Condensed Matter Physics

The area of major concentration is condensed matter physics, which includes the work of two experimentalists, David Carroll and Richard T. Williams, and three theorists, Natalie A. W. Holzwarth, William C. Kerr, and G. Eric Matthews.

The group working with Richard Williams uses ultrafast spectroscopy to investigate the immediate consequences of absorption of a photon by a crystalline solid. They use ultrashort (~80 fs) laser pulses to create an electronic excitation of the solid and monitor its evolution within the coupled electron-lattice system. Phenomena of interest include self-trapping of excitons and of charge carriers in insulators, relaxation of hot carriers and surface state dynamics in semiconductors, excitonic processes of lattice defect formation, and desorption from surfaces. Photoelectron spectroscopy and related surface science techniques are conducted both with ultrafast excitation/probe laser techniques for dynamic studies and in conventional synchrotron radiation experiments at the National Synchrotron Light Source.

Concerns with surfaces, lattice defects, and laser-solid interactions lead naturally to a parallel interest in the spatial atomic structure of surfaces and surface defects, observed directly by scanning tunneling microscopy and atomic force microscopy. The scanning tunneling microscope is used to characterize laser effects on metals and semiconductors, as well as more general atomic-scale studies.

While the focus is basic research, the department is extensively involved in laser processing of materials. Richard Williams, Eric Matthews, and their students have produced and characterized thin film high-Tc superconductors of good quality. Applied research in laser planarization and bonding of metals and in photoablation of polymers is conducted by the Williams group.

William Kerr leads a group in the study of statistical physics. The research in this group concentrates on systems in which nonlinearity is the dominating feature. In some situations nonlinearity produces highly organized behavior, which is described by solitons. In other cases nonlinearity produces seemingly random behavior, which is often characterized as chaos. This group studies these phenomena by computer simulations. Their emphasis recently has been on studying soliton mechanisms underlying the dynamics of first- and second-order phase transitions. The experimental impetus for these studies comes from structural phase transitions in solids. The major issues here include the existence or nonexistence of soft modes and of precursor fluctuations of the product phase occurring within the parent phase. Such fluctuations would be described by nonlinear partial differential equations similar to the sine-Gordon or nonlinear Schrodinger equations, perhaps with extensions which describe bond anharmonicity in addition to the site anharmonicity. In addition to these important physical questions, employing computer simulation to study these problems raises a set of methodological problems. These deal with writing computer codes that efficiently use vector supercomputer architectures and with developing methods of analysis, including visualization schemes, that effectively extract the important physical processes occurring in the system from the large volume of numbers produced by the simulations .

Natalie Holzwarth and Rick Matthews lead research groups that conduct first principles computer modeling of electronic and structural properties of materials. They work closely with experimental groups on a variety of projects including the study of new materials, surfaces, and defects in crystals.

Most of the modeling is based on density functional theory, developed by Walter Kohn and his collaborators in 1964 (for which he shared the 1998 Nobel prize in Chemistry). The work is divided between a focus on code and algorithm development and a focus on materials physics.

The group has developed some general purpose electronic structure code which is made available on the web at http://pwpaw.wfu.edu.

Some of the recent materials studied by the group include scintillating crystalssuch as PbWO₄, F-centers in alkali halide crystals, and surface states related to photoemission experiments performed in Richard Williams' lab. Other projects include studies of molecular crystals such the one illustrated in the figure and Li ion battery materials such as the cathode materials (FeLiPO₄, FePO₄) and electrolytes (Li₃PO₄).

Ball and stick drawing of organic molecule (BEDT-TTF) with superposed contours of electron density in indicated planes from calculations of P. Tang and N. Holzwarth with experimental advice from J. Qualls.