Atomic Dynamics in Real Space and Time

Atomic and molecular dynamics in strongly disordered matter, such as liquid, cannot be fully described in terms of phonons, because they are marginalized and often overdamped. Their dynamic and transport properties depend on local atomic rearrangements which are strongly correlated. To describe such local dynamics, the usual representation in momentum (Q) and energy (E) space in terms of the dynamic structure factor, S(Q, E), is not effective. We discuss an alternative approach in real space (r) and time (t), with the van Hove function, G(r, t), and show how this approach facilitates understanding of real-space local dynamics of liquids and other disordered systems in the length scale of Å and time scale of pico-second

Spinor and Twistor Formulations of Tensionless Bosonic Strings in Four Dimensions

Spinor and twistor formulations of tensionless bosonic strings in 4-dimensional Minkowski space are constructed. We begin with a first-order action that is equivalent to the Nambu-Goto action in the tensionful case and that leads to a spinorial action in the tensionless case. From this spinorial action, we find an alternative spinorial action useful for constructing a simple twistor formulation of tensionless strings. The twistor formulation is steadily constructed in accordance with a fundamental concept of twistor theory. We investigate local internal symmetries inherent in the twistorial action for a tensionless string and carry out some classical analyses of the tensionless string expressed in a twistorial form.Comment: 30 pages, no figures, minor corrections, a footnote added, published versio

Glass Dynamics at High Strain Rates

We present a shear-transformation-zone (STZ) theoretical analysis of molecular-dynamics simulations of a rapidly sheared metallic glass. These simulations are especially revealing because, although they are limited to high strain rates, they span temperatures ranging from well below to well above the glass transition. With one important discrepancy, the STZ theory reproduces the simulation data, including the way in which those data can be made to collapse onto simple curves by a scaling transformation. The STZ analysis implies that the system's behavior at high strain rates is controlled primarily by effective-temperature thermodynamics, as opposed to system-specific details of the molecular interactions. The discrepancy between theory and simulations occurs at the lower strain rates for temperatures near the glass transition. We argue that this discrepancy can be resolved by the same multi-species generalization of STZ theory that has been proposed recently for understanding frequency-dependent viscoelastic responses, Stokes-Einstein violations, and stretched-exponential relaxation in equilibrated glassy materials.Comment: 9 pages, 6 figure

Atomic dynamics in fluids: Normal mode analysis revisited

Developing microscopic understanding of the thermal properties of liquids is challenging due to their strong dynamic disorder, which prevents characterization of the atomic degrees of freedom. There have been significant research interests in the past few decades to extend the normal mode analysis for solids to instantaneous structures of liquids. However, the nature of normal modes that arise from these unstable structures is still elusive. In this work, we explore the instantaneous eigenmodes of dynamical matrices of various Lennard-Jones argon liquid/gas systems at high temperatures and show that the normal modes can be interpreted as an interpolation of T \to \infty (gas) and T = 0 (solid) mode descriptions. We find that normal modes become increasingly collisional and translational, recovering atomistic gas-like behavior rather than vibrational with increase in temperature, suggesting that normal modes in liquids may be described by both solid-like and gas-like modes