## Events at Physics |

### Events on Friday, November 16th, 2018

**Theory Seminar (High Energy/Cosmology)****Dynamical field range and mass hierarchies****Time:**2:00 pm**Place:**5280 Chamberlin Hall**Speaker:**Aitor Landete, University of Wisconsin-Madison**Abstract:**Several swampland conjectures suggest that there is a critical field range beyond which the effective field theory description breaks down in quantum gravity. In this talk we will review applications of these conjectures to axion monodromy models. We argue that the field range of interest is the field space distance traced by the physical trajectory that solves the equations of motion. A sufficiently large mass hierarchy can delay the breakdown of the effective field theory and allow simple techniques of moduli stabilization. In absence of such hierarchy multi-field techniques should be used. We illustrate these subtleties in Type II string compactifications.**Physics Department Colloquium****Terahertz frequency topological switches****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