Experimental control of Weyl physics in van der Waals topological materials

Materials with topological properties are at the forefront of today’s materials science; they have the potential to produce extraordinary advancements in our present nanotechnology and envision innovative and empowered applications in electronics, spintronics, energy storage and production, and quantum computing. Indeed, topologically protected states cannot be washed away by external perturbations and are usually linked to extraordinary features in transport properties. In particular, the highly studied Weyl fermions, which can be realized as low energy excitations in bulk Weyl semimetals, are linked to the appearance of topologically protected phenomena ranging from the chiral anomaly to the anomalous Hall effect. These features represent new fundamental physics which on the one hand extends the boundary of our knowledge of the matter, and on the other fosters the implementation of devices with radically new, enhanced functionalities. Topological properties are often difficult to be experimentally detected and demonstrated, not to mention manipulated, since deeply ‘buried’ in the fundamental behaviour of the electrons, and interlaced with many other, non-topological characteristics of the matter. Hence, to fully explore the potential of the systems by a transport investigation, a set of external knobs must be implemented, isolating the topological states from trivial ones. For instance, a band filling control could be crucial to enhance the contribution of Weyl fermions, but the metallic nature of the compounds is not allowing the creation of field effect devices. In this context, the versatile nature of the van der Waals systems is the perfect playground to control the sample properties, taking advantage of the possibility to finely control their dimensionality. Indeed, a thick exfoliated flake of tens of nanometers shows the same band structure of bulk single crystals, whereas it allows the creation of a field effect device. In this thesis, I will investigate the electric and thermoelectric transport properties of van der Waals materials with emergent topological properties to unravel the presence of Weyl physics. In particular, I will firstly address the magnetic topological insulators MnBi2Te4, MnBi4Te7, since recent theoretical reports have been suggesting that a proper control of the magnetic ground state can make Weyl physics emerge. Finally, I will turn to the highly debated t-PtBi2, a non-magnetic Weyl semimetal that is also hosting superconducting Fermi arcs. The intrinsic combination of non-trivial topology with superconductivity could open the route to novel quantum devices. The experimental study will be carried out both in bulk single crystals and in micro/nano-structures, realizing field effect devices.