# CM Seminar: Three superconducting phases of 1111-type iron-based superconductor RFeAs1-xPnxO1-y(F,H)y (R=La and Nd, Pn=P and Sb)

In one of 1111-type iron-based superconducting (SC) system LaFeAsO, the electron doping level and the local crystal structure can be controlled by the F/H substitution for O and P for As. By these chemical substitution effects, Fermi surface (FS) topology has been changed, and three different SC phases appears in LaFeAs_{1-x}P* _{x}*O

_{1-y}(F/H)

*system. [1,2] The first and second SC phases (SC1 and SC2) are observed in the low F concentration region around LaFeAsO*

_{y}_{0.9}F

_{0.1}((

*x*,

*y*) ~ (0, 0.1)) and LaFeAs

_{0.6}P

_{0.4}O

_{0.9}F

_{0.1}((

*x*,

*y*) ~ (0.4, 0.1)), respectively. On the other hand, the third SC phase (SC3) appears in the highly electron doping region of (

*x*,

*y*) ~ (0, 0.3). In the SC1 and SC2 states, the spin fluctuations due to the FS nesting in LaFeAsO-type FS with

*xy*hole FS or LaFePO-type FS without

*xy*hole FS play an important role in order to stabilize the superconductivity. [3,4] In the SC3 state, however, the heavily electron doping destabilizes the FS nesting. In this SC3 phase, the next nearest neighbor magnetic interaction in the

*xy*direction in real space induces the superconductivity.

We also have investigated the transport properties and structural parameters of NdFeAs_{1-x}P_{x}O_{1-y}(F/H)* _{y}* (

*y*=0~0.4) and LaFeAs

_{1-x}Sb

_{x}O

_{1-y}(F/H)

*(*

_{y}*y*=0~0.3) to clarify the correlation between the stability of superconductivity and the change of the FS accompanied by the P/Sb and F/H substitutions. In the Nd-1111 system with P substitution, the result of structural analysis revealed that the pnictogen height from the Fe plane

*h*is larger than that in the La-1111 system. The Sb substitution in the La-1111 system also increases

_{Pn}*h*. With increasing

_{Pn}*h*, the FS nesting has been improved by the enlarging the

_{Pn}*xy*FS near

*R*FeAsO in the phase diagram, and the strength of the next nearest neighbor magnetic interaction is also enhanced in heavily H doping region. As a result, these effects has stabilized the SC1 and SC3 states, and merges the SC1 phase near

*R*FeAsO and the SC3 one in the highly H doping region in the phase diagram. The phase diagram for the present systems can be explained by the scenario for FS nesting and next nearest neighbor magnetic interaction.

[1] S. Miyasaka *et al*., Phys. Rev. B **95**, 214515 (2017).

[2] K. T. Lai, S. Miyasaka *et al*., Phys. Rev. B **90**, 064504 (2014).

[3] S. Miyasaka *et al*., J. Phys. Soc. Jpn. **82**, 124706 (2013).

[4] A. Takemori, S. Miyasaka *et al*., Phys. Rev. B **98**, 100501(R) (2018).