WEB Strain-Driven Metal-to-Insulator Transition and Charge Ordering in a Geometrically Frustrated Spinel Oxide LiV2O4
The coupling of local atomic configurations and electronic degrees of freedom plays a fundamental role in understanding metal-insulator transitions and the formation of charge ordering. In particular, such competing interactions become more pronounced in the geometrically frustrated pyrochlore lattice in the spinel structure, due to fluctuations in the charge, spin and orbital channels. Here, using atomic-resolution scanning transmission electron microscopy imaging and electron energy-loss spectroscopy, we investigate mixed-valence spinel LiV2O4 thin films grown on SrTiO3 and MgO (001) substrates. We show that the epitaxial strain strongly affects the spatial configurations of valence states in LiV2O4, and the local valence distributions are resolved at atomic-scale resolution. Two competing phases are detected on the thin films, a metallic charge-disordered heavy-fermion state on SrTiO3 and an insulating charge-ordered state on MgO. Importantly, our result shows that the out-of-plane lattice compression relieves the charge frustration and induces a Verwey-type-like charge-ordering pattern in LiV2O4. This observation provides atomic-scale insight into the strong charge-order correlation and the tuneable electronic-phase transitions in related frustrated systems.
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|Presentation||Figure 1||Scanning transmission electron microscopy (STEM) measurements of the spinel oxide LiV2O4. Low-magnification high-angle annular dark-field (HAADF) images of the LiV2O4 thin films grown on the SrTiO3 substrate (a) and the MgO substrate (b). (c) Crystal structure of LiV2O4 along the  axis. (d,e) Simultaneously required STEM-HAADF (left) and annular bright-field (right) images of LiV2O4 on SrTiO3 and MgO, respectively, resolving all the chemical elements including V, O and Li ions.||702 KB||Download|