• Title:Double photoemission with megahertz high-order harmonics
  • Start Date/Time:2020-10-20 / 13:30
  • End Date/Time :2020-10-20 / 15:10
    • Speaker:Dr. Cheng-Tien Chiang (IAMS)
    • Place:R019, Phys. Building
    • Host:Prof. Po-Yao Chang (NTHU)
    • Abstract:
      Studying the interaction between electrons in solids has been a major research topic since the beginning of solid state physics. Despite the central role of electron-electron interaction in magnetism, superconductivity, as well as heavy Fermion systems, a direct spectroscopy on interacting electron pairs is still missing. Aiming at an ultimate tool to quantify the strength of electron-electron interaction in solids, double photoemission (DPE) spectroscopy has been developed since decades. In this talk, laser-based laboratory DPE experiments will be introduced [1]. As a laboratory light source operating at megahertz repetition rates and having a widely tunable photon energy range, high-order harmonic generation using a high power femtosecond laser has been established [2]. Based on this unique light source, band-resolved DPE experiments were performed for the first time [3].

      In the DPE experiments, a pair of photoelectrons is excited upon the absorption of one single photon. The photoelectron pair is analyzed by a pair of time-of-flight electron spectrometers with angular and energy resolution [4]. On the paradigmatic metal surfaces Ag(001) and Cu(111), the characteristic binding energy of the d bands can be observed in the two-dimensional DPE spectra. As a consequence, the band character of the photoelectron pairs can be directly assigned [3]. In contrast, the intensity of photoelectron pairs from the strongly correlated material NiO increases rapidly towards lower kinetic energies [1,4]. These results will be discussed in terms of electron- electron interaction in these materials.

      [1] C.-T. Chiang et al., Prog. Surf. Sci. 95, 100572 (2020).
      [2] C.-T. Chiang et al., New J. Phys. 17, 013035 (2015).

      [3]  A. Türtzschler, M. Huth, and C.-T. Chiang et al., Phys. Rev. Lett. 118, 136401 (2017).
      [4]  M. Huth, C.-T. Chiang et al., Appl. Phys. Lett. 104, 061602 (2014).

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