Separation between Low-Energy Hole Dynamics and Spin Dynamics in a Frustrated Magnet

An angle-resolved photoemission spectroscopy (ARPES) study is reported on a Mott insulator NiGa 2 S 4 in which Ni 2+ (S = 1) ions form a triangular lattice and the Ni spins do not order even in its ground state. The first ARPES study on the two-dimensional spin-disordered system shows that low-energy hole dynamics at high temperatures is characterized by wave vectors Q E which are different from wave vectors Q M dominating low-energy spin excitations at low temperatures. The unexpected difference between Q E and Q M is deeply related to charge fluctuation across the Mott gap in the frustrated lattice and is a key issue to understand the spin-disordered ground states in Mott insulators.