David Santos-Cottin, University of Fribourg, Switzerland

Thursday Feb. 11 2021, 2:00 PM

Finding Weyl and Dirac fermions by optical and magneto-optical spectroscopy

Topological semimetals are a new class of materials that show relativistic-like energy dispersion in three dimensions, and in this way, may be regarded as three-dimensional analogs of graphene [1, 2]. Their band structures present linear band dispersions in the three directions of momentum, and they cross each other at the Dirac or Weyl points [3, 4]. These topological semimetals exhibit huge carrier mobilities and low carrier densities. Moreover, they exhibit a complex and appealing optoelectrical response because their properties are tunable by the magnetic field, temperature, and carrier doping [5–7]. The most interesting physics in such linearly dispersing systems happens at the charge neutrality point, where the two cones merge into an hourglass shape. At this precise point, the carrier mobility spikes, while the carrier density plummets. We studied diverse types of topological semimetal materials through optical and magnetooptical spectroscopy combined with theoretical calculations. Our results provide indirect proof of Dirac/Weyl fermions’ presence linked with characteristic optical and magneto-optical response of linear band dispersions. Often this response is hidden by other contributions – multi band character, intraband transitions, finite linear dispersion range, or band anisotropy. To optimize the particular optical response and access the most interesting material properties, we will discuss different ways to shift the chemical potential into the vicinity of the charge neutrality point.

[1] H. Weng et al., Phys. Rev. X 5, 011029 (2015).

[2] N. M. R. Peres et al., Phys. Rev. B 73, 125411 (2006).

[3] Z. K. Liu et al., Science 343, 864 (2014).

[4] M. Neupane et al., Nature Communications 5, 3786 (2014).

[5] S.-Y. Xu et al., Science 349, 613 (2015). [6] Q. Zhou et al., Phys. Rev. B 94, 121101 (2016).

[7] Y. Zhang et al., Nature Communications 8, 15512 (2017).

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