  The single crystal of RuAs obtained by Bi-flux method shows obvious
successive metal-insulator transitions at T_MI1~255 K and T_MI2~195$ K. The
X-ray diffraction measurement reveals a formation of superlattice of 3x3x3 of
the original unit cell below T_MI2, accompanied by a change of the crystal
system from the orthorhombic structure to the monoclinic one. Simple
dimerization of the Ru ions is nor seen in the ground state. The multiple As
sites observed in nuclear quadrupole resonance (NQR) spectrum also demonstrate
the formation of the superlattice in the ground state, which is clarified to be
nonmagnetic. The divergence in 1/T_1 at T_MI1 shows that a symmetry lowering by
the metal-insulator transition is accompanied by strong critical fluctuations
of some degrees of freedom. Using the structural parameters in the insulating
state, the first principle calculation reproduces successfully the reasonable
size of nuclear quadrupole frequencies for the multiple As sites, ensuring the
high validity of the structural parameters. The calculation also gives a
remarkable suppression in the density of states (DOS) near the Fermi level,
although the gap opening is insufficient. A coupled modulation of the
calculated Ru d electron numbers and the crystal structure proposes a formation
of charge density wave (CDW) in RuAs. Some lacking factors remain, but it shows
that a lifting of degeneracy protected by the non-symmorphic symmetry through
the superlattice formation is a key ingredient for the metal-insulator
transition in RuAs.
