Tl-NMR Study of Pyrochlore Oxide Tl₂Ru₂O₇: Observation of Spin-Singlet Formation

Abstract

Pyrochlore oxide Tl₂Ru₂O₇, which shows a metal-insulator transition at T_MIT∼ 120 K, has been investigated using Tl-NMR measurements. The very sharp Tl-NMR spectrum in the insulating phase, whose shift is almost zero, suggests that the ground state of this compound takes a nonmagnetic spin-singlet state. The spin-lattice relaxation rate T₁^-1 immediately below T_MIT suggests the presence of a spin gap between the ground and excited states. This spin-singlet state may originate from inherent geometrical frustration of pyrochlore oxide.