Topological Semimetals

The realization of material hosting symmetrical-protected massless bulk excitations is at the hearth of research in condensed matter physics. Such excitations originate from band touching points or nodes at the Fermi energy, where two or more bands are exactly degenerate at specific points inside the first Brillouin zone. Those crossing points are the defining feature of topological semimetals (TS), which should not be confused with the topological insulator. The former has massless excitation directly in the bulk band structure, whether instead the latter is an insulator in the bulk and hosts massless excitation at their surface only. Depending on the degeneracy and momentum space distribution of the nodal points, this class of materials are then divided into other subclass as Dirac (DSM), Weyl semimetals (WSM) and nodeline semimetal (NLSM). The first one is in the presence of 4-fold degeneracy point, which excitations can be described by Dirac Hamiltonian, gaining the equivalence of the high energy physics Dirac particle. The second one is in the case of concomitant presence of two 2-fold degeneracy point, described by Weyl Hamiltonian and equivalent to a couple of Weyl particle and anti-particle. Finally, in a NLSM the band touch points form a closed ring in momentum space. The interest on these massless quasiparticles has started with graphene, where the first two-dimensional Dirac cone was observed. Then it was the case of threedimensional (3D) Dirac semimetals as $Cd_3As_2$ and $NiBi_3$. Finally, there was the discovery of a physical realization of Weyl semimetals in $TaAs$ family. This kind of material holds the promise for new device application as: low-dissipation transport, low-consumption spintronic devices and magnetic memory devices, in ultrafast photodetectros, and for high-efficiency energy converters or thermal detectors.

This project is on the discovery of new TS and the possible tunability of their topological phase by alloy engineering.