We report 75As NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca1-xPrxFe2As2 . In both cases of x = 0.075, 0.15, a large increase of νq upon cooling is consistent with the tetragonal-collapsed tetragonal structure transition. A sharp drop of the Knight shift is also seen just below the structure transition, which suggests the quenching of Fe local magnetism, and therefore offers important understanding of the collapsed tetragonal phase. At even low temperatures, the 1/75T1 is enhanced and forms a peak at T ≈ 25 K, which may be caused by the magnetic ordering of the Pr3+ moments or spin dynamics of mobile domain walls.
The discovery of high temperature superconductivity in single-layer FeSe/SrTiO3 provides a new platform for exploring superconductivity and pursuing higher Tc(superconducting transition temperature) through fabricating artificial heterostructures. In this paper, we review the recent progress in studying and tuning the interfacial superconductivity in single-layer FeSe, through the combined in-situ spectroscopic studies and atomic-scale engineering. By fabricating artificial heterostructures, various interfacial factors were tuned, and the corresponding evolutions of electronic structure and superconducting gap behavior were investigated. These studies enrich the current understanding on the interfacial superconductivity, and provide clues for further enhancing Tc through interface engineering.
The newly discovered iron-chalcogenide superconductorKxFe2ySe2exhibits a distinct electronic structure from other iron-based superconductors.Exploiting polarization-dependent angle-resolved photoemission spectroscopy,we have determined the orbital characters of band structure in aKxFe2ySe2superconductor.To a large extent,we find thatKxFe2ySe2superconductor shares similar orbital characters with other iron-based superconductors,but with its own characteristics.For example,we have resolved two highly degenerate electron cylinders around the zone corner in the s and p geometries,respectively,indicating negligible interactions between them.Moreover,in contrast to the band calculation results,the small electron pocket around Z is found to be mainly consisted of the d z 2 orbital.The determined orbital characters would help to construct a realistic model forKxFe2ySe2.
The newly discovered iron-based superconductors have triggered renewed enormous research interest in the condensed matter physics community. Nuclear magnetic resonance (NMR) is a low-energy local probe for studying strongly correlated electrons, and particularly important for high-T C superconductors. In this paper, we review NMR studies on the structural transition, antiferromagnetic order, spin fluctuations, and superconducting properties of several iron-based high-T C superconductors, including LaFeAsO1-xF x , LaFeAsO1-x , BaFe2As2 , Ba1-xKxFe2As2 , Ca0.23Na0.67Fe2As2 , BaFe2 (As1-xPx )2 , Ba(Fe1-xRux )2As2 , Ba(Fe1-x Cox )2 As2 , Li1+xFeAs, LiFe1-xCoxAs, NaFeAs, NaFe1-xCoxAs, KyFe2-xSe2 , and (Tl,Rb)yFe2-x Se2.
