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The single crystal of RuAs obtained by Bi-flux method shows obvioussuccessive metal-insulator transitions at T_MI1~255 K and T_MI2~195$ K. TheX-ray diffraction measurement reveals a formation of superlattice of 3x3x3 ofthe original unit cell below T_MI2, accompanied by a change of the crystalsystem from the orthorhombic structure to the monoclinic one. Simpledimerization of the Ru ions is nor seen in the ground state. The multiple Assites observed in nuclear quadrupole resonance (NQR) spectrum also demonstratethe formation of the superlattice in the ground state, which is clarified to benonmagnetic. The divergence in 1/T_1 at T_MI1 shows that a symmetry lowering bythe metal-insulator transition is accompanied by strong critical fluctuationsof some degrees of freedom. Using the structural parameters in the insulatingstate, the first principle calculation reproduces successfully the reasonablesize of nuclear quadrupole frequencies for the multiple As sites, ensuring thehigh validity of the structural parameters. The calculation also gives aremarkable suppression in the density of states (DOS) near the Fermi level,although the gap opening is insufficient. A coupled modulation of thecalculated Ru d electron numbers and the crystal structure proposes a formationof charge density wave (CDW) in RuAs. Some lacking factors remain, but it showsthat a lifting of degeneracy protected by the non-symmorphic symmetry throughthe superlattice formation is a key ingredient for the metal-insulatortransition in RuAs.

Superlattice formation lifting degeneracy protected by non-symmorphic symmetry through a metal-insulator transition in RuAs