Brief Summary of Research Programs

1. Statistical mechanics of self-assembly of amphiphiles, Structural and electronic properties of nanoporous materials containing proto-zeolitic seeds.

A major part of my NSF CRG (Chemical Research Group) research program (Billinge, Hogan, Kanazidis, Mahanti, Pinnavaia, Thorpe) involves a fundamental understanding of the physics of self-assembly of amphiphiles and physical properties of confined particles inside meso- and nanoporous media. The types of porous media we are concerned with have pore sizes ranging from several angstroms (as in Zeolites) to about 50 angstroms (as in MCM41 and novel systems discovered at MSU by Prof. Pinnavaia's group). We have developed a very efficient model to study via computer simulation the self-assembly problem and have been able to understand the physics of micellar formation and both co-operative and non co-operative routes to the formation of ordered structures. With Aniket Bhattacharya, who was a research associate here and is now an Assistant professor at the University of Central Florida, I am working on the temperature dependence of the critical micelle concentration using 3-dimensional lattice simulations. We have proposed a new way to look at this problem by monitoring the peak in the heat capacity as a function of temperature near the micellar transition. With Prof. Pinnavaia, we are exploring the possibility of using proto-zeolitic nano-clusters in enhancing the structural stability of nanoporous materials. Hoang Khang (grad student) and Hong Li (research associate) are involved in this project.

2. Spin dynamics, tunnelling and localization properties of electrons in doped manganites (systems showing colossal magneto-resistance) within Double-Exchange model

The observation of metal-insulator and ferromagnetic-paramagnetic transitions near the same temperature in some doped perovskite manganites La1-xMxMnO3 (M is a divalent atom) has made these systems potentially exciting because they exhibit colossal magneto-resistance. The fundamental physics of these systems is governed by the so-called Double-Exchange (DE) model. Tom Kaplan and I have been carrying out exact calculations of the eigenstates of this model to understand unusual neutron scattering data which show, contrary to expectations, spin-wave excitations describable by a nearest-neighbour Heisenberg Hamiltonian.  We have discovered unusual low-energy excitations (we call these non-Stoner excitations) in this DE model, which arise from the quantum mechanical nature of the localized spins. We are currently looking at the question of the stability of the fully saturated ferromagnetic state in the DE model as it has important implications in the tunneling measurements between a DE metal and a superconductor using Andreev reflection.

3. Electronic and Structural Studies of Narrow-band gap Semiconductors

Prof. Kanazidis of Chemistry, Prof. Tim Hogan of EE, and I have a joint synthesis, measurement, and theory program (ONR-MURI) to study novel ternary and quaternary systems containing both rare-earth and transition metal atoms for their potential thermoelectric properties. I am concerned with the theoretical understanding of the electronic structure and transport in these new systems. Since optimum thermoelectric response near room temperature and below is expected in narrow-gap semiconductors (gaps of the order of 0.25 eV or less), my major focus has been to understand the electronic structure of different classes of systems and the physics behind the gap formation.      Daniel Bilc, Salameh Ahmad, and Keyur Desai (three grad students) are working on the structural and electronic properties of several ternary and quaternary systems. We use either LDA (local density approximation) or its improved version, GGA (generalized gradient expansion) methods in our calculations. In several systems, theoretical predictions regarding the nature of the ground state (metal or semiconductor) have been verified experimentally. In contrast to Mott insulators and other large gap semiconductors like Si, Ge, GaAs etc, the LDA and GGA seem to be doing better in predicting the gap structure in many narrow-gap semiconductors.  We are trying to understand the reason behind this. In addition to the electronic structure studies we have recently generalized the Faraday Field line method to carry out Monte Carlo simulation of Coulomb Lattice gas systems to understand the charge ordering in ternary and quarternary alloys of thermoelectric significance. I expect these studies will have applications in the areas of structure of mixed perovskite ferroelectrics.      In addition to the above bulk system studies, we are trying to understand the scanning tunneling spectroscopic measurement results of Sergei Urazhdin and Stuart Tessmer on Bi2Se3 surfaces.