Abstract
75 As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe 2 As 2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependence of nuclear-spin–lattice relaxation rate (1/ T 1 ) measured at tetragonal phase show no coherence peak just below T c and decrease with decreasing temperature. The superconductivity is of gapless at P = 4.7 kbar but evolves to multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.
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