J. A. Krumhansl

Bio: J. A. Krumhansl is an academic researcher. The author has contributed to research in topics: Quantum field theory & Path integral formulation. The author has an hindex of 1, co-authored 1 publications receiving 50 citations.

Mean field and exact results for structural phase transitions in one-dimensional and very anisotropic two-dimensional and three-dimensional systems

Abstract: Previous theoretical descriptions of one-dimensional structural phase transitions are extended to two- and three-dimensional systems. The model has relevance to transitions in ferroelectrics, to highly anisotropic organic and polymeric materials, and possibly to current discussions of nonlinear quantum field theories (following from the analogy between functional integral representations of statistical-mechanical partition functions for continuous fields, and Feynman path-integral formulation of quantum mechanics). (auth)

Structural phase transitions III. Critical dynamics and quasi-elastic scattering

Abstract: The dynamical behaviour of systems exhibiting structural phase transitions is reviewed, with particular emphasis on the problems posed by the simultaneous existence of quasi-elastic and phonon features in the dynamic response of the soft coordinates. The results of scattering experiments revealing this phenomenon are surveyed in detail, and are used in an assessment of current theories, subdivided into three categories: those which seek to describe the phenomenon within an extended phonon-based theory; those which attempt to trace the effect to the formation of long-lived clusters of precursor order; and those which associate the quasi-elastic scattering with crystal imperfections.

Kink, breather and asymmetric envelope or dark solitons in nonlinear chains. I. Monatomic chain

Abstract: The authors have examined and obtained analytic expressions for the basic nonlinear excitations in a monoatomic chain with cubic and quartic interatomic potentials and verified their stability under collision. They determined bounds on the potential parameters for which kink, breather, envelope and dark solitons exist. The kink and the envelope of a soliton are determined in the long-wavelength approximation, while the oscillations of the carrier in the envelope soliton are treated exactly. The introduction of second-neighbour interactions (SNI) changes drastically the character of the solutions. Kinks are supersonic if the dispersive term is positive and subsonic if it is negative. In a cubic potential the envelope solitons are asymmetric (there is a net displacement). There is a switch from a kink to a symmetric envelope as the potential parameters are changed; these parameters also determine the regions for which plane waves are unstable. The above excitations can be created by using arbitrary initial conditions which, however, have a net displacement for kinks and the correct Fourier components for an envelope soliton. The asymmetric envelope solitons explain earlier computer simulations using as initial conditions optical gaussian pulses.