G Venkataraman
Articles written in Pramana – Journal of Physics
Volume 5 Issue 4 October 1975 pp 175-184 Solids
Dynamics of a crystal containing a molecular impurity—I. Rigid molecule approximation
The dynamics of a crystal containing a rigid, isolated substitutional molecular impurity is discussed using the Green’s function method. The dynamical matrix for the problem is set up, and the various constraints on the force constants are pointed out. The application of the matrix partitioning technique is then indicated, and the possibility of a resonance mode arising from molecular librations is pointed out. Comparison is made with the earlier work of Wagner, and finally the relevance of the present formalism to the interpretation of a recent neutron scattering experiment is discussed.
Volume 5 Issue 4 October 1975 pp 185-210 Solids
Dynamics of a crystal containing a molecular impurity—II. Molecular vibrations included
The dynamics of a crystal containing a molecular impurity is discussed with allowance for the effects of internal vibrations of the molecule. Cartesian co-ordinates are introduced to describe internal vibrations, angular oscillations and centre of mass motions of the impurity, and the displacements of the atoms of the host crystal. Next the Hamiltonian is set up and the equations of motion derived. In this process, use is made of Dirac brackets when dealing with coordinates having redundancy and constraints. From the dynamical matrix, some of the familiar results of the crystal-field approximation are recovered. The application of the partitioning technique is then discussed, and finally comparison is made with results of other approaches.
Volume 13 Issue 3 September 1979 pp 205-218 Solids
Analysis of ultrasonic anomaly in V_{3}Si
V_{3}Si exhibits an ultrasonic anomaly when cooled well below its martensitic and superconducting transition temperatures (
Volume 16 Issue 1 January 1981 pp 1-16 Solid State Physics
Raman study of temperature dependence of lattice modes in calcite
A K Sood A K Arora V Umadevi G Venkataraman
The temperature dependence of the line width and the peak position of the
Volume 16 Issue 2 February 1981 pp 109-130 Statistical Physics
Two-state random walk model of lattice diffusion. 1. Self-correlation function
Diffusion with interruptions (arising from localized oscillations, or traps, or mixing between jump diffusion and fluid-like diffusion, etc.) is a very general phenomenon. Its manifestations range from superionic conductance to the behaviour of hydrogen in metals. Based on a continuous-time random walk approach, we present a comprehensive two-state random walk model for the diffusion of a particle on a lattice, incorporating arbitrary holding-time distributions for both localized residence at the sites and inter-site flights, and also the correct first-waiting-time distributions. A synthesis is thus achieved of the two extremes of jump diffusion (zero flight time) and fluid-like diffusion (zero residence time). Various earlier models emerge as special cases of our theory. Among the noteworthy results obtained are: closed-form solutions (in
Volume 16 Issue 6 June 1981 pp 437-455 Statistical Physics
Two-state random walk model of diffusion. 2. Oscillatory diffusion
Continuing our study of interrupted diffusion, we consider the problem of a particle executing a random walk interspersed with localized oscillations during its halts (
Volume 22 Issue 5 May 1984 pp 387-405 Statistical Physics
Phase transitions of a feedback amplifier
The phase transition behaviour of an amplifier with positive feedback is experimentally studied. The results are interpreted using catastrophe theory language. Zero (in Gilmore’s classification), first and second-order transitions are demonstrated by driving the system along appropriate trajectories in control parameter space and the cusp and the spinodal are mapped. The fluctuations of the order parameter are investigated and their relationship to system response time established. Quench experiments analogous to those familiar in condensed matter have also been performed and with similar results.
Volume 23 Issue 5 November 1984 pp 573-593 Solid State Physics
Infrared studies on ion irradiated quartz
A K Sood V Umadevi R Kesavamoorthy G Venkataraman
Damage produced in
Volume 24 Issue 1-2 January 1985 pp 317-350 Solid State Physics And Materials Science
Melting in two dimensions—the current status
The current status of the controversy relating to melting in two dimensions is surveyed. To begin with, a review is given of the seminal work of Kosterlitz and Thouless. This is followed by a discussion of the modifications introduced by Nelson and Halperin. The search for the continuous transitions and the intermediate hexatic phase predicted by these theories is then described, covering both the laboratory as well as simulation experiments. Alternate viewpoints to the
Volume 36 Issue 1 January 1991 pp 1-77 Review
Spin glass, the travelling salesman problem, neural networks and all that
This paper presents an overview of diverse topics that are seemingly different but interrelated, with strong connections to statistical mechanics on the one hand and spin glass physics on the other. Written primarily for an inter-disciplinary audience, we start with a brief recapitulation of the relevant aspects of statistical mechanics, particularly those needed for understanding the recently-popular simulated-annealing technique used in optimization studies. Then follows a survey of the spin glass problem, with particular attention to the consequences of quenched randomness. The travelling-salesman problem is considered next, as also the impact made on it by the spin glass problem. Several examples are then presented of optimization studies wherein the simulated-annealing concept has been profitably used. Attention is also drawn in this context to the lessons provided by the spin glass problem. Finally, a brief survey of neural networks is made, essentially from a physicist’s point of view. The different learning schemes proposed are discussed, and the relevance of spin models and their statistical mechanics is also discussed.
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