Abstract Body

The iron chalcogenide superconductor, FeSe, has been extensively studied due to its diverse and complex properties, such as the absence of magnetic order even at low temperatures and an extremely small Fermi surface[1]. Moreover, by performing isovalent-substitutions in FeSe and applying negative chemical pressure to create FeSe_1-xTe_x, it is possible to suppress the phase separation which was considered inevitable in certain compositions of bulk crystals through thin film fabrication. Reports have even shown that within this region, the superconducting transition temperature can exceed 1.5 times that of bulk single crystals[2]. To investigate these behaviors unique to thin films, this study focused on complex conductivity measurements using microwaves. Particularly, complex conductivity measurements spanning the entire temperature range, including the vicinity of T_c, were conducted on systematically varied FeSe_1-xTe_x thin films.

Furthermore, novel phenomena are expected to emerge in these thin films. In particular, the study of magnetic flux quanta is not only of interest for the fundamental science of multiband superconductors, but also for engineering applications such as wire materials. Hence, this study also investigated the behavior of magnetic flux quanta in the mixed state (vortex phase) and examined the motion and electronic states of magnetic flux quanta in these thin films.

The cavity resonator perturbation method was employed to measure the complex conductivity of the thin film samples. In the cavity resonator perturbation method, changes in the resonant characteristics upon sample insertion are measured. In this study, the thin film samples were positioned in the cavity resonator at locations with magnetic field nodes and maximal electric field (Fig. 1), enabling measurements of the complex conductivity in the vicinity of the thin film's T_c[3]. Systematic measurements of the complex conductivity were carried out on FeSe_1-xTe_x thin films (x = 0 - 0.5) to investigate the nature of thin films.

Additionally, measurements were conducted under a magnetic field (1T-5T). Complex conductivity data was used to obtain the surface impedance of the thin films, aiming to investigate the motion of magnetic flux quanta. Parameters related to sample characteristics, such as crossover frequencies and pinning constants, were analyzed.

As a result, complex conductivity near T_c was obtained for all compositions, and the obtained data exhibited consistency with past low-temperature data. Moreover, the superfluid density and quasiparticle scattering probability could be analyzed from the complex conductivity. Fig. 2 presents the temperature dependence of superfluid density near T_c. The differences in the rise of superfluid density due to the Te content are attributed to differences in gap structure. Additionally, the motion of magnetic flux quanta was examined, and a comparison with the bulk was made regarding the electronic state within the vortex core.

References

[1] T. Shibauchi,et al ., J. Phys. Soc. Jpn. 89, 102002 (2002)
[2] Y. Imai,et al ., Proc. Natl. Acad. Sci. U.S.A. 112, 1937 (2015).
[3] D. -N. Peligrad, et al ., Phys. Rev. B 64, 224504 (2001)