The cuprates which have high superconducting critical temperatures were discovered by Bednolz and Muller in 1986 and are generally called as high-Tc superconductors (HTSCs). About thirty years have passed since the discovery of HTSCs, but the mechanism of the high-Tc superconductivity has not yet been understood. Recently iron-based high-Tc superconductors were discovered by Hosono group in Japan. In order to clarify the mechanism of the iron-based high-Tc superconductivity, much studies of HTSCs are being carried out all over the world.
In the cuprate high-Tc superconductors, doping carriers into two-dimensional insulating planes consisting of Cu and O (CuO2 plane) induces the superconductivity. In the iron-based high-temperature superconductors, similarly carrier doping for the metallic antiferromagnetic FeAs layers causes the superconductivity.
Mapping the intensity of photoelectrons at the Fermi level into momentum space allows us to visualize Fermi surfaces (FSs). In underdoped region, part of the FSs disappears, which is an unexpected phenomenon for conventional metals.It is called pseudogap phenomenon and the remaining part of the FSs is called Fermi arc. Because the pseudogap and the Fermi arc may be deeply related to the origin of the high-Tc superconductivity, studies on their properties have been carried out extensively.
The transition temperature of the high-Tc superconductors tends to rise as the number of CuO2 layers increases. The optimally doped Bi2Sr2Ca2Cu3O10+δ (Bi2223) which has three CuO2 planes (two outer layers, one inner layer) has a higher transition temperature Tc =110 K. In our study, it has been revealed that the magnitude of the superconducting gap at the outer planes is different from that at the inner layer in the case of Bi2223. This results will give us clues to clarify relation between the number of CuO2 planes and the superconductivity.
In the electron-doped cuprate high-Tc superconductors, strongly antiferromagnetic correlation still persists in the superconducting phase. Therefore, it has been considered that a large gap, so-called pseudogap, opens (cut2). However, in the case of optimally-annealed samples, the pseudogap was not observed at all by ARPES with maintaining high Tc, indicating antiferromagnetic correlation is strongly suppressed. ARPES spectra of Pr1.3−xLa0.7CexCuO4 with and without protect annealing are shown in left figures. (a-c: Fermi surface mappings of as-grown, weakly annealed, and annealed respectively. d-f: Intensity plot in energy-momentum space for each sample. g-h: EDCs in each cut.) The results will compel us to reconsider relationship between the high-Tc superconductivity and the antiferromagnetism.
It has been well known that electronic structure of the cuprate high-Tc superconductors is two-dimensional (2D). On the contrary, strong three dimensionality in FSs of the iron-based superconductors has been identified by recent studies. Fermi surface mapping of optimally doped BaFe2(As1-xPx)2 in the k//-kz plane is shown in a left figure. The sizes of FSs change along the kz axis, indicating the three-dimensional FSs. It is thought that the three dimensionality in FSs dramatically affects the symmetry of the order parameter.
In the iron-based superconductor SFe2(As1-xPx)2 (S=Ba, Sr), as the antiferromagnetic order is suppressed by P doping, superconductivity appears due to chemical pressure effects without carrier doping. The superconducting transition temperature reaches approximately 30 K. These systems have attracted particular attention since the presence of line nodes in the superconducting gap was suggested. We have revealed the FS shapes of SrFe2(As0.65P0.35)2 and kz dependence of the FSs using ARPES and local-density-approximation (LDA) band-structure calculations. Furthermore, our results indicate that interlayer magnetic interactions play a more important role than FS nesting in stabilizing the AFM order. The results will give us clues to clarify the mechanism of the superconductivity in the iron-based superconductors.