Superconductivity of Iron-based Superconductors, in Particular Fe Chalcogenides Research in Film Form

Fe chalcogenide (FeCh) is mine of interesting phenomena even in Fe based superconductor family. In addition to nematic state without magnetic order, amazing flexibility of increasing superconducting T_c 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.  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 may be realized.  However, even after 12 years of the report, T_c 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.

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.

We have studied superconductivity of FeCh in films grown by PLD technique and investigated superconductivity of all three categories described above.  For the first category, we succeeded in preparing a series of FeSe_1-xTe_x samples, avoiding phase separation which used to be inevitable in bulk crystals.  By the investigations of electronic properties both in normal stare and in superconducting state by ARPES, optical, THz and microwave conductivity, dc electromagnetic studies, m-SR and DFT 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.  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 different from that of the first category.  During that procedure, we realized zero-resistance of 46 K, which is the highest value among all available reliable data of FeCh.  For the third category, we succeeded in realizing the interface superconductivity also by the PLD technique down to 4-layer thickness.  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 by introducing coupling between interfaces and killing 2 dimensionality.

Reference
[1] A. Maeda, T. Kobayashi and F. Nabeshima: “Studies of superconductivity of Fe chalcogenides in films grown by PLD technique”, Supercond. Sci. Tech., submitted.