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2D materials

Ordered crystal structures may display long range coherence even if lacking translational symmetry. Materials within this class are quasicrystals, misfit compounds, and incommensurate charge density waves (CDWs). In many cases, the lack of a periodic structure gives rise to unconventional transport properties and collective modes. Indeed, not much is known about the nature of the electronic states when the Bloch theorem no longer applies. Surely the electronic scattering with an aperiodic potential favors a strong electronic localization. According to the common view, a pseudogap should result as it increases the cohesive energy of the aperiodic structure by a Hume-Rothery mechanism. Photoemission experiments on quasicrystals may be consistent with this scenario. Nonetheless, the connection between the pseudogap, electronic localization, and electron-phonon coupling still has to be settled. The misfit compound (LaS)1.196VS2 is a model system where one can study the coupling of an aperiodic potential to the electronic states. As shown in Fig. A and B, the sudden photoexcitation at 50 K induces a partial filling of the electronic pseudogap within less than 80 fs. By looking to the time evolution of the electronic population 200 meV above the Fermi level (see Fig. C) we deduce that the electronic energy flows into the lattice modes on a comparable time scale. This result reveals the essential role of strong electron-phonon coupling for the electronic localization in quasiperiodic structures.

[1] Ultrafast filling of an electronic pseudogap in an incommensurate crystal; V. Brouet, J. Mauchain, E. Papalazarou, J. Faure, M. Marsi, P.H. Lin, A. Taleb-Ibrahimi, P. Le Févre, F. Bertran, L. Cario, E. Janod, B. Corraze, V. Ta Phuoc, and L. Perfetti, Phys. Rev. B 86,  041106 (2013).