Nearly linear orbital molecules on a pyrochlore lattice

Krajewska, A.*; Tsuji, Takuya  ; Matsumura, Daiju   ; 15 of others*

The interplay of spin-orbit coupling with other relevant parameters gives rise to the rich phase competition in complex ruthenates featuring octahedrally coordinated Ru. While locally, spin-orbit coupling stabilizes a non-magnetic = 0 state, intersite interactions resolve one of two distinct phases at low temperatures: an excitonicmagnet stabilized by the magnetic exchange of upper-lying = 1 states or Ru molecular orbital dimers driven by direct orbital overlap. Pyrochlore ruthenates ARuO (A = rare earth, Y) are candidate excitonic magnets with geometrical frustration. We synthesized InRuO with covalent In-O bonds. This pyrochlore ruthenate hosts alocal = 0 state at high temperatures; however, at low temperatures, it forms a unique nonmagnetic ground state with nearly linear Ru-O-Ru molecules, in stark contrast to other ARuO compounds. The disproportionation of covalent In-O bonds drives RuO molecule formation, quenching not only the local spin-orbit singlet but also geometrical frustration.