Low-energy excitations in 3D Dirac and Weyl semimetals – DIRAC3D

Within the DIRAC3D project, we have formed a consortium gathering four laboratories with complementary competences in experimental and theoretical physics. These were used to approach unique properties of topological materials in basic, curiosity driven research, but also with respect to possible applications. The employed experimental methods comprised techniques directly addressing electrical, and mainly, optical properties of topological materials - those which are most relevant for their future use in electronics and optoelectronics. In this respect, we departed from the mainstream in solid-state physics which dominantly uses experimental methods sensitive to the surface of explored topological materials, such as scanning tunneling spectroscopy or angle resolved photoemission spectroscopy, often combined with ab initio calculations. In particular, we have used advanced tools of optical spectroscopy - from the terahertz up to visible spectral range - and electrical transport, in a wide range of magnetic fields which allowed a genuine inspection of electronic states in a series of topological materials and thus collect information often missing in the scientific community working these systems. In several cases, the results our work guided beyond the originally intended scope of the projects. For instance, when a topological semimetal subjected to an externally applied magnetic field was shown to be an efficient source of THz radiation that could serve as a possible active medium in a new generation of infrared lasers.

Within the DIRAC3D project, we have explored a series of materials from the topological class, primarily three-dimensional Weyl and Dirac semimetals, the primary target of this project, but we also extended our scope towards other systems such as topological insulators or nodal-line semimetals. These studies, both experimental and theoretical, provided us with detailed insights into low-energy electronic states in these materials and will serve to the topological community.

Most interesting results, however, were obtained when the achieved understanding of explored materials allowed us to venture beyond the goals predicted or anticipated in the original proposal. This was the case of our magneto-optical studies of nodal-line semimetals, with result interpreted in term of a Lorentz boost, creating thus yet another link between solid-state physics and relativity (J. Wyzula et al, Advanced Science, 2022). The observed emission of terahertz radiation from the Landau-quantized HgCdTe-based semimetals is another example (D. B. But et al., Nature Photonics, 2019 and S. Gebert et al., Nature Photonics 2023)]. This was the very first cyclotron emission from massless relativistic-like charge carriers that bought us closer to the long-searched Landau level lasers.

At present, despite undeniable potential of topological materials for applications in electronics and optoelectronics, we are still at too early a stage to judge their final impact, if any, and the interest in them is driven mostly by scientific curiosity. The real impact of results achieved within the project can only be evaluated retrospectively, after a certain period. Still, the work realized within the DIRAC3D project revealed at least one additional area where topological materials might be used in the future – as active media of future quasi-monochromatic sources of radiation in the THz spectral range, so-called Landau level lasers. It is our plan to pursuit this specific research direction in future. To this end, we have formed a new consortium, basically extending the one from the DIRAC3D project. Our proposal has been submitted to ANR 2023 call as international collaborative project (PRCI).

The DIRAC3D project resulted so far in 27 publications in peer-reviewed international journals with high impact factor, other papers are under review or preparation. Among them, one may find 2 publications in Physical Review Letters and 2 publications in Nature Photonics.

The DIRAC3D project aims at expanding radically the experimental and theoretical knowledge of 3D Dirac and Weyl semimetals, which have recently emerged as a new intriguing playground for both fundamental and applied condensed matter physics. We propose a genuine inspection of their electrical and optical properties in a wide range of magnetic fields using advanced tools of electrical transport and optical spectroscopy, from terahertz up to visible spectral range. We will search for their unique physical properties due to the presence of three-dimensional massless particles in their Dirac and Weyl nodes as well as their specific surface states. The proposed experiments, supported by appropriate theoretical modelling, will allow us to characterize in detail the three-dimensional conical bands (shapes, isotropies, energy scales…) in selected Dirac and Weyl materials (e.g., in Cd3As2, TaAs, ZrTe5 or ZrSiS). Further experimental activities will include, for instance, the search for specific optical excitations related to the presence of Fermi arcs and massive states on the surfaces of these 3D Dirac and Weyl semimetals, or the exploration of the phenomenon of linear magnetoresistance observed generically in 3D materials with conical band structures. Our ultimate goals include the discovery of unambiguous evidence for the chiral anomaly in Weyl semimetals and the exploration of the fascinating optical and electrical properties of the Dirac and Weyl semimetals, whose long-term application potential is also relevant.