Molecular dynamic simulation of a homogeneous bcc -> hcp transition

We have performed molecular dynamic simulations of a Martensitic bcc->hcp transformation in a homogeneous system. The system evolves into three Martensitic variants, sharing a common nearest neighbor vector along a bcc direction, plus an fcc region. Nucleation occurs locally, followed by subsequent growth. We monitor the time-dependent scattering S(q,t) during the transformation, and find anomalous, Brillouin zone-dependent scattering similar to that observed experimentally in a number of systems above the transformation temperature. This scattering is shown to be related to the elastic strain associated with the transformation, and is not directly related to the phonon response.Comment: 11 pages plus 8 figures (GIF format); to appear in Phys. Rev.

Charge kinks as Raman scatterers in quarter-filled ladders

Charge kinks are considered as fundamental excitations in quarter-filled charge-ordered ladders. The strength of the coupling of the kinks to the three-dimensional lattice depends on their energy. The integrated intensity of Raman scattering by kink-antikink pairs is proportional to $\phi ^{5}$ or $\phi ^{4},$ where $\phi$ is the order parameter. The exponent is determined by the system parameters and by the strength of the electron-phonon coupling.Comment: To be published in Phys. Rev.B (june 2001

Stochastic Production Of Kink-Antikink Pairs In The Presence Of An Oscillating Background

We numerically investigate the production of kink-antikink pairs in a $(1+1)$ dimensional $\phi^4$ field theory subject to white noise and periodic driving. The twin effects of noise and periodic driving acting in conjunction lead to considerable enhancement in the kink density compared to the thermal equilibrium value, for low dissipation coefficients and for a specific range of frequencies of the oscillating background. The dependence of the kink-density on the temperature of the heat bath, the amplitude of the oscillating background and value of the dissipation coefficient is also investigated. An interesting feature of our result is that kink-antikink production occurs even though the system always remains in the broken symmetry phase.Comment: Revtex, 21 pages including 7 figures; more references adde

Theory of structure and dynamics in condensed matter. Progress report, 1 April 1973--31 March 1974

Work during the report period is summarized. Data and information are included from studies of the properties of disordered alloys and similar materials. Results are also included on the macroscopic engineering properties of polycrystalline or otherwise heterogeneous materials, theory and interpretation of heat pulse experiments and second sound in crystals, theory of neutron scattering from solid helium, studies of mechanisms of structural phase transitions, and dynamics of classical quadrupole solids. (auth

Viewpoints on Microscopic and Mesoscopic Transformation Processes in Martensite

The purpose of this presentation is to provide some perspective gained from recent studies in condensed matter physics on the microscopic processes by which martensitic transformations take place to supplement the highly developed crystallographic and thermodynamic descriptions. Atomic scale dynamics can be related to mesoscale pattern formation, in idealized systems, and the energies of twin boundaries and ferroelastic patterns can be computed from such data, at least in principle. An overview of the formalism used utilizing nonlinear and strain-gradient elasticity is given. Yet in reality probably all transformations of laboratory or manufactured materials are controlled by defects. An approach to the defect situation based on the framework developed for the idealized martensite is given. Finally a few remarks are made relating this general viewpoint to shape memory materials

Fine scale mesostructures in superconducting and other materials

We review experimental and theoretical literature on mesoscale features and structures in a variety of displacively transforming materials. These include twinning in martensites and the much finer scale tweed; both may have significant effects on both normal and superconducting properties. Elastic properties are particularly strongly affected. Recent x-ray neutron experiments suggest them in hi-{Tc} materials