Change of Fermi surface states related with two different Tc-raising mechanisms in iron pnictide superconductors

The evolution of Fermi surface (FS) states of NdFeAs1−xPxO0.9F0.1 single crystals with As/P substitution has been investigated. The critical temperature Tc and the power-law exponent (n) of temperature-dependent resistivity [ρ(T)=ρ0+ATn] show a clear correlation above x=0.2, suggesting that Tc is enhanced with increasing bosonic fluctuations in the same type of FS state. Around x=0.2, all the transport properties show anomalies, indicating that x∼0.2 is the critical composition of a drastic FS change. Angle-resolved photoemission spectroscopy has more directly revealed the distinct change of FS around x=0.2, that one hole FS disappears at the Brillouin zone center and the other FS with a propellerlike shape appears at the zone corner with decreasing x. These results are indicative of the existence of two types of FS states with different nesting conditions that are related with two Tc-raising mechanisms in this system.