|Date||April 23, 2019||Time||3:30 - 5:00 pm|
|Location||Baylor Sciences Building, Room E.125|
David J. Hilton, Ph.D.
Materials in Extreme Environments: Unlocking New Materials Physics in High Magnetic Fields
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 high mobility two-dimensional semiconducting systems and our extensions of these techniques to higher magnetic field spectroscopy. We model our results using the Optical Bloch Equations to determine the dephasing lifetime as a function of temperature and explain our low temperature results using ionized impurity and bound interface charge scattering in the conducting layer. 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 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 materials lattice constants.
|Publisher||Department of Physics|
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