Fe chalcogenide (FeCh)[1] is mine of interesting phenomena even in Fe based superconductor family, In addition to nematic state without magnetic order, amazing flexibility of increasing superconducting Tc by various kinds of pressure application and interesting phenomena associated with extremely small Fermi energy comparable to superconducting gap attract much attention. Particularly interesting is “high-temperature superconductivity” at around 65 K or at higher temperatures reported to take place in ultrathin, such as monolayer, film grown on some sort of oxide substrate grown by MBE technique[2,3]. Other than electron doping in bulk bands, extra carrier doping from the substrate, and/or interaction with phonons of the substrate are considered to play important roles, which reminds us that the so-called interface high-temperature superconductivity proposed by Ginzburg half century ago[4] may be realized. However, even after 10 years of the report, Tc shown up in resistivity is very low (30-45 K in onset, and 10-30 K in zero resistivity), which I believe is a very serious problem.
In my view, the most characteristic aspect of superconductivity in FeCh is that three different category of superconductivity appear in the same material; (1) 10-40 K class superconductivity with both hole and electron Fermi surface, (2) 40-50 K superconductivity with electron Fermi surface alone, and (3) high-temperature superconductivity realized only in ultrathin films having almost the same electronic structure as that of the 2nd category, as described above.
We have studied superconductivity of FeCh in films grown by PLD technique and investigated superconductivity of all three categories described just above. For the first category, we succeeded in preparing a series of FeSe1-xTex samples, avoiding phase separation which used to be inevitable in bulk crystals[5,6]. By the investigations of electronic properties both in normal stare and in superconducting state by ARPES[7,8], optical[9,10], THz[11] and microwave conductivity[12,13], dc electromagnetic studies[14], m-SR[15] and DFT[12] calculation, we conclude that a characteristic behavior of Tc as a function of Te content is the consequence of the change in the electronic structure at the Fermi level taking place caused by the disappearance of the novel pure nematic state without lattice distortion[16]. For the second category, the investigation of the isovalent substitution on the electron doped samples prepared by the electron-double-layer-transistor (EDLT) technique confirmed that the mechanism of superconductivity is definitely different from that of the first category[17,18]. During that procedure, we realized zero-resistance of 46 K[18], which is the highest value among all possible data of FeCh, except for a singular report by Ge et al.[19] For the third category, we succeeded in realizing the interface superconductivity also by the PLD technique[20,21]. However, zero resistivity temperature, nor the onset temperature of superconductivity fluctuation measured by the Nernst effect have not reached 65 K, but stays around 30 K.
In my talk, I will discuss details on the above mentioned properties in detail, together with results obtained by other groups. I also introduce our recent challenge to raise Tc in resistivity for the interface superconductivity.
[1] F. C. Hsu et al.: Proc. Natl. Acad. Sci. USA. 105 (2008) 14262.
[2] Q. Y. Wang et al.: Chin. Phys. Lett. 29 (2013) 037402 .
[3] S. H. He et al.: Nat. Mater. 12 (2013) 605-610, S. Y. Tan et al.: ibid. 634-640.
[4] V. L. Ginzburg: Phys. Lett. 13 (1964) 101.
[5] Y. Imai et al., Proc. Natl. Acad. Sci. USA. 112 (2015) 1937.
[6] Y. Imai et al., Sci. Rep. 7 (2017) 46653.
[7] G. N. Phan et al.: Phys. Rev. B95 (2017) 224507.
[8] K. Nakayama et al.: Phys. Rev. Res. 3 (2021) L012007.
[9] M. Nakajima et al.: Phys. Rev. B95 (2017) 184502.
[10] M. Nakajima et al.: Phys. Rev. B104 (2021) 024512.
[11] N. Yoshikawa et al.: Phys. Rev. B100 (2019) 035110.
[12] H. Kurokawa et al.: Phys. Rev. B104 (2021) 014505.
[13] G. Matsumoto et al.: J. Phys. Conf. Ser. 2776 (2024) 012002, also arXiv. 2402.18082.
[14] F. Nabeshima et al., Phys. Rev. B101 (2020) 184517.
[15] F. Nabeshima et al., Phys. Rev. B103 (2021) 184504.
[16] Y. Kubota et al.: Phys. Rev. B108 (2023) L100501.
[17] N. Shikama et al., Appl. Phys. Express 13 (2020) 083006.
[18] N. Shikama et al.: Phys. Rev. B104 (2021) 094512.
[19] J. F. Ge et al.: Nat. Mater. 14 (2015) 285.
[20] T. Kobayashi et al.: Supercond. Sci.Tech. 35 (2022) 07LT01.
[21] T. Kobayashi et al: J. Phys: Cond. Mater. 35 (2023) 41LT01.