Within the Landau paradigm, phases of matter are distinguished by their symmetries. There exists however a finer level of classification based on the topological entanglement of their electronic wavefunctions. This gives rise to new quantum phases such as topological insulators (TIs), Dirac and Weyl semimetals (DSMs, WSMs) which are the natural 3-Dimensional extension of graphene. In DSMs conduction and valence bands cross near the Fermi energy forming 3D degenerate linearly dispersive electronic bands (Dirac cones). This degeneracy is removed in WSMs, due to broken inversion or time reversal symmetry, and each Dirac cone splits into two cones separated in energy or in momentum. The low-energy electronic excitations in WSMs are Weyl electrons which are massless chiral fermions considered a building block of quantum field theory. Although Weyl fermion was intended as a model of elementary particles, in nearly 90 years, no candidate Weyl fermions have ever been established in high-energy experiments, so WSMs represents the first example of Weyl physics.
DSMs and WSMs show exotic electronic properties as highly conductive, non-dissipative surface states and extremely non linear optical response functions stemming from geometric (Berry) phase effect. In this project, we plan to measure the linear and unlinear optical properties of high-quality Weyl and Dirac thin-films from microwaves (1 GHz) to ultraviolet (1000 THz). These high-quality films will be obtained from already established collaborations (S. Oh, Rudgers University, USA; A. Molle, CNR-IMM, Italy). In particular, we will investigate the electromagnetic response of WTe2 and LaAlGe Weyl and Cd3As2 Dirac semimetals and their exploitation for photonic and plasmonic devices.
The project, strongly innovative in the international scenario, is based on a well composed group combining recognized experts in topological materials, microwaves, terahertz, infrared spectroscopies and non linear optics.