C. Shekhar, MPI CPfS Dresden and Y. Skourski, HLD Dresden. In 1929, Hermann Weyl discovered that massless spin-1/2 particles are solutions of the Dirac equation. After many decades, these Weyl particles were finally experimentally revealed in 2015 in simple semimetallic materials such as TaAs. Weyl fermions are low-energy quasiparticle excitations in the vicinity of the unavoidable touching points of a valence band and a conduction band: these materials are “Weyl semimetals”.

M. van Delft, S. Pezzini, T. Khouri, C. Müller, N. Hussey, S. Wiedmann, HFML Nijmegen. Researchers from Germany, USA, UK, and the HFML Nijmegen have found evidence for electron-hole tunneling in momentum space in the nodal-line semimetal HfSiS. This specific tunneling phenomenon is revealed in quantum oscillations of the electrical resistance at low temperatures and in high magnetic fields, and can be illustrated as a ‘figure-of-eight orbit’ enclosing one electron and one hole pocket.

Cyril Proust, LNCMI-Toulouse and Louis Taillefer, University of Sherbrooke. Measuring the electrical resistance of a new material is often the first experiment that researchers do, but also often the last to be understood. Nevertheless, the temperature dependence of the electrical resistance gives essential information on the ground state of materials.