  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.
