CWRU Physics Faculty

I define the area of condensed matter physics in which I am active as "theory of materials." This is because the emphasis of my work is on the study of real materials, rather than that of general condensed matter phenomena on model systems. Paraphrasing P. A. M. Dirac, one could say that all chemistry and condensed matter physics was solved since the laws of quantum mechanics and the electromagnetic forces were established. Today's computational materials physics attempts to make this bold statement a reality not only in principle, but in practice as well. A major step towards this goal was the development of the density functional method, which has become a "standard model" for reducing the enormous complexity of the many body problem of the electrons in interaction with the nuclei and external fields of a solid state or molecular system to a tractable one. Along with it, highly efficient methods were developed to solve Schrödinger's equation for the electronic states in an effective potential and computers became ever more powerful. Together, these developments have made it possible to predict a wide range of materials properties (e.g. structural, vibrational, mechanical, optical, magnetic, electronic transport) from first principles with an amazing degree of confidence and precision.

Currently, the two main topics of research in my "Electronic Structure Group" are spintronics and nonlinear optical materials. Spintronics is a new term for spin-dependent electronics. Its main idea is to utilize both the spin and charge degrees of freedom of an electron in semiconductor devices. In practice it means merging magnetic materials with semiconductors and faces severe materials challenges. How to create magnetic semiconductors with above room temperature Curie temperature? How to create suitable magnetic or and possibly half-metallic spin-injection materials matched to semiconductors with nearly perfect interfaces? In our group we focus on the transition metal nitrides and their potential in this area.

In terms of nonlinear optics, we work on chalcopyrite semiconductors. These materials have high second order susceptibilities as well as birefringence, thus making them suitable for frequency conversion applications such as second harmonic generation and optical parametric oscillators. We have developed a method for calculating the second order susceptibilities from the band structures and studied the trends in these materials system. Our current main emphasis, however, is on studying the point defects in these materials because they present a road block for their further development. In particular, we have recently focused on studies of the various native point defects in ZnGeP₂.

An important continuing theme of our research group is optical properties and optical materials. By calculating the optical response functions from the underlying electronic band structure, and analyzing them, we assist in the interpretation of various types of optical spectroscopy: X-ray, ultraviolet and visible reflectivity and absorption. Our phonon calculations are used to interpret Raman spectroscopy.

Another topic in our recent research has been the study of high-pressure phase transitions. Our work here has focused on the transformation between tetrahedrally bonded structures such as zincblende and the octahedrally bonded rocksalt structure and has attempted to describe the transition path ways from one structure to another and their link to anisotropic strains.

We also continue some work on wide-band gap semiconductors such as silicon carbide and the group-III nitrides. In particular, we have recently studied the problem of stacking fault growth in silicon carbide and the properties of nanoscale cubic SiC inclusions in a hexagonal SiC matrix. We also studied transitional metal impurities in SiC.

Finally, I would like to add that in all my work I have enjoyed close collaborations with related experimental work. In spite of the predictive power of the methods that I mentioned above, this is important to keep in touch with reality.