Pressure and K doping induced superconductivity in BaFe₂As₂

We have measured electrical resistivity under pressure P and ⁷⁵As NMR on single crystal and poly crystalline BaFe₂As₂ and (Ba_1−xK_x)Fe₂As₂ to elucidate mechanism of antiferromagnetic/superconducting phase transition in BaFe₂As₂. This system is an antiferromagnet, AF, with a transition temperature of 141 K, which shows superconductivity by applying P or by a carrier doping of K for Ba. From high P resistivity measurements, we understood that the transition was highly sensitive to the P homogeneity. Small uniaxial stress strongly suppresses AF order and stabilizes superconductivity at much lower pressure. ⁷⁵As NMR study in (Ba_1−xK_x)Fe₂As₂ shows existence of large AF spin fluctuations in tetragonal phase having a superconducting ground state. The AF spin fluctuations decrease gradually with increasing K concentration.