Faculty: Hongxing Jiang, Bruce Law, Jingyu Lin, Michael O'Shea, and Chris Sorensen

Condensed matter physics is a broad area of inquiry into the manifold properties of matter in the solid, liquid, and (despite the name "condensed") gaseous states. We are trying to understand the wide variety of phenomena seen in material objects all around us. What is the source of magnetism? How does it depend on the size of a system? What determines the electronic and optical properties of semiconductors? Can we tailor these properties to meet our needs? What happens during phase transitions? Why do materials boil or condense, melt or freeze? How do small particles nucleate and then aggregate together? How do these aggregates scatter light?

Due to its breadth and intellectual challenge, condensed matter physics occupies the largest number of physicists in the world. It is directly responsible for giving us major technological advances such as transistors, integrated circuits, superconductors, and new materials of nearly every possible description. Experimental condensed matter physics at K-State is an established and growing enterprise. Our program consists of lone investigators or small groups working with state-of-the-art equipment to wrestle secrets from nature. We offer our students challenging and thorough training while they explore a wide variety of research problems.

Semiconductor Physics--Optical Properties: Semiconductors have changed our world profoundly in the last 30 years. Most of our semiconductor technology is based upon silicon or gallium arsenide and designed to accurately control the flow of charge in a circuit. An important new area of semiconductor technology is the development of devices that emit or absorb radiation in the visible and ultraviolet regions. The II-VI and III-V wide bandgap semiconductors perform this function.

Jingyu Lin is leading a research group studying the optical properties of these types of semiconductors. The optical properties are determined by the underlying structure. The influence of disorder on the optical properties is important here.

Disorder is a nearly universal feature of all real materials. It causes the electronic wavefunctions in semiconductors to be localized in space, which affects the electronic, optical and transport properties. Dr. Lin generates electron-hole pairs by the absorption of a laser pulse; these pairs are localized by the disorder and a study of their recombination provides valuable information on the effects of this disorder. These processes occur on timescales of the order of picoseconds, and therefore picosecond, laser pulses and fast detection electronics are required.

Semiconductor Physics--Electronic Transport Processes: Equally important to a comprehensive understanding of II-VI and III-V wide bandgap semiconductors is a study of the electronic transport processes within these materials. This research effort is being led by Hongxing Jiang.

The transport of optically excited electrons and holes through such a material is governed to a large extent by the nature of the disorder and the local environment. The development of integrated circuits over the last twenty years has been to make the packing density of individual transistors ever greater. Consequently there is a great need to understand how semiconductor properties are modified by finite size effects. The semiconductor research group is also investigating the effects of dimensionality and size on semiconductor properties by studying small particles of semiconductor material immersed in a nonconducting matrix.

Magnetism and Magnetic Materials: Another area of importance in today's technological world is the field of magnetism. Thin films of magnetic materials are found in everyday products; i.e., audio tapes and computer memory devices. It is important to understand magnetism on a molecular scale both in the form of thin films and small magnetic particles.

Michael O'Shea studies the magnetism of magnetic multilayers, granular solids, and ultra fine particles. In such systems finite-size effects and interfaces significantly modify the materials properties. The structure of such materials is determined using either an electron microscope or x-ray diffraction, while the magnetization is determined using a SQUID magnetometer.

Dr. O'Shea has found that the anisotropy of a material at an interface is important in determining the magnetic properties of ultrathin multilayers while single domain effects dominate the behavior of ultra-fine particles. In granular solid materials he has discovered unusual hysteretic behavior where it is believed that the interface plays a significant role.

Liquid Surfaces: Surfaces and finite size effects are important not only in solids, but also in liquids. Bruce Law's research is centered around liquid surfaces and the structural phase transitions that can occur on them. The physics of liquid surfaces, thin liquid films, and competitive surface adsorption is rather poorly understood despite its importance in many technological and biological processes such as catalysis, electrolysis, and osmosis. Dr. Law studies how surface forces and bulk liquid phase transitions influence surface structural transitions and competitive adsorption at liquid surfaces. He studies the dynamics of surface nucleation, growth, and coalescence of microscopic liquid droplets into uniform liquid films using the techniques of multi-angle light scattering, high precision video microscopy, and surface-induced polarization of laser light.

Magnetic Particles, Soot, Supercooled Water, Gels: Chris Sorensen is involved in a number of different projects, most of which center around the statistical nature of matter. He is interested in very small magnetic particles and how the magnetization varies with particle size. Dr. Sorensen also studies the aggregation of fractal soot particles in a flame by examining the spectrum, intensity, and polarization of scattered laser light. Small particles aggregate to form random, branched, chain-like structures because of the interactive forces between particles. These random structures can now be described using the concepts of fractals where the branches of the chain appear the same at different length scales.

Dr. Sorensen also studies the anomalous properties of supercooled water (below 0øC) using Raman spectroscopy and small angle x-ray scattering. Water is one of our most unusual and important substances; it plays a central role in all biological materials. The kinetics of gelation is also being studied within Dr. Sorensen's research group using the technique of dynamic light scattering. Long polymer chains dissolved in water form a gel on cooling and exhibit unusual relaxation behavior because of the entanglement of the polymer chains.

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Law.Gif

Bruce Law points to a critical connection on his ellipsometer while Craig Caylor and Dan Smith watch.