Abstract Body

Superconductors with broken inversion symmetry have offered a rich playground to explore unconventional superconductivity and/or non-trivial functionalities. One of the archetypal phenomena in such superconductors is the superconducting rectification effect, which has been observed as nonreciprocal resistance in the superconducting transition region around the critical temperature (T_c) [1] or superconducting diode effect [2], i.e., nonreciprocal superconducting critical current, in the fully superconducting region. Recently, various mechanisms for these phenomena have been discussed among Rashba spin-orbit interaction [3], asymmetric pinning potential of vortices [4], and surface or edge barrier effect [5], depending on the materials and measurement system of interests. Although the nonreciprocal resistance around T_c and the superconducting diode effect should be related through the common origin, the link between these two phenomena has not been explicitly addressed to date probably because the broadness of resistive transition around T_c generally impedes the magnitude of critical current density under magnetic fields.

In this talk, we report that tellurium doped iron selenide Fe(Se,Te) provides a good platform for the comprehensive study of the superconducting rectification effect. Thanks to its high superconducting critical parameters, such as T_c, critical field, and critical current, and the strong spin-orbit interaction, we demonstrated both the superconducting diode effect and nonreciprocal resistance in a wide range of magnetic field and temperature in a superconducting Fe(Se,Te)/FeTe heterostructure. From these data set, it was revealed that the nonreciprocal coefficient of the resistance, which is strongly enhanced around T_c , and rectification efficiency of the critical current exhibit a scaling relationship regardless of applied magnetic field and temperature. Our results point out the important role of the spin-orbit interaction on the asymmetric pinning potential of vortices for emergence of supercurrent rectification [6].

[1] J. E. Villegas et al., Science 302, 1188 (2003). [2] F. Ando et al., Nature 584, 373 (2020). [3] A. Daido, Y. Ikeda, and Y. Yanase, Phys. Rev. Lett. 128, 037001 (2022). [4] Y. M. Itahashi et al., Sci. Adv. 6, eaay9120 (2020). [5] Y. Hou et al., Phys. Rev. Lett. 131, 027001 (2023). [6] Y. Kobayashi, J. Shiogai et al., submitted.

Abstract Body

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).

Abstract Body

The anisotropy is one of the fundamentally important parameters to understand the electoronic properties of high-temperature superconductors including iron-based superconductors. However, for the LnFeAsO (Ln = Nd, Sm,...) system, which exhibits the highest superconducting transition temperature T_c among the iron-based superconductors [1], detailed studies on the anisotropy are limited mainly due to the difficulty in crystal sinthesis with large size. Recently, our group fabricated thin films of NdFeAs(O,H) (hereafter Nd1111:H) [2] and investigated the anisotropy of the normal resistivity γ_ρ and that of the upper critical field H_c2 (γ_Hc2) as a function of the H content [3,4]. Our findings indicated that γ_ρ varies dramatically with the H content, while γ_Hc2 only weakly. However, the H content dependence of g_Hc2 was still not fully explored, especially in the low H content region. In this work, therefore, we prepared several additional samples with different H contents, and measured γ_Hc2.

Thin films of the parent phase NdFeAsO were prepared by MBE, followed by a H-substitution process with topotactic reaction using CaH₂. The H content was adjusted by the processing temperature/time of the topotactic reaction. Since the c-axis length of Nd1111:H systematically decreases with increasing H-substitution, the c-axis length can be used as a measure of the H content. We measured the field angle dependence of resistivity under various magnetic fields at temperatures below the zero field T_c and determined γ_Hc2. The samples with c < 8.5 Å exhibited a γ_Hc2 of ~ 4 at 0.95T_c, which is consistent with our previous report [3,4]. On the other hand, for specimens with c > 8.5 Å (lower H content specimens), γ_Hc2 was about 8-9 at 0.95T_c, which is larger than that of other iron-based superconductors. In LaFeAs(O,H) [5] and SmFe(As,P)(O,H) [6], the T_c dome has a two-peak shape, which can be ascribed to the existence of two superconducting phases in the LnFeAsO system. The observed abrupt change in γ_Hc2 as a function of H content may indicate that there are two superconducting phases in NdFeAs(O,H) thin films as well, although not manifested itself in the variation of T_c.

[1] e.g. K. Tanabe and H. Hosono, Jpn. J. Appl. Phys. 51, 010005 (2011).

[2] K. Kondo et al., Supercond. Sci. Technol. 33, 09LT01 (2020).

[3] M. Chen et al., Phys. Rev. Mater. 6, 054802 (2022).

[4] T. Hatano et al., PC8-4, ISS2022 (2022).

