Unconventional Superconductivity and Antiferromagnetic Quantum Critical Behavior in the Isovalent-Doped BaFe2(As1-xPx)2

Spin dynamics evolution of BaFe2(As1-xPx)(2) was probed as a function of P concentration via P-31 NMR. Our NMR study reveals that two-dimensional antiferromagnetic (AF) fluctuations are notably enhanced with little change in static susceptibility on approaching the AF phase from the superconducting dome. Moreover, the magnetically ordered temperature theta deduced from the relaxation rate vanishes at optimal doping. These results provide clear-cut evidence for a quantum-critical point, suggesting that the AF fluctuations associated with the quantum-critical point play a central role in the high-T-c superconductivity.