Topological phases are not limited to gapped systems, sparking theoretical prediction that there can be metallic phases or nodal superconductors with topologically protected properties and exotic quasiparticles. One example of a gapless topological system is that of a three-dimensional Dirac semimetal in which crystalline symmetry protects the band crossing points, when time reversal symmetry or inversion symmetry is not broken. However, breaking either of these symmetries, the Dirac points in the three-dimensional band structure split up into pairs of so-called Weyl points in the band structure. Our group has investigated a variety of Dirac and Weyl semimetals and have uncovered their novel properties using our high-resolution spectroscopic techniques with the STM.
Using Landau level spectroscopy, with our dilution fridge STM operating at high magnetic fields, we have used the quantum oscillation of the tunneling density of states to demonstrate the Dirac dispersion of the electronic state in Cd3As2.[S. Jeon et al, Nature Materials 13 (2014)] In general, high magnetic field STM spectroscopic studies (such those we used on these materials) open the door to using quantum oscillations of tunneling density of states as a powerful probe of quantum materials and their phase transitions.
In another study we have uncovered the remarkable surface bulk connectivity of Weyl semimetals. Weyl semimetals are characterized by unusual topologically protected surface states, which have an arced Fermi surface. By performing high resolution STM measurements of these Fermi arc surface states, we demonstrated that the surface and bulk electronic states are connected at the termination of the Fermi arcs in momentum space. Our experiments show that when surface electrons have momentum that matches these points, which are also the Weyl momentum point of the bulk states, they sink into the sample.[H. Inoue et al, Science 351 (2016)] This connectivity is a theoretically predicted property of Weyl semimetals.