Speaker: : Prof. Shigeki Miyasaka, Department of Physics, Osaka University, Japan
Time: July 18, 2019, 2:00 PM - 3:30 PM
Abstract: 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 LaFeAs1-xPxO1-y(F/H)y system. [1,2] The first and second SC phases (SC1 and SC2) are observed in the low F concentration region around LaFeAsO0.9F0.1 ((x, y) ~ (0, 0.1)) and LaFeAs0.6P0.4O0.9F0.1 ((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 NdFeAs1-xPxO1-y(F/H)y (y=0~0.4) and LaFeAs1-xSbxO1-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 hPn is larger than that in the La-1111 system. The Sb substitution in the La-1111 system also increases hPn. With increasing hPn, the FS nesting has been improved by the enlarging the xy FS near RFeAsO 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 RFeAsO 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.
 S. Miyasaka et al., Phys. Rev. B 95, 214515 (2017).
 K. T. Lai, S. Miyasaka et al., Phys. Rev. B 90, 064504 (2014).
 S. Miyasaka et al., J. Phys. Soc. Jpn. 82, 124706 (2013).
 A. Takemori, S. Miyasaka et al., Phys. Rev. B 98, 100501(R) (2018).