Millikelvin scanning tunneling spectroscopy: electronic features of semimetals and putative topological superconductors at very low energies

Scanning tunneling microscopy and spectroscopy (STM/S) down to 100 mK is an efficient tool to study superconductors and semimetals. The local electronic density of states is obtained at atomic level with a resolution in energy of a few tens of micro eV. Friedel like oscillations are observed close to defects like vacancies or interstitials. Real space maps of the electronic density of states close to defects provide thus a visual account from which we can measure the wavelength of the electronic wavefunctions at the energy corresponding to the applied bias voltage. The Fourier transform of the real space maps provides the constant energy contour of the electronic dispersion relation. By making maps at different energies, below and above the Fermi level, we can in principle trace the full bandstructure. In this talk, I will discuss recent visualization experiments of scattering effects in superconductors and semimetals. I will focus on the typeII Weyl semimetal WTe${}_2$, where we directly observe the signature of edge states associated to the Weyl points and recent efforts in the putative chiral d-wave superconductor URu${}_2$Si${}_2$.