No abstract

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 + AT n ) show a clear correlation above x =0.2, suggesting that Tc is enhanced with increasing bosonic fluctuation 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 drastic FS change. The angle resolved photoemission spectroscopy has more directly revealed the distinct change of FS around x =0.2, that one hole FS disappears at Brillouin zone center and the other FS with propeller like shape appears at zone corner with decreasing x. These results are indicative of the existence of two types of FS state with different nesting conditions that are related with two Tc-rising mechanisms in this system.

We have investigated the electronic structure of Sr 1−y Ca y Fe 2 (As 1−x P x ) 2 (x = 0.25, y = 0.08) by means of angle-resolved photoemission spectroscopy. From the comparison with the results of BaFe 2 (As 1−x P x ) 2 , the effects of smaller structural anisotropy (c/a) on the Fermi surfaces (FSs) and the gap structures are discussed. The observed FSs have three dimensional shapes. One of the hole FSs is strongly warped between the Γ and Z points, and the innermost FS observed at the Z point disappears at the Γ point, which is similar to the FS features of SrFe 2 (As 1−x P x ) 2 (x = 0.35). In the superconducting state, the node like gap-minimum is present for the d xy electron FS near the X point, while the gaps around the other high symmetry points are isotropic. Several theoretical models based on the spin and/or the orbital fluctuation are examined to explain all these experimental results.

No abstract