The project evolves around the concept of topological phases in condensed matter physics, with a focus on topological semimetals, whose theory has had a steep development in the past decade with the discovery of materials like graphene, topological insulators, and Weyl/Dirac semimetals. Focus will be given to the latter exotic electromagnetic response, in particular in the THz electromagnetic spectral region. The goal of the project will be the study of specific nonlinear effects predicted theoretically in the physics of Weyl semimetals. Fundamental research on topological semimetals optical responses and their possible practical applications will be discussed, along with the interest of opening such a research line at Sapienza University.
All the emergent transport effects already described are screened by hypothetical measurements due to the difficulty in finding Weyl semimetals that host the nodes near the Fermi level, and that have a very low density of states at the nodes energy. Due to this latter problem, all the transport measurements will have a nonzero contribution coming from trivial metallic states, that may cover completely the nodes response. These problems call for a rapid development in the growth and optical measurements techniques of Weyl semimetals. Optimistic results for the growth technologies have been obtained recently with the production of high-quality thin films using molecular beam epitaxy (MBE) [2D Mater. 4 025044 (2017)], epitaxial growth [ACS Nano 2020, 14, 4, 4405-4413, ACS Appl. Electron. Mater. 2, 1, 126-133 (2020)] and pulsed laser deposition [Applied Physics Letters 111: 031906 (2017)]. On a parallel path, the recent discovery of new TRS breaking Weyl semimetals opens the way for the study of the interplay between the magnetic and electrical nature of the Weyl quasiparticles [Ann. Phys. (Berlin), 1900287 (2019)]. Optical studies remain the last problem to tackle. Recent researches have only highlighted briefly the responses coming from the Drude or interband behaviour, and are mostly limited to the infrared spectral range. A complete characterisation of the Weyl phases in the THz spectral region is instead required. Therefore, the future contribution of our facility would be the THz exploration of the semimetal phase, developing optical experiments under different structural, electro-magnetic and thermodynamic conditions.
These characterizations are the required starting point for a future application of Weyl semimetals as electro-optical devices. THz control can happen through nonlinear signatures, in particular from photogalvanic effects. These responses are strictly related to the topological signature of the Berry curvature monopole (Weyl node) through the second order interband (or intraband) nonlinear response [Science Advances 2(5) : e1501524 (2016), Physical Review B, 94(24) (2016)]. Recent studies have also highlighted how the tilting property can enhance massively this process, permitting a net separation from background photo-thermal effects [Nature Materials 18 : 476-481 (2019)]. The practical applications that exploit this process are plenty; for instance, highly sensitive photodetection in the technically important mid-infrared and terahertz region, over a broad wavelength range. The electromagnetic wave emission induced by ultrafast currents may also prove useful for generation in the terahertz (THz) frequency regime, where control of the ellipticity and chirality over a broad spectral range is known to be notoriously difficult. The THz Sapienza laboratory, directed by prof. Stefano Lupi, will offer the right experimental tools required for these studies.
The main reason in investing in such a research line at Sapienza University is the already well-established THz facility (Terahertz Sapienza laboratory), permitting a direct study of the open questions in the field. The contributions of such a novel argument will span across theoretical and experimental strengthening of the fundamental physics of topological materials and their electro-optic application to overcome the THz gap, also giving a solid background for further topics like topological spintronic and superconductivity. THz spectroscopy, imaging and communication, quantum computing, and rapid memory storage systems are just a few of the technological outcomes that could be achieved from the deeper study of topological semimetals. Such a rich research field is thus expected to acquire higher relevance in the near future.