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

English (英語)

Physical Review Materials

2(5)

055001-055001

American Physical Society (APS)

2018-05-04

2020-02-21

A single crystal of RuAs obtained with the 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 the formation of a superlattice of 3 x 3 x 3 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 not seen in the ground state. The multiple As sites observed in the nuclear quadrupole resonance spectrum also demonstrate the formation of the superlattice in the ground state, which is clarified to be nonmagnetic. The divergence in 1/T1 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-principles calculation reproduces successfully the reasonable size of nuclear quadrupole frequencies ν(Q) 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 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 the formation of a charge density wave in RuAs. Some lacking factors remain, but it is shown that a lifting of degeneracy protected by the nonsymmorphic symmetry through the superlattice formation is a key ingredient for the metal-insulator transition in RuAs.

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