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The discovery of topological insulators (TIs) has fuelled the search for novel topological phases of matter, by combining theory, computational high-throughput screening and experiments [1, 2]. From the experimental point of view, angle-resolved photoelectron spectroscopy (ARPES) has played a key role due its direct access to the material band structure. The advent of efficient spin detector, and ultrafast light source in the UV energy range, has further extended our capability to map also the vectorial spin texture and the out-of-equilibrium dynamics of the topological states. We have recently exploited these techniques to investigate TIs and several topological semimetal. We have tracked the evolution of the band structure of MoTe2 across the topological phase transition between a trivial semimetal and a type-II Weyl semimetal, as a function of the sample temperature [3]. Our results show that the presence of several bulk and surface states hamper a direct determination of the topological phase. However, the temporal dynamics of the photoexcited electrons reveals clear signatures of the Weyl point formation in the unoccupied band structure [4]. In the second part of my talk, I will show that optical excitation can be also used to alter the material band dispersion, as for the case of the nodal line semimetal ZrSiSe [5], which displays both spin polarized surface states and a bulk hidden spin polarization. [1] M. Z. Hasan and C. L. Kane, Reviews of Modern Physics 82, 3045 (2010). [2] B.-J. Yang and N. Nagaosa, Nature Communications 5, 1 (2014). [3] A. Crepaldi, et al., Phys. Rev. B 95, 041408 (2017). [4] A. Crepaldi, et al., Phys. Rev. B 96, 241408(R) (2017). [5] G. Gatti, et al., in preparation.

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Alberto Crepaldi, École polytechnique fédérale de Lausanne
Seminar Room of the Institute of Physics II
Contact: Paul van Loosdrecht