Physics & Astronomy Colloquium

Speaker:  David Hilton (UA Birmingham)

Title: Terahertz Spectroscopy of Two-dimensional Materials at High magnetic field

Abstract:
There has been a growing interest in the physics of two-dimensional materials, both from a fundamental materials physics perspective as well as with an eye towards new materials platforms for next generation microelectronics.  Of these, graphene (monolayer carbon) is the best studied, but there have been a large number of alternate monolayer materials like monolayer transition metal dichalcogenides (i.e. MoS2) that have been discovered within the past decade.  From a materials physics perspective, there is are a wealth of novel physical phenomena that occur and that can be distinct from the parent three-dimensional compounds, but there is still much work that needs to be done to develop both the experimental and theoretical tools needed to understand these novel materials.

In this talk, I will discuss our recent work characterizing the ultrafast far-infrared and terahertz properties of two-dimensional gallium arsenide quantum wells using a custom-designed cyclotron resonance spectrometer using an ultrafast terahertz spectrometer.  We use this system first to study a high mobility  (µ= 3.6 ×106 cm2 V-1 s-1) two-dimensional electron gas due to the extremely high materials quality inherent to MBE-grown two-dimensional samples.  Below 1.5 K, we observe a dramatic slowing of the dephasing rate for cyclotrons at a filling factor of 6.6 that is concomitant with a reduction in oscillation amplitude that we attribute to the dominance of small angle scattering in these materials that limit the deposing rate to 60× the single particle scattering rate.  In the second part of this talk, I will discuss our recent work studying dynamics in two-dimensional materials under external magnetic field in a series of single and multiple quantum well samples.  Mixing of the Landau levels in the multiple well samples dramatically shortens dephasing both exciton and cyclotron lifetimes relative to comparable single quantum wells due to the additional decay pathways. Finally, our recent experiments have demonstrated the extension of this technique to high magnetic fields using the new Split-Florida Helix 25 Tesla magnet system. We have studied multiple quantum well samples in this broadband terahertz spectrometer and observe long lived oscillations with a magnetic-field dependent line width.

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).  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: Dave 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.