K. R. Subbaswamy

Structural and vibrational properties of fullerenes and nanotubes in a nonorthogonal tight-binding scheme

A simple, computationally efficient method for the evaluation of structural and vibrational properties of carbon is presented. The scheme is based on the generalized tight‐binding molecular dynamics technique applicable to covalent systems. The force constants for the evaluation of vibrational modes are obtained by employing analytic second derivatives of the electronic structure Hamiltonian matrix elements. This method, while providing better accuracy than conventional schemes, greatly expedites the determination of vibrational modes for large size clusters. The efficacy of the method is demonstrated by application to fullerenes and nanotubes. Good agreement is obtained with experiment for bond lengths and vibrational frequencies for these systems.

Structure of Si60. Cage versus network structures

A generalized tight-binding molecular dynamics technique which was used to obtain complete agreement with ab initio results for small silicon clusters is used to optimize the Si60 cage without any symmetry restrictions. The perfect icosahedral cage is found to be unstable, distorting to a lower symmetry C2h structure with no change in the threefold coordination. The relaxed geometry shows increasing tendency for tetrahedral arrangement of atoms, and is energetically unfavorable compared to relaxed bulk-like fragment. This is in contrast to carbon, where the cage is favored.

Optimized structures of C60O and C60O2 calculated by a damped molecular dynamics optimization scheme

Abstract A newly developed universal parameter tight-binding molecular dynamics technique is used to optimize the structure of C60O and C60O2. The most stable complex for C60O is found to be the epoxide, in agreement with experimental results. Incorporation of two oxygens is found to break an underlying carbon-carbon bond. The electronic structures and Mulliken populations are also presented.

Edge localized vibration modes on a rectangular ridge

We present a theory of elastic waves which propagate along a semi‐infinite rectangular ridge made up of a nonpiezoelectric elastic medium, and which are localized at the top and sides of the ridge. Stress‐free boundary conditions are incorporated into the calculation by the assumption of position‐dependent elastic moduli. The equations of motion of the ridge are solved by expanding each displacement component in a double series, each term of which is a product of a Laguerre function and an orthogonal function specially constructed to be exponentially localized at each end of a finite interval. Dispersion curves for the speeds of these ridge waves, as functions of qd are presented, where q is the wave vector describing their propagation along the ridge and d is the half‐width of the ridge. In the limit of large qd, two of these modes approach symmetric and antisymmetric linear combinations of the displacement fields associated with an edge mode localized at each of the two edges of the wedge.

Local density theory of ionic hyperpolarizability

A self‐consistent formulation for the computation of nonlinear susceptibilities within the local density approximation is presented. The formulation correctly takes account of wave function normalization. Numerical results for helium are presented as illustration.

Light scattering spectra of guided wave polaritons in thin crystals: Theory☆

Abstract We present theoretical calculations which explore light scattering from guided wave polariton modes in thin crystals. Under certain conditions, these modes scatter with intensity comparable to that produced by the volume LO phonons.

Attracting solitons and discontinuous lock-in transition in a Peierls-Fröhlich condensate

Abstract We explore the exact soliton solutions of a model Hamiltonian that describes the nonlinear phase-amplitude excitations in a Peierls-Frohlich condensate. Several novel features of soliton physics emerge: the existence of static multi-soliton bound states (solitonic “molecules”) in the commensurate system and solitonic “molecular” lattice in the incommensurate system. We show that a discontinuous transition to an incommensurate ground state can occur. Implications of these results to doped polyacetylene and to the spin-Peierls system are discussed.

Intriguing properties of kinks in a simple model with a two-component field

Abstract We present a summary of the topological and non-topological solitons of a two component field in 1+1 dimensions with application in field theory and in condensed matter physics. We note several intriguing analytical and numerical relationships between these solutions, which we believe to suggest that the relevant coupled nonlinear differential equations may be integrable exactly.

Elastic plates model of domain walls in intercalated graphite

Abstract The recently proposed model of thin elastic plates coupled via harmonic forces for the calculation of domain wall energies in intercalated graphite is extended to higher stages. It is found that whether coherent domains bind to each other depends on the degree of stagger. The effect of the loss of coherence and the interface between different stages are discussed.

Raman scattering from potassium iodide near the melting point

Abstract We have measured the Raman spectrum of potassium iodide in the crystalline phase at high temperatures, and in the melt. As temperature increases the various second order features in the low temperature spectra coalesce into a broad feature, which persists into the melt. A smooth, quasi-exponential wing also appears in the melt. Thus, the spectrum is structurally similar to those found in solid electrolytes and silver halide melts. Possible interpretations of the spectrum are indicated.