Faculty: Amit Chakrabarti, Nate Folland, Talat Rahman, and Gary Wysin

Theoretical condensed matter physics covers a broad range of topics related to predicting and designing properties of materials such as solids, polymers, and liquids. Some interest is in structural properties, such as the arrangement of atoms or molecules in the bulk or near a surface, while other interest is in electronic, optical, magnetic, and other responses to applied fields. A broad knowledge of classical and quantum mechanics, electromagnetism, and statistical physics is used to describe a diverse collection of phenomena such as how solar cells work, how atoms stick on surfaces and react, how to make very strong magnets, and how jello solidifies.

The types of systems studied in our group are principally liquids and polymers, metal and semiconductor surfaces, magnetic layers, and fine magnetic particles. Our theorists rely heavily on computer calculations that "simulate" the laws of physics applied to "models" for these condensed matter systems. They can use computer memory elements to represent individual atoms and design programs that have these atoms exerting forces on each other.

This kind of "molecular dynamics" is used to calculate many properties, such as diffusion, dynamic growth of clusters of atoms, and magnetic hysteresis. Other calculational schemes attempt to deduce macroscopic properties from microscopic motions.

Fluid Mixture and Polymers: Amit Chakrabarti studies how a two-fluid mixture or a binary polymer blend undergoes a separation into distinct phases. This phase separation is drastically modified if the mixture is kept inside a porous medium, such as certain complex porous glasses. Phase separation kinetics are studied to see how the porous medium geometry creates barriers to the growth of one phase at the expense of the other. Dr. Chakrabarti also studies the configurations of long-chain polymer molecules adsorbed or grafted to a solid-liquid interface. When such grafted layers are made of two incompatible polymer chain molecules, the phase separation process is again radically different due to the confinement of the molecules to the grafting surface, and the system forms many microdomains rich in one component or the other.

Stress and Electronic Structure: Nate Folland investigates stress in quantum fluids and electronic structure of solids. The studies analyze stress contributions due to mechanical, magnetic, and electric energies and can be applied as a sort of wave function "analyzer."

Atom-Surface Interactions: Using molecular dynamics techniques, Talat Rahman wants to understand how materials such as metals, alloys, and layered structures are formed and how the atoms in the bulk and at the surfaces arrange themselves as their environment and temperature is changed. How do atoms at surfaces become disordered and eventually melt before the atoms in the bulk? These simulations are also used to mimic the process of epitaxial growth and probe the roles that steps, kinks, and other defects play in controlling the growth process.

Dr. Rahman's research also focuses on the mechanisms by which atoms and molecules stick to surfaces and by which they detach from them. Is a chemical bond formed or broken in the process? Or are the atoms and molecules only loosely bound to the surface? Here quantum mechanical calculations are performed to obtain the potential energy surfaces seen by impinging gas molecules. The sensitivity of the chemical process to surface geometry, structure, and temperature can be realized in this way.

Theory of Magnetization: Gary Wysin considers how magnetic models can be used to describe small magnetic particles. He wants to understand what determines how a small magnetic particle can be magnetized, and how its magnetization depends on its previous history. Does it have more to do with the shape of the particle or with special forces on the magnetic moments near the particles surface? How does temperature affect the particle? Energy minimization for magnetic models gives clues to questions like these. The microscopic magnetic moments can have a net twist about some point, called vortices. The vortices are especially interesting because they act like particles with effective charges and mass. Wysin uses spin dynamics simulations to understand what it takes to create these special configurations and how they affect the macroscopic magnetic properties.