  We studied the quantum critical behavior of the second antiferromagnetic (AF)
phase in the heavily electron-doped high-$T_c$ pnictide, LaFeAsO$_{1-x}$H$_x$
by using $^{75}$As and $^{1}$H nuclear-magnetic-resonance (NMR) technique. In
the second AF phase, we observed a spatially modulated spin-density-wave-like
state up to $x$=0.6 from the NMR spectral lineshape and detected a low-energy
excitation gap from the nuclear relaxation time $T_1$ of $^{75}$As. The
excitation gap closes at the AF quantum critical point (QCP) at $x \approx
0.49$. The superconducting (SC) phase in a lower-doping regime contacts the
second AF phase only at the AF QCP, and both phases are segregated from each
other. The absence of AF critical fluctuations and the enhancement of the
in-plane electric anisotropy are key factors for the development of
superconductivity.
