RUI: Using Coherent Phonons for Ultrafast Control of the Dirac Node of SrMnSb2

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Non-technical:In the recently discovered 'Dirac semimetals', electrons behave as though massless and exhibit very high mobilities. This study aims to investigate one such semimetal, SrMnSb2. It has been theoretically predicted that exposure to a short pulse of laser light might change the material's electronic properties, radically, rapidly and reversibly. This project exposes SrMnSb2 to laser pulses, following them up with several advanced optical and X-ray measurements to learn the pulses' influence on the material's atomic positions and electronic properties. This project supports undergraduate and post-doctoral researchers at a primarily-undergraduate institution, who operate laser experiments, participate in experiments at free-electron lasers and other major facilities, write computer code, and analyze complex sets of data. Because of the scientific and industrial relevance of condensed-matter physics, and the rapid growth of ultrafast technology, the students and post-doc become prepared for a wide variety of scientific and technical careers. Technical:The newly discovered Dirac and Weyl semimetals have linearly-dispersing electronic bands, which cross at a node, but this node may sometimes be gapped. The ability to open and close a gap on sub-picosecond timescales could, if achieved, provide an ultrafast on-off switch for many of the materials' exotic optical effects; it could allow researchers to create Dirac fermions on demand and to explore and tune electronic states beyond those available in static materials. In one gapped Dirac material, SrMnSb2, electronic-structure calculations have identified a particular phonon mode which, if excited to sufficiently high amplitude, would shift the atoms' positions enough to close the gap, briefly and periodically, as the phonon oscillates. The purpose of this project is to optically excite the phonon, coherently and to high amplitude, and to observe the subsequent dynamics, thereby enabling sub-picosecond optical control of the gap of this Dirac semimetal and elucidating the physics that governs the gap's closing. The research consists of several optical experiments, each exciting the coherent phonon and then probing its effect with ultrafast resolution. Transient-grating spectroscopy explores the use of multiple laser pulses to control and amplify the phonon. Ultrafast X-ray diffraction measures the time-dependent, absolute displacement of atoms from equilibrium during the phonon's oscillation. Finally, optical-pump, mid-infrared-probe spectroscopy measures the resulting oscillation of the nodal gap, possibly showing the gap's closure at high oscillation amplitude. Together, these measurements can establish the quantitative relations between atomic position, nodal gap, and the pulses that drive the oscillation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.