Importance of dxy orbital for high-Tc superconductivity in 1111 type iron-based superconductors via investigations of electronic phase diagram and angle-resolved photoemission spectroscopy

In one of the iron-based superconducting systems, $R$FeAsO ($R =$ rare earth elements), the structural flexibility at three substitution sites enabled investigation of the relation among superconductivity, local structure parameters, and electronic structure over a wide range of crystal compositions. In this study, the substitutions of Nd for La, Sb or P for As, and F or H for O were performed, and we synthesized $R$FeAs$_{1-x}$(P/Sb)$_x$O$_{1-y}$(F/H)$_y$ samples ($R =$ La and Nd). All these substitutions modify the local structural parameters, while the F/H-substitution also changes band filling of 3d electron bands in Fe ions. It was observed that the pnictogen height from Fe-plane ($h_{Pn}$) increases with increasing As/Sb composition and electron doping by F or H substitution effects, and that $h_{Pn}$ in the Nd-system is larger than that in the La-system. The changes in the superconducting transition temperature ($T_c$) with composition can be understood in terms of the contribution of the $d_{xy}$ hole Fermi surface (FS), which is controlled by $h_{Pn}$.
Figure 1 shows the contour plots illustrating how $T_c$ changes with $x$ and $y$ for the La- and Nd-systems, respectively. [1] In these phase diagrams, we distinguish three superconducting phases (SC1, SC2 and SC3), particularly in the La-system. Compared to the change from SC1 to SC2 with the disappearance of the $d_{xy}$ FS, the change from SC1 to SC3 is not clearly defined. In the La-system, although these two phases (SC1 and SC3) are separated at $x =$ 0, when Sb is substituted, they merge with each other, as in the Nd-system with larger $h_{Pn}$, owing to the enlargement of the $d_{xy}$ FS by the increase in $h_{Pn}$. We therefore think that the superconductivity mechanisms in phases SC1 and SC3 have the same origin. According to this scenario, the $T_c$ suppression near $x =$ 0 for the intermediate doping range (0.14 $< y < $0.25) in the La-system is not caused by the bad FS nesting condition, but by the loss of $d_{xy}$ FS due to electron doping. The lost $d_{xy}$ FS and suppressed $T_c$ can be recovered by Sb substitution and/or electron doping. While FS nesting controlled by band filling plays an important role in the P-rich compositions (SC2 phase), as $h_{Pn}$ increases with increasing As/Sb-content, superconductivity regime switches to SC1/SC3 with $d_{xy}$ orbital playing an important role. In the present work, we confirmed the change of band structure related to the phase change from SC3(/SC1) to SC2 in NdFeAs$_{1-x}$P$_x$O$_{0.9}$F$_{0.1}$ using angle-resolved photoemission spectroscopy (ARPES). [2,3] The results of ARPES studies revealed that the SC3(/SC1) can be defined as the phase in which the $d_{xy}$ hole FS contributes to superconductivity, while this is not expected in SC2. In the SC3(/SC1) ($x < $0.2), it was experimentally confirmed that the $d_{xy}$ band approaches the Fermi level, forming an incipient band.

[1] T. Kawashima $et al$., Scientific Reports, 11, 10006 (2021).
[2] A. Takemori $et al$., Physical Review B, 98, 100501(R) (2018).
[3] Z. H. Tin $et al$., submitted to Physical Review B.