ABSTRACT
An angle-resolved photoemission spectroscopy (ARPES) study is reported on a Mott insulator NiGa2S4 in which Ni2+ (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.
ABSTRACT
We report synchrotron-based operando soft X-ray microscopic photoelectron spectroscopy under charge-discharge control of single crystalline LiCoO2 (LCO) particles as an active electrode material for an all solid-state lithium-ion battery (LIB). Photoelectron mapping and the photoelectron spectrum of a selected microscopic region are obtained by a customized operando cell for LIBs. During the charge process, a more effective Li extraction from a side facet of the single crystalline LCO particle than from the central part is observed, which ensures the reliability of the system as an operando microscopic photoelectron analyzer that can track changes in the electronic structure of a selected part of the active particle. Based on these assessments, the no drastic change in the Co 2p XPS spectra during charge-discharge of LCO supports that the charge-polarization may occur at the oxygen side by strong hybridization between Co 3d and O 2p orbitals. The success of tracking the electronic-structure change at each facet of a single crystalline electrode material during charge-discharge is a major step toward the fabrication of innovative active electrode materials for LIBs.