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Physics & Astronomy Colloquium
March 21, 2018 @ 3:30 pm - 5:00 pm
Speaker: David Hilton (University of Alabama, Birmingham)
Title: Materials in Extreme Environments: Unlocking New Materials Physics in High Magnetic Fields
Abstract: We are constantly pushing materials into new regimes and extremes to try to understand how they function. How fast can the electronic or optical properties of a material be modulated? How do they operate under thermodynamic extremes of temperature, pressure, and/or magnetic field? As we push these materials to these new extremes, are we elucidating new physics or can they be explained using extensions to conventional descriptions of their properties?
Novel two-dimensional materials are one promising platform for next-generation devices that push the limits of both speed and size, but they also require new descriptions and experimental tools to describe their novel properties. In these newer two-dimensional materials like graphene and transition metal dichalcogenides, the relatively short coherence times of these still-developing materials masks some of their unique capabilities for next generation novel electronics. The modulation doped gallium arsenide two-dimensional electron gas (2DEG), in contrast, has seen and continues to see extensive study as one of the more “traditional” platforms for 2D materials. High quality 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.
Traditional measurement in these materials have included a variety of electrical transport measurements [e.g. Phys. Rev. Lett. 48, 1559 (1982)] and time-integrated optical measurements [e.g. Phys. Rev. B 31, 5253 (1985)], while the study of their dynamic properties on subpicosecond time-scales is relatively recent [e.g. Phys. Rev. B 93, 155437 (2016)]. Ultrafast spectroscopic techniques are a powerful technique that can be used to unravel complex and often competing processes in condensed matter systems on a femtosecond time scale. High magnetic field spectroscopy is also 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 in two-dimensional electron gases. In this talk, I will discuss our work using terahertz time-domain spectroscopy to study Landau level populations and coherences in a range of high mobility two-dimensional semiconducting systems and our extensions of these techniques to higher magnetic field spectroscopy. We determine the cyclotron decay lifetime as a function of temperature and explain our low temperature results using ionized impurity and bound interface charge scattering in the conducting layer. We find a striking and substantial deviation in cyclotron decay dynamics below 1.2K in different mobility samples, which is related to the formation of an alternate (spin) ordered electronic phase. In the second part of my talk, I will discuss our recent work to study these materials in high magnetic field using the 25 Tesla Split-Florida Helix at the National High Magnetic Field Lab. Our results reveal a complex interplay between conventional (electron transport) and complex (many-body) electronic interaction on an ultrafast time scale..
This work is funded by NSF CAREER (2DEG Materials Physics, DMR-1056827) and the Department of Energy/Basic Energy Sciences (Instrument Development, DE-SC0012635). 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.
Short Biography: David Hilton received B.S. (1997) and M.S. (1999) degrees in Optics from the University of Rochester. He received a M.S. (2001) and Ph.D. (2002) in Applied Physics from Cornell University. From 2002 to 2006, he was a postdoctoral researcher at Los Alamos National Laboratory in New Mexico, where his research focus shifted to terahertz spectroscopy of correlated electronic systems. From 2006 to 2007, he was a postdoctoral researcher at Rice University, where his interests included the development of novel spectroscopic measurement techniques for high-resolution spectroscopy in high magnetic fields. He joined the faculty as Assistant Professor of Physics at the University of Alabama at Birmingham and was promoted to Associate Professor in 2013. His research program focuses on the study of insulator-to-metal phase transitions in transition metal oxides and ultrafast investigations of high mobility 2DEGs and dichalcogenides.
[Cancelled] Speaker: Duane Lee (Vanderbilt/MIT)
[Cancelled] Title: What Can Modeling Chemical Abundance Ratio Distributions in Dwarf Galaxies Tell Us?
[Cancelled] Abstract: The chemical abundances found in stars convey a wealth of information on stellar evolution from stellar nucleosynthesis (e.g., stellar core burning, AGB phase burning, explosive supernova phase fusion, etc.) to stellar remnant explosive nucleosynthesis (e.g., white dwarf and neutron star mergers). Their chemical abundances also convey information on their likely galactic origins, i.e., are they formed in the bulge/disk of the Milky Way (in situ) or are they originally from an accreted dwarf galaxy or major merger (ex situ)? To make use of the information stellar chemical abundances provide I have constructed semi-analytic models of the chemical abundance ratio distributions (CARDs) found in ultra-faint dwarf (UFD) galaxies (Lee et al. 2013) to better constrain the relative contributions to r-process element enrichment (e.g., Barium, Europium, etc.) from core-collapse supernova and neutron star mergers (Lee & Frebel 2018 [in prep]). These CARDs can also provide useful information for reconstructing the accretion history of Galaxy (Lee et al. 2015). In my talk, I will explain how these CARD models and statistical analysis methods can be used to gain better insights into the origins of the Galactic halo, origins of individual r-process enhanced stars in the UFDs and the origin of r-process elements themselves.
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