J. P. Rino

The structure and spectrum of the anisotropically confined two-dimensional Yukawa system

We have studied the structural and spectral properties of the classical system consisting of a finite number of charged particles, moving in two dimensions (2D), and interacting through a screened Coulomb potential and held together by an anisotropic harmonic potential. It is known that for the bare Coulomb interaction, the system crystallizes in well defined ordered configurations in which the particles are distributed in shells. However, we have found that the occupation of the shells changes considerably as a function of the screening parameter, and for large screening, the shell structure disappears and the particles form a Wigner lattice. We have shown that the eigenmodes of the system stiffen with increasing screening. By increasing the anisotropy of the confining potential, we were able to drive the system from 2D to 1D; this change occurs through a series of structural transitions. These transitions are reflected in the mode spectrum which collapses into a narrower frequency region with increasing anisotropy.

Dynamic behaviour of silicon carbide nanowires under high and extreme strain rates: a molecular dynamics study

Molecular dynamics simulations are used to investigate the dynamic behaviour of SiC nanowires under strain rates between 2 × 109 s−1 and 2 × 1011 s−1. Nanowires of different cross sections in the wurtzite (WZN) and zinc blende (ZBN) phases are considered under tensile and compressive deformation. Results show contrasts and similarities in the behaviour of WZNs and ZBNs for the lowest strain rate. (i) WZNs present a continuous structural transformation in the elastic regime under compressive deformation, to a h-MgO structure, while ZBNs display a similar kind of transformation to the β-Sn structure under tensile deformation. (ii) Under tensile deformation WZNs fail by brittle fracture while ZBNs display complex plasticity before failure. (iii) Under compressive deformation both ZBNs and WZNs show buckling and plasticity. For the highest strain rate the mechanical behaviour is similar: both WZNs and ZBNs show induced amorphization for both tensile and compressive deformations.

Structural and dynamic properties of vitreous and crystalline barium disilicate: molecular dynamics simulation and Raman scattering experiments

In this paper, we use a molecular dynamics simulation and Raman scattering measurements to study the vibrational and structural characteristics of barium disilicate, BaSi2O5, in vitreous and crystalline states. Our proposed atomic interaction potential describes the structural and dynamic behaviour of this material very well and can also be used for further extended studies. In addition, Raman spectroscopy enabled validation of the predictions of the potential by comparing the simulated vibrational density of states with the spectrum of the material in its vitreous state. Furthermore, we characterized the kinetics of the crystallization process through in situ Raman measurements as a function of temperature.

Modeling ferroelectric permittivity dependence on electric field and estimation of the intrinsic and extrinsic contributions

The nonlinear dependence of the dielectric permittivity on electric field e (E) is modeled by the relationship e(E) = e00 + 1/(α + β Eeff 2 ), where Eeff = E + κ P(E). Using an appropriate mathematical representation of the polarization loop P(E), it is possible to describe correctly the hysteretic behavior of the curve e(E). This model allows the separation of the intrinsic and extrinsic contributions to the dielectric response from the experimental measurement of e(E). The model is validated for PZT and PLZT ferroelectric thin films. It is shown that the polarization hysteresis loop can be reconstructed from the curve e(E), which can be very useful for separating polarization current from leakage current in ferroelectric samples.

Low-temperature elastic anomalies in CaTiO3: dynamical characterization.

Pulse-echo ultrasonic measurements of elastic coefficients of CaTiO(3) show anomalous behavior around 200 K, with a notable rise in the attenuation coefficient. Molecular dynamics simulation is used to simulate the elastic response of a mono-domain (MDm) and a poly-domain (PDm) configuration of CaTiO(3) using the Vashishta-Raman interatomic potential. The PDm is obtained by cooling the melt from 3600 to 300 K at a rate of 0.5 K ps(-1), so that it recrystallizes to the PDm orthorhombic configuration. The elastic behavior is simulated in the temperature range from 300 to 20 K. In the MDm, it is observed that the bulk modulus varies linearly with temperature, while in the PDm an anomalous hardening is seen around 210 K. The bulk modulus of the PDm fluctuates strongly and is lower than that of the MDm. Neither the pair correlation function nor the Ti-Ti-O bonding angle indicate a true structural phase transition in this range of temperatures. Given the absence of any apparent change in the structure, a possible explanation for this phenomenon is the emergence of a certain class of dynamical instability associated with domain wall motion. Curiously, the pressure fluctuations in both the MDm and PDm configurations follow a power law distribution f ~ P(-α), with the exponent independent of applied strain and temperature. Time series for pressure are used to analyze the dynamics by time-delay reconstruction techniques. The calculus of embedding and correlation dimension indicates that in the polycrystalline configuration, low-dimension dynamics (<26) appears, which tend to disappear at higher temperatures.

STATIC STRUCTURE FACTOR OF HARD-SPHERE YUKAWA SYSTEMS

We have applied the Singwi, Tosi, Land and Sjolander approximation for the two-particle distribution function in the BBGKY hierarchy equations to investigate the properties of the hard-sphere Yukawa systems. The static structure factor and the radial distribution function are evaluated and compared with other approximations of the theory of liquids and computer simulations.

MOBILITY OF SURFACE ELECTRONS OVER THE 3HE-4HE MICROSTRATIFIED SOLUTION

Abstract We have calculated the ripplon-limited mobility of electrons localized over the 3He–4He solution, whose surface contains a thin film enriched with 3He, by taking into account viscous damping in the evaluation of the dispersion relation of interfacial modes. We have considered the electron–ripplon scattering from the low-damped capillary-like mode, which is similar to that of ripplons in uniform liquid. The temperature dependence of the electron mobility is shown for some values of the film thickness. The mobility is also calculated for electrons over pure bulk 3He and an excellent agreement is found with the experimental results.

Domain wall contribution to the nonlinear dielectric response: effective potential model

Domain wall displacement has an important contribution to the different nonlinear dielectric responses observed in ferroelectrics. For a moderated alternating electric field, domain walls perform a small displacement around their equilibrium positions. Such motion of the domain walls can be modelled as a body moving in a viscous medium under the action of an effective potential W(l). From this model the dispersion relationships are derived. The exact expression for the effective potential is found assuming that the dielectric permittivity depends on the electric field strength as . The effect of multidomain structure and polarization hysteresis are introduced through the effective field approximation . An important merit of the model is that it allows the simulation of transient polarization processes for the arbitrary input signal, predicting a power law for the polarization and depolarization currents. An analytic expression is found for the dependence of the permittivity on the electric field strength that correctly reproduces its hysteretic behaviour. The polarization loop and nonlinear dielectric response for subswitching the alternating electric field are simulated and compared with experimental data obtained from PZT thin films. It was observed that the simulated dielectric loss was lower than the experimental one, which can be explained as a result of the interaction of domain walls with defects. Point defects are introduced into the model as a perturbation of the effective potential, showing the dependence of the dielectric loss on the concentration of the defects.

MOLECULAR-DYNAMICS STUDIES OF CHARGE COMPLEXES IN LIQUID HELIUM

Abstract Molecular-dynamics simulations were employed to describe the solid–liquid phase transition of charge complexes in helium films. These two-dimensional atomic-like states, called diplons, arise from the coupling between surface electrons on helium film and positive ions localized in the helium film–substrate interface. The effects of the distance between the helium film and the substrate on the structural properties of the diplon system are investigated. The melting temperature was determined by analyzing quantities like the total energy per particle, the self-diffusion coefficient, and the pair correlation function.