TOTEM: engineering TOpological quantum phases in hexagonal TErnary coMpounds

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 Cd₃As₂ and NiBi₃. 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 concerns the discovery of new TSs and the tuning of their topological phase through alloy engineering. The project is led by Prof. Federico Bisti in collaboration with: - Dr. Paola Gentile, from CNR-SPIN in Salerno (Italy); - Prof. Fabio Berardini and Prof. Alessio Filippetti, from University of Cagliari (Italy); - Prof. Daniele Marré, from University of Genova (Italy); - Dr. Ilaria Pallecchi and Dr. Federico Caglieris, from CNR-SPIN in Genova (Italy).

The project started in 29/11/2024 and it will end in 28/02/2026. The project recieved funding from the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, funded by the European Union (NextGenerationEU) (CUP E53D23001710006 - Call for tender No. 104 published on 2.2.2022 and Grant Assignment Decree No. 957 adopted on 30/06/2023 by the Italian Ministry of Ministry of University and Research (MUR)).