[5] S. Iimura et al., Nature Commun. 3, 943 (2012).

[6] S. Matsuishi et al., Phys. Rev. B 89, 094510 (2014).

Abstract Body

The iron-chalcogenide superconductor FeSe has garnered significant attention because the monolayer film on oxides shows a large enhancement of superconducting transition temperature (T_c) [1]. The enhancement is attributed to interface effects, including electron transfer from the oxide and possible electron-phonon coupling via the interface. In FeSe/SrTiO₃ (STO), superconducting transitions with the onset T_c around 40 K and zero resistivity below T_c^zero (=10–29 K) was observed by the resistivity measurements [1], which are higher than T_c of 9 K in bulk form. The enhanced superconductivity has been realized through molecular beam epitaxy [1] and pulsed laser deposition [2]. Furthermore, gap opening around Fermi energy below 65 K, which may suggest superconducting transition, was reported using angle-resolved photoemission spectroscopy (ARPES) [3]. However, recent studies of in-situ resistivity measurement reported that the T_c^zero and T_c^onset was still lower than gap opening temperatures (T_g) even in the same sample [4,5]. The discrepancy between resistive T_c and T_g is believed to be attributed to pseudogap phase due to the superconducting fluctuations enhanced by the two-dimensional(2D) nature of the superconductivity, however, the pseudogap can also result from a density wave state. Thus, other probes are needed to elucidate the existence of superconducting fluctuations well above T_c. One promising technique for detecting superconducting fluctuations with high sensitivity is the Nernst effect which is the generation of transverse thermoelectric voltage by the thermal gradient in the presence of perpendicular magnetic field [6]. In superconductors, the movement of vortices induced by a thermal gradient produce the Nernst signals. In addition, the Nernst effect caused by superconducting fluctuation is observed above T_c. In amorphous superconductors, Nernst signals was detected far above T_c, and the magnetic field and temperature dependence are good agreement with the theory based on amplitude fluctuations of superconducting order parameter [7]. The Nernst effect has also been employed to investigate potential phase fluctuations in high T_c superconductors [8].

In this study, we performed the Nernst effect measurements on ultrathin FeSe/STO. We fabricated 4- and 9-layer films using the Pulsed laser deposition. The Nernst effect measurement was performed using one-heater-two-thermometer configuration under 0.4–9 T. Figure (a) shows temperature dependence of sheet resistance R_sq (upper panel) and \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) (lower panel), where \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) is the transverse thermoelectric coefficient and B is the magnetic field, for the 4-layer film. The 4-layer film showed superconducting transition with T_c^onset of 29 K and zero resistance below 8.0 K at 0 T. As B increases, resistive transition became broader. Notably, in this temperature regime, \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) started to increases at T^* = 30 K, and peak structure was observed at all magnetic fields. Furthermore, B dependence of \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) changed below T^*. Above T^*, \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) is independent of B while it varies with B at lower temperatures. These results suggest that the existence of superconducting fluctuations just below T^* and vortex movement in lower temperatures. The similar behavior was observed in the 9-layer film as shown in Fig. (b). While the 9-layer films exhibited T_c^onset = 23 K and T_c^zero=7.3 K, \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) started increasing at T^*= 28 K. Finally, let us discuss the relationship between \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) as a function of T and fundamental properties: resistance and diamagnetism which is measured by the two-coil mutual inductance technique. Figure (c) shows temperature dependence of R_sq in 0 and 1 T(upper panel), real part of mutual inductance M₁ between drive and pick up coils in 0 T (middle panel), and \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) in 1 T (lower panel). Although R_sq decreased around T_c^onset, the Meissner effect was observed only below 8 K which is close to T_c^zero. This result is similar to the previous report [9] while a singular study reported observation of the Meissner effect below 65 K [10]. Above T_c^zero, the vortex movement was detected, and \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) caused by superconducting fluctuations persist up to T^.*, which is about 1.2 times T_c^onset. These results are similar to other iron-chalcogenide thin films [11].

References

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[11] F. Nabeshima et al., Phys. Rev. B 97, 024504 (2018).

(a),(b)Temperature dependence of sheet resistance (upper panel) and \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \) (lower panel) for the 4-layer(a) and 9-layer films, respectively. (c) Temperature dependence of R_sq in 0 and 1 T (upper), M₁ in 0 T, and \( \scriptsize{\alpha} \begin{subarray}{rl} \scriptsize{2D} \\ \small{xy} \end{subarray} \ /\!B \)  in 1 T.

Keywords: Iron-based superconductor, Interface superconductivity, Nernst effect, Superconducting fluctuations