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 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.
