Abstract — Materials design and discovery require a thorough knowledge of the underlying physics. Incomplete understanding can lead to misguided searches, both experimentally and computationally. I will illustrate these points with two case studies.
The Mott-Hubbard gaps of rare-earth titanates are commonly reported to be 0.2-0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Rigorous computational and experimental studies for GdTiO3 indicate that the gap is significantly larger, and that the previously identified feature in the optical absorption is due to the excitation of small hole polarons. These findings likely apply to a broader set of materials, and impact the design of complex-oxide heterostructures as well as the search for materials in which the metal-insulator transition can be exploited.
Defect-assisted nonradiative recombination can severely affect the efficiency of electronic and optoelectronic devices. The rule of thumb for assessing whether a defect will lead to strong nonradiative recombination has been based on whether the defect level is close to mid-gap. However, we have found that strong nonradiative recombination can occur for defects that fail to meet this criterion. These insights also impact the search for novel qubits or single photon emitters for quantum information science.
Work performed in collaboration with A. Alkauskas, L. Bjaalie, C. Dreyer, L. Gordon, B. Himmetoglu, A. Janotti, E. Kioupakis, G. Kresse, J. Lyons, J. Shen, J. Speck, J. Varley, J. Weber, D. Wickramaratne, and Q. Yan, and supported by DOE, NSF, and ONR.
About the speaker — Chris Van de Walle is a Distinguished Professor of Materials and the inaugural recipient of the Herbert Kroemer Endowed Chair in Materials Science at the University of California, Santa Barbara. Prior to joining UCSB in 2004, he was a Principal Scientist at the Xerox Palo Alto Research Center (PARC). He received his Ph.D. in Electrical Engineering from Stanford University in 1986, and was a postdoc at IBM Yorktown Heights (1986-1988) and a Senior Member of Research Staff at Philips Laboratories in Briarcliff Manor (1988-1991). He has published over 400 research papers, holds 24 patents and has given 175 invited and plenary talks at international conferences. Van de Walle is a Member of the National Academy of Engineering, a Fellow of the APS, AVS, AAAS, MRS, and IEEE, as well as the recipient of a Humboldt Award for Senior US Scientist, the David Adler Award from the APS, the Medard W. Welch Award from the AVS, and the TMS John Bardeen Award.