Transport properties of PLD-grown ultrathin films of FeSe under magnetic field

Iron chalcogenide superconductor, FeSe, has attracted much attention because monolayer films on SrTiO₃ (STO) grown by molecular beam epitaxy (MBE) exhibit interface-enhanced superconductivity with superconducting transition temperature (T_c) of 40-65 K, which is much higher than that of 9 K in bulk form [1,2]. The ultrathin films of FeSe had been grown only by the MBE technique, however, we reported the successful realization of interface-enhanced superconductivity in films thicker than 7 nm using the PLD technique. In the early stage, a thinner film with thickness d of 5 nm did not show superconductivity probably due to degradation by air exposure [3].

Here, we report the successful growth of superconducting ultrathin films of FeSe/STO with d ≦ 4 nm with a capping layer of amorphous Si. The ultrathin film with d = 4 nm showed T_c^onset of 30 K and zero resistivity below 10 K. Negative hall coefficient, which is in contrast to thick FeSe films, was observed at low temperatures, suggesting the electron transfer from STO. The film showed large anisotropy of the upper critical fields as high as 14.8 and the GL coherence length along the c-axis ξ_c of 0.2 nm, which is smaller than the c-axis lattice parameter of FeSe. Similar short coherence length was also observed in thicker film (d  = 19 nm), suggesting a confinement of superconductivity at FeSe/STO interface [4]. Furthermore, we will discuss the vortex state and superconducting fluctuation using Nernst effect measurement.

Reference

[1]. Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012).

[2]. S. He et al., Nat. Mater. 12, 605 (2013).

[3]. T. Kobayashi et al., Supercond. Sci. Technol. 35, 07LT01 (2022).

[4]. T. Kobayashi et al., J. Phys.: Condens. Matter 35, 41LT01 (2023).