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Institute: search.filters.UBSInstitut.4. Physikalisches InstitutStart date: 2018End date: 2018Publication Type: search.filters.UBSTyp.Zeitschriftenartikel

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    Photo-excited dynamics in the excitonic insulator Ta2NiSe5
    (2018) Werdehausen, Daniel; Takayama, Tomohiro; Albrecht, Gelon; Lu, Yangfan; Takagi, Hidenori; Kaiser, Stefan
    The excitonic insulator is an intriguing correlated electron phase formed of condensed excitons. A promising candidate is the small band gap semiconductor Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally developed by Kabanov et al for superconductors (Kabanov et al 1999 Phys. Rev. B 59 1497), we show that the material’s intrinsic gap can be described as almost temperature independent for temperatures up to about 250 K to 275 K. This behavior supports the existence of the excitonic insulator state in Ta2NiSe5. The onset of an additional temperature dependent component to the gap above these temperatures suggests that the material is located in the BEC-BCS crossover regime. Furthermore, we show that this state is very stable against strong photoexcitation, which reveals that the free charge carriers are unable to effectively screen the attractive Coulomb interaction between electrons and holes, likely due to the quasi 1D structure of Ta2NiSe5.
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    Modeling of second-harmonic generation in periodic nanostructures by the Fourier modal method with matched coordinates
    (2018) Defrance, Josselin; Schäferling, Martin; Weiss, Thomas
    We present an advanced formulation of the Fourier modal method for analyzing the second-harmonic generation in multilayers of periodic arrays of nanostructures. In our method, we solve Maxwell’s equations in a curvilinear coordinate system, in which the interfaces are defined by surfaces of constant coordinates. Thus, we can apply the correct Fourier factorization rules as well as adaptive spatial resolution to nanostructures with complex cross sections. We extend here the factorization rules to the second-harmonic susceptibility tensor expressed in the curvilinear coordinates. The combination of adaptive curvilinear coordinates and factorization rules allows for efficient calculation of the second-harmonic intensity, as demonstrated for one- and two-dimensional periodic nanostructures.