Anisotropy of superconductivity is one of the fundamental physical parameters for understanding layered iron-based superconductors (IBSs). Here we investigated the anisotropic response of resistive transition as a function of thickness (d) in iron selenide (FeSe) based electric-double-layer transistors (EDLTs) on SrTiO3, which exhibit superconducting transition temperatures Tc as high as 40 K below d=10nm. According to the analyses of the in-plane (Hc2//) and out-of-plane (Hc2) upper critical fields (Hc2) and the magnetic field angle dependence of the resistance (Rs-θ) in ultrathin condition, we found that the anisotropy factor 0=Hc2///Hc2 is 7.4 in the thin limit of d∼1nm, which is larger than that of bulk IBSs. In addition, we observed the shorter out-of-plane coherence length ξc of 0.19 nm compared to the c-axis lattice constant, which implies the confinement of the order parameter in the one unit cell FeSe. These findings suggest that high-Tc superconductivity in the ultrathin FeSe-EDLT exhibits an anisotropic three-dimensional (3D) or quasi-two-dimensional (2D) nature rather than the pure 2D one, leading to the robust superconductivity. Moreover, we carried out the systematic evaluation of the anisotropic Hc2 against thickness reduction in the FeSe channel. The in-plane Hc2 as a function of normalized temperature T/Tc is almost independent of d until the thin limit condition. On the other hand, the out-of-plane Hc2 near T/Tc∼1 decreases with increasing d, resulting in the increase of 0 at around Tc to 32.0 at the thick condition of d=9.3nm, which is also confirmed by Rs-θ measurements. The counterintuitive behavior can be attributed to the degree of coupling strength between two electron-rich layers possessing a high superconducting order parameter induced by electrostatic gating at the top interface and charge transfer from SrTiO3 substrates at the bottom interface. Besides a large Hc2 for d=9.3nm exceeding 20 T even at T=0.8Tc, we observe the decoupling crossover of the two superconducting layers at low temperature, which is a unique feature for the high-Tc FeSe-EDLT on SrTiO3.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics