Variational Monte Carlo and Configurational Interaction Studies of C60C_{60} and its Fragments

The $C_{60}$ molecule and its fragments are studied using Configuration Interaction (CI) and Variational Monte Carlo (VMC) techniques, within the Hubbard model. Using benzene as a test case, we compare the results of the approximate calculations with exact calculations. The fragments of $C_{60}$ studied are pyracylene, fluoranthene and corannulene. The energies, bond orders, spin-spin and charge-correlation functions of these systems are obtained for various values of the Hubbard parameter, $U$. The analysis of bond orders and correlation functions of these individual molecules allow us to visualise pyracylene as a naphthalene unit with two ethylenic moieties and fluoranthene as weakly bridged benzene and naphthalene units. Corannulene is the largest fragment of $C_{60}$ that we have studied. The hexagon-hexagon(h-h) bond orders are slightly larger than those of the hexagon-pentagon bonds(h-p), a feature also found in other fragments. We also find bonds between two co-ordinated carbon sites to be stronger than bonds involving three coordinated carbon sites. In $C_{60}$, the h-h bonds are stronger than in corannulene and the h-p bonds weaker than in corannulene for all correlation strengths. Introducing bond alternation in the buckyball enhances this difference.Comment: 42 pages, 5 figures available on request, to appear in J. Phys. Che

Internal avalanches in models of granular media

We study the phenomenon of internal avalanching within the context of recently introduced lattice models of granular media. The avalanche is produced by pulling out a grain at the base of the packing and studying how many grains have to rearrange before the packing is once more stable. We find that the avalanches are long-ranged, decaying as a power-law. We study the distriution of avalanches as a function of the density of the packing and find that the avalanche distribution is a very sensitive structural probe of the system.Comment: 12 pages including 9 eps figures, LaTeX. To appear in Fractal

Spectral properties in the charge density wave phase of the half-filled Falicov-Kimball Model

We study the spectral properties of charge density wave (CDW) phase of the half-filled spinless Falicov-Kimball model within the framework of the Dynamical Mean Field Theory. We present detailed results for the spectral function in the CDW phase as function of temperature and $U$. We show how the proximity of the non-fermi liquid phase affects the CDW phase, and show that there is a region in the phase diagram where we get a CDW phase without a gap in the spectral function. This is a radical deviation from the mean-field prediction where the gap is proportional to the order parameter

Dynamics and transport properties of Kondo insulators

A many-body theory of paramagnetic Kondo insulators is described, focusing specifically on single-particle dynamics, scattering rates, d.c. transport and optical conductivities. This is achieved by development of a non-perturbative local moment approach to the symmetric periodic Anderson model within the framework of dynamical mean-field theory. Our natural focus is the strong coupling, Kondo lattice regime; in particular the resultant `universal' scaling behaviour in terms of the single, exponentially small low-energy scale characteristic of the problem. Dynamics/transport on all relevant ($\omega, T$) scales are considered, from the gapped/activated behaviour characteristic of the low-temperature insulator through to explicit connection to single-impurity physics at high $\omega$ and/or $T$; and for optical conductivities emphasis is given to the nature of the optical gap, the temperature scale responsible for its destruction, and the consequent clear distinction between indirect and direct gap scales. Using scaling, explicit comparison is also made to experimental results for d.c. transport and optical conductivites of Ce_3Bi_4Pt_3, SmB_6 and YbB_{12}. Good agreement is found, even quantitatively; and a mutually consistent picture of transport and optics results.Comment: 49 pages, 23 figure

Low-Lying Electronic Excitations and Nonlinear Optic Properties of Polymers via Symmetrized Density Matrix Renormalization Group Method

A symmetrized Density Matrix Renormalization Group procedure together with the correction vector approach is shown to be highly accurate for obtaining dynamic linear and third order polarizabilities of one-dimensional Hubbard and $U-V$ models. The $U-V$ model is seen to show characteristically different third harmonic generation response in the CDW and SDW phases. This can be rationalized from the excitation spectrum of the systems.Comment: 4 pages Latex; 3 eps figures available upon request; Proceedings of ICSM '96, to appear in Synth. Metals, 199

Self-Structuring of Granular Media under Internal Avalanches

We study the phenomenon of internal avalanching within the context of recently proposed ``Tetris'' lattice models for granular media. We define a recycling dynamics under which the system reaches a steady state which is self-structured, i.e. it shows a complex interplay between textured internal structures and critical avalanche behavior. Furthermore we develop a general mean-field theory for this class of systems and discuss possible scenarios for the breakdown of universality.Comment: 4 pages RevTex, 3 eps figures, revised version to appear in Phys. Rev. Let

Phase Diagram of the Spin-One Heisenberg System with Dimerization and Frustration

We use the density matrix renormalization group method to study the ground state properties of an antiferromagnetic spin-$1$ chain with a next-nearest neighbor exchange $J_2 ~$ and an alternation $\delta$ of the nearest neighbor exchanges. We find a line running from a gapless point at $(J_2 , \delta) = (0, 0.25 \pm 0.01)$ upto an almost gapless point at $(0.725 \pm 0.01, 0$ such that the open chain ground state is $4$-fold degenerate below the line and is unique above it. A disorder line $2 J_2 + \delta = 1$ runs from $\delta =0$ to about $\delta =0.136$. To the left of this line, the peak in the structure factor $S(q)$ is at $\pi$, while to the right of the line, it is at less than $\pi$.Comment: 11 pages, plain TeX, 3 figures available on reques

Immunobiology of a synthetic luteinizing hormone receptor peptide 21-41

Immunization of adult male rabbits with a synthetic luteinizing hormone-receptor peptide (LH-RP; representing amino-acids 21-41 of the extracellular domain of the rat LH receptor) resulted in production of high-titer antibodies capable of interacting with particulate and cell-based LH receptors. The antibody produced was able to inhibit binding of 125I-labeled human chorionic gonadotropin (hCG) to a particulate sheep luteal LH receptor preparation by 40%-50%. Maximal inhibitory activity was correlated with high antibody titer. Immunocytometry revealed that the antibody could directly bind to cells having LH receptors, such as rat granulosa and Leydig cells. The antibodies recognized a 77-kilodalton membrane protein in Western blots of mouse testicular extracts. Interaction of endogenous Leydig cell LH receptor with the LH-RP antibody resulted in both hormone agonist and antagonistic activities. The hormone-mimicking activity (increase in serum testosterone over control) was confined only to the early phase of immunization when the antibody titer was low. Blockade of LH receptor during the later part of immunization resulted in a significant reduction in serum testosterone over controls and inhibition of spermatogenesis. DNA flow cytometry showed that a specific and significant inhibition of meiosis (transformation of primary spermatocytes to round and elongated spermatids P < .01) and spermiogenesis (transformation of round spermatids to elongated spermatids P < .0001) occurred following blockade of LH function