Nearly linear orbital molecules on a pyrochlore lattice | Science Advances
Abstract
The interplay of spin-orbit coupling with other relevant parameters gives rise to the rich phase competition in complex ruthenates featuring octahedrally coordinated Ru
4+
. While locally, spin-orbit coupling stabilizes a nonmagnetic
J
eff
= 0 state, intersite interactions resolve one of two distinct phases at low temperatures: an excitonic magnet stabilized by the magnetic exchange of upper-lying
J
eff
= 1 states or Ru
2
molecular orbital dimers driven by direct orbital overlap. Pyrochlore ruthenates
A
2
Ru
2
O
7
(
A
= rare earth, Y) are candidate excitonic magnets with geometrical frustration. We synthesized In
2
Ru
2
O
7
with covalent In─O bonds. This pyrochlore ruthenate hosts a local
J
eff
= 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
A
2
Ru
2
O
7
compounds. The disproportionation of covalent In─O bonds drives Ru
2
O molecule formation, quenching not only the local spin-orbit singlet but also geometrical frustration.