Speaker: Nicholas Sirica (LANL)

Title: Shedding Light on Topological Semimetals

Abstract: Much of our understanding in physics derives from the study of symmetry, and how such symmetry underlies the various conservation laws found in nature. Condensed matter systems are naturally understood by their symmetries, and it is through symmetry, or more importantly symmetry breaking, that technologically relevant emergent properties, including magnetism, ferroelectricity, and superconductivity, can be understood. More recently, symmetry is seen to play a prominent role in a new class of materials known as topological semimetals. In Dirac semimetals, which are three-dimensional analogs to graphene, symmetry protects the degeneracy of the Dirac point, while for Weyl semimetals, the breaking of time-reversal or inversion symmetry acts to lift the degeneracy of the Dirac point, giving rise to two Weyl nodes. By sensitively probing symmetry change through use of both conventional diffractive techniques as well as less-conventional nonlinear optical methods, we will explore how light fields themselves can drive symmetry breaking on ultrafast timescales. In this talk, I will outline our most recent work on transient symmetry breaking in the Weyl semimetal TaAs, and make the case that the spectroscopic signatures probed in our study are completely generic and can be extended to many other material systems beyond this class of materials. 

Short Bio: Nicholas Sirica (Los Alamos National Laboratory) is a staff member working in the Laboratory for Ultrafast Material and Optical Science as a part of the Center for Integrated Nanotechnologies. A condensed matter experimentalist, his work focuses on the use of static and ultrafast optical, x-ray, and electron spectroscopies to study novel material systems. A graduate of the University of Tennessee, Knoxville, he came to LANL in 2017 as a Seaborg Fellow and joined MPA as a staff member in 2019. Since his arrival at LANL his research has predominately focused on topologically non- trivial materials and their excitations.