Lecturer: Dr. Haim Beidenkopf Affiliation: Weizmann Institute of Science Abstract: A defining property of a topological material is the existence of surface bands that cannot be realized but as the termination of a topological bulk. In a Weyl semi-metal these are given by the surface Fermi-arcs, whose open-contour Fermi-surface curves between pairs of surface projections of bulk Weyl points of opposite chirality. We show first scanning tunneling spectroscopic visualization of these Fermi arc states on the surface of the recently discovered Weyl semi-metal tantalum arsenide (TaAs). Its surface hosts 12 Fermi arcs alongside several surface bands of non-topological origin. Using the distinct structure and spatial distribution of the wave functions associated with the different bands we detect all possible scattering processes in which Fermi arcs are involved (intra- and inter arc and arc-trivial). Each of these imaged scattering processes entails information on the unique nature of Fermi arcs in TaAs: their contour, their dispersion and its relation with the Weyl points, the relative uniform structure of their Bloch wave function, and their association with tantalum sites which indicates their close relation with the tantalum derived bulk Weyl cones. The analysis technique we demonstrate, based on the structure of the Bloch wave function within the unit cell, is applicable to other electronic systems of interest such as high temperature superconductors and topological crystalline insulators. ‏האירוע הזה כולל שיחת וידאו ב-Google Hangouts. הצטרף: https://hangouts.google.com/hangouts/_/mail.huji.ac.il/condensed?hceid=b...