Formation of orbital molecules on a pyrochlore lattice induced by A-O bond covalency

A. Krajewska; A. Yaresko; J. Nuss; A. S. Gibbs; S. Bette; M. Blankenhorn; R. E. Dinnebier; D. P. Sari; I. Watanabe; J. Bertinshaw; H. Gretarsson; K. Ishii; D. Matsumura; T. Tsuji; M. Isobe; B. Keimer; H. Takagi; T. Takayama

paper

Formation of orbital molecules on a pyrochlore lattice induced by A-O bond covalency

arXiv:2111.05057

Abstract

The pyrochlore ruthenate In $_{2}$ Ru $_{2}$ O $_{7}$ displays a subtle competition between spin-orbital entanglement and molecular orbital formation. At room temperature, a spin-orbit-entangled singlet state was identified. With decreasing temperature, In $_{2}$ Ru $_{2}$ O $_{7}$ undergoes multiple structural transitions and eventually forms a nonmagnetic ground state with semi-isolated Ru $_{2}$ O units on the pyrochlore lattice. The dominant hopping through the Ru-O-Ru linkage leads to molecular orbital formation within the Ru $_{2}$ O units. This molecular orbital formation is unique in that it involves the O $^{2 -}$ anions, unlike the transition-metal dimers observed in systems with edge-sharing octahedra. We argue that the covalent character of In-O bonds plays a pivotal role in the structural transitions and molecular orbital formation and such bonding character of " $A$ -site" ions is an important ingredient for electronic phase competition in complex transition metal oxides.