FeSe provides a rich playground to study a plenty of exotic phenomena that arise from the interplay between superconductivity, electronic nematicity, and magnetism [1]. From the very small effective Fermi energy of FeSe, it is expected that a change in structural parameters significantly affect the low-energy band structure and hence the physical properties. In this talk, we show how in-plane strain and isovalent substitution of Te for Se alter the normal-state charge dynamics in FeSe and discuss the evolution of the electronic structure.
The low-energy optical conductivity spectrum of FeSe is described by the sum of narrow and broad Drude components, associated with coherent and incoherent charge dynamics, respectively. Below the nematic transition temperature, the weight of the narrow Drude component decreases with decreasing temperature, indicative of a gradual suppression of the coherent carrier density. This indicates a peculiar metallic state in FeSe that the Fermi surface gradually modified with temperature [2]. From the measurement on thin films of FeSe with different substrates, it turned out that the carrier density decreases with tensile strain, which is likely related to the suppression of the superconductivity. For FeSe with a tensile strain, we observed a transfer of the spectral weight below ~ 400 cm-1 to a higher energy region in the nematic phase. This behavior can be explained by the change in the band structure associated with the Lifshitz transition. These results are also supported by magneto-transport measurement [3,4]. With substituting Te for Se, the fraction of the narrow Drude component severely decreases, indicating that Te substitution leads to stronger electronic correlations [5]. Our result suggests that superconductivity is closely related with nematicity and electronic correlations.
This work was done in collaboration with K. Yanase, Y. Senoo, Y. Ohata, S. Tajima (Department of Physics, Osaka University), M. Kawai, D. Asami, T. Ishikawa, N. Shikama, Y. Sakishita, F. Nabeshima, A. Maeda (Department of Basic Science, The University of Tokyo), and Y. Imai (Department of Physics, Tohoku University).
[1] T. Shibauchi, T. Hanaguri, and Y. Matsuda, J. Phys. Soc. Jpn. 89, 102002 (2020).
[2] M. Nakajima, K. Yanase, F. Nabeshima, Y. Imai, A. Maeda, and S. Tajima, Phys. Rev. B 95, 184502 (2017).
[3] F. Nabeshima, M. Kawai, T. Ishikawa, N. Shikama, and A. Maeda, Jpn. J. Appl. Phys. 57, 120314 (2018).
[4] M. Nakajima, Y. Ohata, and S. Tajima, Phys. Rev. Mater. 5, 044801 (2021).
[5] M. Nakajima, K. Yanase, M. Kawai, D. Asami, T. Ishikawa, F. Nabeshima, Y. Imai, A. Maeda, and S. Tajima, Phys. Rev. B 104, 024512 (2021).