Frustrated random-singlet state with ice-type structural fluctuation in spinel titanates

花咲 徳亮*; 服部 崇幸*; 薦田 匠*; 源 拡栄*; 鳥越 秀平*; 山下 智史*; 中澤 康浩*; 中野 岳仁*; 吉見 光平*; 八島 光晴*; et al.

Proceedings of the National Academy of Sciences of the United States of America, 123(1), p.e2517926123_1 - e2517926123_6, 2026/01

https://doi.org/10.1073/pnas.2517926123

In ice, it is well-known that the orientation of HO molecules is disordered by geometrical frustration. Ice-analogous materials having a pyrochlore lattice display interesting phenomena such as the spin-ice state and the magnetic monopole. In the spinel titanate MgTiO, the Ti ions have a quantum spin in the pyrochlore lattice. The Ti ions are displaced, accompanied by the spin-singlet formation. Since this displacement pattern follows the ice rule, the title compound is a material analogous to ice. When a small quantity of Ti ions are replaced with Mg ions, the ice-type structural fluctuation exists. In this structural ice-type state, the spins are also fluctuating at a nanosecond scale down to 0.3K. We ascribed this phenomenon to the gapless frustrated random-singlet state, in which the spin-singlet pairs are resonating, and the orphan spins are hopping.