⁷⁵As-NMR/NQR studies on new iron-based 122 superconductors (La_0.5-xNa_0.5+x)Fe₂As₂

New iron-arsenide compounds (La_0.5-xNa_0.5+x)Fe₂As₂ (x =-0.5, 0, +0.3) were investigated by ⁷⁵As-NMR/NQR measurements. The parent compound (x = 0) exhibits a stripe-type antiferromagnetic (AFM) order below T_N = 130 K, analogous to BaFe₂As₂. In hole-doped state of x = +0.3 (T_c = 27 K), we observed strong AFM spin fluctuations in the normal state and the unconventional superconducting (SC) state characterized by multiple gaps, which is comparable to the typical hole-doped (Ba,K)Fe₂As₂. In heavily electron-doped state of x = -0.5 that is equivalent to non-SC compound Ba(Fe_0.5Co_0.5)₂As₂ in the formal valence, we detected weak AFM spin fluctuations in the normal state and unconventional multiple-gapped SC state characterized by obscure gap structure. These results reveal the close relationship of AFM spin fluctuations for the onset of unconventional multiple-gapped SC state from the hole-doped to heavily electron-doped states of (La_0.5-xNa_0.5+x)Fe₂As₂. (author)