Probing and manipulating Dirac & Weyl semimetals with terahertz light – TERA-DIRAC

Quantum materials have attracted a lot of attention in the past decade due to recent discoveries of condensed matter systems where several phases coexist or are in competition. Probing these quantum phases and controlling their properties using light excitation stand as exciting challenges in condensed matter, aiming towards on-demand properties and devices with tunable functionalities.
The TERA-DIRAC project focuses on rising materials, the Dirac and Weyl semimetals (D&W SM). They have been predicted to host specific states of matter called topological states, which are strongly dependent on the symmetry of the system. Hence, by altering the symmetry of the lattice, one can expect to modify their topological nature and induce/suppress the related properties. These materials exhibit indeed unprecedented properties resulting from their topology, in particular a specific electronic band structure showing a 3D-linear dispersion around nodes. Considered as 3D-analog of graphene, these materials have become extremely promising, both on the fundamental level regarding their unusual topology, and on the application level opening perspectives for optoelectronics (IR sensors). Moreover, the intertwinement between electronic, structural and topological degrees of freedom makes these materials particularly interesting for light-manipulation.
This project intends to investigate carrier and structural dynamics in these novel D&W SM compounds, addressing two scientific scopes, making use of the remarkable properties of terahertz (THz) radiation. The first will explore the carrier dynamics and the exact band structure specifically in the low energy range. This goal is crucial both to envision optoelectronic applications which performances will be dictated by the carrier dynamics, and to shed new light on the D&W SM topology, of which the linear dispersion is a direct signature. So far, only high energy carrier dynamics has been explored, following an optical pump exciting the carriers well above the linear range. Relying on low-energy photons provided by a THz pump, this scope will address the linear dispersion dynamics. It will enable a clear identification of the relaxation processes and their associated time constants, probing the carrier conductivity and identifying individually the phonon modes implicated, as well as a fine description of the low-energy band structure, still under debate for some compounds.
The second scope focuses on exploring light-induced structural modifications, identifying the responsible mechanisms. Since topological properties are dependent on the lattice symmetry, inducing structural distortions, i.e. driving atoms out of equilibrium, could expectedly lead to changes in the topology. It is essential to clarify the mechanisms at play after a light excitation of D&W SM, leading to structural changes, in order to conceive a proper light-control of materials with on-demand properties. This scope will offer thorough insights in the light-excited degrees of freedom, following simultaneously the temporal evolution of the carrier and the phonon populations. Their specific behaviors will allow to identify the mechanisms triggered by optical light excitations, but also by THz light excitations which are expected to show a more efficient structural driving.
To achieve these goals, the project intends to implement an original experimental approach, relying on versatile ultrafast techniques, associating low-energy photons but high electric field of THz pump excitations with the complementary THz and time-resolved (tr-)Raman spectroscopies. The THz spectroscopy will be adequate for probing carriers (conductivity) and IR-active phonons, while tr-Raman spectroscopy gives access to electronic responses and Raman-active phonons. This project will achieve the first demonstration a unique platform combining THz pumping and tr-Raman spectroscopy, extremely relevant to explore D&W SM, but also other quantum materials in the future.