Electronic properties across metal-insulator transition in \beta-pyrochlore-type CsW2O6 epitaxial films

TL;DR

This study reports the successful epitaxial growth of CsW2O6 films, revealing a metal-insulator transition driven by charge disproportionation and rattling motion, with detailed electronic property analysis across the transition.

Contribution

First demonstration of epitaxial CsW2O6 films enabling detailed study of its electronic properties and transition mechanisms.

Findings

Sharp MIT observed around 200 K in epitaxial films

Charge disproportionation of W ions identified in the insulating phase

Rattling motion of Cs+ influences electronic structure changes

Abstract

In CsW2O6, which undergoes a metal-insulator transition (MIT) at 213 K, the emergence of exotic properties associated with rattling motion of Cs is expected owing to its characteristic \beta-pyrochlore-type structure. However, a hurdle for crystal growth hampers elucidation of detailed properties and mechanisms of the MIT. Here we report on the epitaxial growth of \beta-pyrochlore-type CsW2O6 films and their electronic properties across the MIT. Using pulsed-laser deposi-tion technique, we grew single-crystalline CsW2O6 films exhibiting remarkably lower resistivity compared with a poly-crystalline bulk and sharp MIT around 200 K. Negative magnetoresistance and positive Hall coefficient were found, which became pronounced below 200 K. The valence-band and core-levels photoemission spectra indicated the drastic changes across the MIT. In the valence band photoemission spectrum, the finite density of states was observed at the Fermi level in the metallic phase. In contrast, an energy gap appeared in the insulating phase. The split of W 4f core-level spectrum suggested the charge disproportionation of W5+ and W6+ in the insulating phase. The change of spectral shape in the Cs 4d core levels reflected the rattling motion of Cs+ cations. These results strongly suggest that CsW2O6 is a novel material, in which MIT is driven by the charge disproportionation associated with the rattling motion.