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Master 2 Internship : Electron dynamics in topological Dirac semimetals investigated by terahertz spectroscopy

Sarah Houver,

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Topological semimetals are a new class of quantum materials, considered as 3D-analogs of graphene since they possess a linear dispersion, protected by topology and symmetry. These novel materials are extremely promising, both on the fundamental level regarding their topological properties, and on the application level
in optoelectronics (infrared sensors), energy conversion or quantum computing. The topology is strongly dependent on the system symmetries, and thus deeply connected to electronic and lattice properties. Direct optical measurements of topological properties are still in their infancy, so it is of crucial interest to study
their fundamental properties such as their electron and lattice dynamics, aiming at understanding their topological properties and manipulate them.

This project aims at exploring fundamental properties of topological Dirac semimetals such as Cd3As2, and ZrTe5, in particular their electron dynamics, taking advantage of a recent technique : the terahertz (THz) time domain spectroscopy.

THz radiations belong to the the far infrared range with photons typically of lambda=300 μm i.e. f=1 THz i.e. E =4 meV. This range of the electromagnetic spectrum is extremely relevant to investigate electronic, lattice and magnetic
properties of condensed matter systems. Implemented in a time-resolved pump-probe scheme, one can combine an optical pulse to excite electrons in the system (see figure) with a THz pulse used to probe the electrons close to equilibium and highlight the different relaxation processes. Identifying these processes in the linear band is a crucial step for further investigations of topological properties and for envisionning possible opto-electornic applications.

Typical linear dispersion of a Dirac smimetal with electrons in the valence band (red) and partially in the conduction band (yellow). After an optical excitation (red arrow), some electrons are excited form the valence to the conduction band, before they relax through various processes we would explore.