the universality class of the Ising model for d>2.Comment: 25 pages, 5 figure

Pressure-dependence of electron-phonon coupling and the superconducting phase in hcp Fe - a linear response study

A recent experiment by Shimizu et al. has provided evidence of a superconducting phase in hcp Fe under pressure. To study the pressure-dependence of this superconducting phase we have calculated the phonon frequencies and the electron-phonon coupling in hcp Fe as a function of the lattice parameter, using the linear response (LR) scheme and the full potential linear muffin-tin orbital (FP-LMTO) method. Calculated phonon spectra and the Eliashberg functions $\alpha^2 F$ indicate that conventional s-wave electron-phonon coupling can definitely account for the appearance of the superconducting phase in hcp Fe. However, the observed change in the transition temperature with increasing pressure is far too rapid compared with the calculated results. For comparison with the linear response results, we have computed the electron-phonon coupling also by using the rigid muffin-tin (RMT) approximation. From both the LR and the RMT results it appears that electron-phonon interaction alone cannot explain the small range of volume over which superconductivity is observed. It is shown that ferromagnetic/antiferromagnetic spin fluctuations as well as scattering from magnetic impurities (spin-ordered clusters) can account for the observed values of the transition temperatures but cannot substantially improve the agreeemnt between the calculated and observed presure/volume range of the superconducting phase. A simplified treatment of p-wave pairing leads to extremely small ($\leq 10^{-2}$ K) transition temperatures. Thus our calculations seem to rule out both $s$- and $p$- wave superconductivity in hcp Fe.Comment: 12 pages, submitted to PR

Developments in synthesis, characterization, and application of large, high-quality CVD single crystal diamond

Single crystal diamond synthesis by microwave plasma chemical vapor deposition at rapid growth rate has considerably advanced in the past few years. Developments have been made in growth, optical quality, and mechanical properties. Of the various types of single crystal diamond that can be produced using these techniques, high quality single crystal CVD diamond can be routinely produced, and this material is playing an increasing role in research on materials under extreme conditions. This article highlights recent developments in single crystal CVD diamond synthesis and characterization, as well as various applications in high-pressure materials research.За останні кілька роки синтез монокристалів алмазу мікрохвильовим плазмовим хімічним осадженням з газової фази з високою швидкістю значно просунувся. Прогрес було досягнуто у рості, оптичних і механічних властивостях. З різних типів монокристалічного алмазу, що можна отримати з використанням цього методу, регулярно виробляють високоякісні монокристалічні CVD-алмази, і цей матеріал відіграє все більш важливу роль при дослідженні матеріалів в екстремальних умовах. У даній роботі висвітлено останні досягнення в синтезі монокристалічного CVD-алмазу і дослідженні його характеристик, а також його різне застосування у дослідженні матеріалів при високому тиску.За последние несколько лет синтез монокристаллов алмаза микроволновым плазменным химическим осаждением из газовой фазы с высокой скоростью значительно продвинулся. Прогресс был достигнут в росте, оптических и механических свойствах. Из различных типов монокристаллического алмаза, которые могут быть получены с использованием этого метода, регулярно производят высококачественные монокристаллы CVD-алмаза, и этот материал играет все более важную роль в исследовании материалов в экстремальных условиях. В этой работе осветлены последние разработки в синтезе монокристаллов CVD-алмаза и исследовании его характерист

Crystal structure of solid Oxygen at high pressure and low temperature

Results of X-ray diffraction experiments on solid oxygen at low temperature and at pressures up to 10 GPa are presented.A careful sample preparation and annealing around 240 K allowed to obtain very good diffraction patterns in the orthorhombic delta-phase. This phase is stable at low temperature, in contrast to some recent data [Y. Akahama et al., Phys. Rev. B64, 054105 (2001)], and transforms with decreasing pressure into a monoclinic phase, which is identified as the low pressure alpha-phase. The discontinuous change of the lattice parameters, and the observed metastability of the alpha-phase increasing pressure suggest that the transition is of the first order.Comment: 4 pages with three figure

Effect of nanostructuration on compressibility of cubic BN

Compressibility of high-purity nanostructured cBN has been studied under quasi-hydrostatic conditions at 300 K up to 35 GPa using diamond anvil cell and angle-dispersive synchrotron X-ray powder diffraction. A data fit to the Vinet equation of state yields the values of the bulk modulus B0 of 375(4) GPa with its first pressure derivative B0' of 2.3(3). The nanometer grain size (\sim20 nm) results in decrease of the bulk modulus by ~9%

Adaptive Robust Fault-Tolerant Control for Linear MIMO Systems with Unmatched Uncertainties

In this paper, two novel fault-tolerant control design approaches are proposed for linear MIMO systems with actuator additive faults, multiplicative faults and unmatched uncertainties. For time-varying multiplicative and additive faults, new adaptive laws and additive compensation functions are proposed. A set of conditions is developed such that the unmatched uncertainties are compensated by actuators in control. On the other hand, for unmatched uncertainties with their projection in unmatched space being not zero, based on a (vector) relative degree condition, additive functions are designed to compensate for the uncertainties from output channels in presence of actuator faults. The developed fault-tolerant control schemes are applied to two aircraft systems to demonstrate the efficiency of the proposed approaches

Equation of state and phonon frequency calculations of diamond at high pressures

The pressure-volume relationship and the zone-center optical phonon frequency of cubic diamond at pressures up to 600 GPa have been calculated based on Density Functional Theory within the Local Density Approximation and the Generalized Gradient Approximation. Three different approaches, viz. a pseudopotential method applied in the basis of plane waves, an all-electron method relying on Augmented Plane Waves plus Local Orbitals, and an intermediate approach implemented in the basis of Projector Augmented Waves have been used. All these methods and approximations yield consistent results for the pressure derivative of the bulk modulus and the volume dependence of the mode Grueneisen parameter of diamond. The results are at variance with recent precise measurements up to 140 GPa. Possible implications for the experimental pressure determination based on the ruby luminescence method are discussed.Comment: 10 pages, 6 figure

High pressure phases in highly piezoelectric Pb(Zr0.52Ti0.48)O3

Two novel room-temperature phase transitions are observed, via synchrotron x-ray diffraction and Raman spectroscopy, in the Pb(Zr0.52Ti0.48)O3 alloy under hydrostatic pressures up to 16 GPa. A monoclinic (M)-to-rhombohedral (R1) phase transition takes place around 2-3 GPa, while this R1 phase transforms into another rhombohedral phase, R2, at about 6-7 GPa. First-principles calculations assign the R3m and R3c symmetry to R1 and R2, respectively, and reveal that R2 acts as a pressure-induced structural bridge between the polar R3m and a predicted antiferrodistortive R-3c phase.Comment: REVTeX, 4 pages with 3 figures embedded. Figs 1 and 3 in colo