Highlights 2016

Electrodynamic response of the type II Weyl semimetal YbMnBi2

Weyl fermions play a major role in quantum field theory but have been quite elusive as fundamental particles. Materials based on quasi two-dimensional bismuth layers were recently designed and provide an arena for the study of the interplay between anisotropic Dirac fermions, magnetism and structural changes, allowing the formation of Weyl fermions in condensed matter. We performed an optical investigation of YbMnBi2, a representative type II Weyl semimetal, and contrasted its excitation spectrum with the optical response of the more conventional semimetal EuMnBi2. Our comparative study allows us disentangling the optical fingerprints of type II Weyl fermions, but also challenge the present theoretical understanding of their electrodynamic response.

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Spectral weight reshuffling in the electronic nematic phase of BaFe2As2

We performed, as a function of uniaxial stress, a temperature-dependent optical-reflectivity investigation of the parent Fe-arsenide compound BaFe2As2 over a broad spectral range, from the far infrared up to the ultraviolet, across the coincident structural tetragonal-to-orthorhombic and spin-density-wave (SDW) phase transitions at Ts,N = 135 K. Our results provide knowledge to the complete electrodynamic response of the title compound over a wide energy range as a function of both tunable variables. We show that the spectral weight distribution in the SDW state occurs for energies below approximately 1 eV, and therefore points towards a correlation mechanism due to Hund’s coupling rather than onsite Coulomb interactions.

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