Prof. Ming Shi from the Paul Schell Institute was invited to give a lecture titled "Massless Dirac and Weyl fermions appearing in semimetals with broken PT symmetry"

Introduction: Dirac semimetals and Weyl semimetals are recently discovered topological quantum states. It is characterized by the intersection of two nondegenerate doubly degenerate energy bands near the Fermi level. These intersections are the source of exotic phenomena including the realization of massless Dirac and Weyl fermions as quasiparticles in bulk states and the formation of surface state Fermi arcs. An important issue in the study of topological quantum states is to study the relationship between different topological phases and the symmetries possessed by different systems. Using our recent angle-resolved photoelectron spectroscopy (ARPES) results, this report will show how Weyl and Dirac semimetallic phases can be realized in symmetric systems with spatial inversion symmetry P and time inversion symmetry T. The report will also discuss whether band degeneracy can be maintained when parity time symmetry is broken. This is crucial for the emergence of massless Dirac fermions as low-energy excitations (quasiparticles) in the system. The report will also briefly describe the progress of the Swiss Light Source's Super ARPES Facility, capable of reaching temperatures below the temperature of liquid helium with high energy and high momentum resolution, enabling SLS's ARPES beamline to lead the global synchrotron ARPES station. !