2019 Fall Physics Colloquium Series: David J. Hilton, Ph.D.

DateNovember 20, 2019Time3:50 - 5:00 pm
LocationBaylor Sciences Building, Room E.125
2019 Fall Physics Colloquium Series

David J. Hilton, Ph.D.
Associate Professor, Department of Physics
Baylor Laboratory for Materials In Extreme Environments (BLMEE)
Baylor University
Ultrafast Spectroscopy of Quantum Materials

Ultrafast spectroscopy is a broadly implemented experimental technique that can be used to understand the behavior of materials away from equilibrium on a time scale as short as a few femtoseconds (10-15 s) or less, depending on the wavelengths of light used. While ultrafast spectroscopic techniques pre-date the invention of the titanium:sapphire laser oscillators and amplifiers, the improved stability and high pulse energies has enabled an entire new generation of experiments that study materials that are impulsively driven from equilibrium on a time scale that is much faster than the individual degrees of freedom (charge, spin, orbital, and lattice) are coupled to each other. This gives us an unprecedented window into light-matter interactions not only to reveal the ground state properties of materials but also using this as a method of controlling properties of materials in ways that cannot be thermodynamically driven. In this talk, I will discuss the research program that we will be building at Baylor to study the nonequilibrium optical and electronic properties of materials in extremes of temperature, laser fluence, and magnetic fields as well as highlight some of our recent results that demonstrate the utility of these techniques for the study of quantum materials. High magnetic field ultrafast spectroscopy is a recently developed extension to these techniques and is a particularly useful optical tool for unraveling complex interactions in these systems, which are a particularly rich source of novel materials physics due to the relative absence of disorder reduced-dimensionality materials. These fall into multiple different classes: quantum-confined semiconductors grown via molecular beam epitaxy and the more recently developed monolayer systems grown via mechanical exofoliation from the bulk. The modulation doped gallium arsenide two-dimensional electron gas (2DEG) continues to see extensive study as one of the more “traditional” platforms for 2D materials. In transition metal dichalcogenides (TMDC), however, the relatively short coherence times of these still-developing materials masks some of their unique capabilities for next generation novel electronics. High quality 2DEG samples with mobilities exceeding >106 cm2 V-1 s-1 are currently available, which provides a model system to study the electronic and optical properties of two-dimensional materials in the “clean” limit. The novel band structure of TMDC’s, however, shows great promise for a new generation of electronics based on the valley degree of freedom. Both systems provide an available platform for studying and controlling quantum coherence in materials for next generation microelectronics applications as well as for emerging devices that will exploit quantum computational techniques as simulators and eventually general-purpose computers. I will discuss our work using high magnetic field terahertz time-domain spectroscopy to study quantum coherence in high mobility two-dimensional semiconducting systems and transition metal. In 2DEG’s, our results reveal a complex interplay between conventional (electron transport) and complex (many-body) electronic interaction on an extremely fast time scale. These results have their origin in the breakdown of the frequency used uniform electron gas description of conductivity in high quality two-dimensional electron gas systems that happens when the magnetic length is on the same order as the material’s lattice constants. In the transition metal dichalcogenides, our results demonstrate the unique role that dimensionality has on the optical and electronic properties of these layered materials. This work has been funded by NSF CAREER (2DEG Materials Physics, DMR-1056827) and the Department of Energy/Basic Energy Sciences (Instrument Development, DE-SC0012635 and DE-SC0019137). Additional funding for graduate students working on these projects comes from the Department of Education GAANN (P200A090143). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490 and the state of Florida. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility.

For more information contact: Dr. B.F.L. Ward, 254-710-4878
PublisherDepartment of Physics
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