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Physics Department Colloquia

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Terahertz frequency topological switches
Date: Friday, November 16th
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Aaron Lindenberg, Standford University
Abstract: Novel characterization techniques developed over the past two decades have revolutionized our ability to visualize the microscopic, atomic-scale processes that determine the functional properties of materials. The overarching challenge here is that the relevant time-scales and length-scales for these processes are typically 10^-13 seconds (100 femtoseconds) and 10^-10 m (1 Angstrom) such that our view of how a material or device functions is often blurred out in time or in space. In this talk I will describe femtosecond-resolution crystallographic measurements probing dynamical switching responses in topological Weyl semimetals. First I will provide a brief introduction to the unique aspects of these materials. I will then show that terahertz frequency light pulses can be used to induce large amplitude interlayer shear oscillations with ~1% strain amplitudes, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements show that this transition is associated with a symmetry change from a non-centrosymmetric to centrosymmetric structure and therefore corresponds to a transition to a topologically trivial phase. We further show that such shear strain serves as an ultrafast, energy-efficient means to induce more robust, well-separated Weyl points or to annihilate all Weyl points of opposite chirality. This work defines new possibilities for ultrafast manipulation of the topological properties of solids and for a topological switch operating at THz frequencies. Reference: "Time-varying shear strain as an ultrafast symmetry switch in a Weyl semimetal,” E. Sie et al., Nature (2018) (in press)
Host: Jim Lawler
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