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

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 β-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 β-pyrochlore-type CsW2O6 films and their electronic properties across the MIT. Using pulsed-laser deposition technique, we grew single-crystalline CsW2O6 films exhibiting remarkably lower resistivity compared with a polycrystalline 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 W 5+ and W 6+ 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.

Electrical and Thermal Transport Properties of the β-Pyrochlore Oxide CsW2O6

“…As a result, Phase II was reported to be a nonmagnetic insulator. The phase transition between Phases I and II was also observed in a study on thin films [12]. Although the electrical resistivity  of Phase I is much smaller than that of Phase II in all the measured samples, the  in Phase I increases with decreasing temperature [9,10,12].…”

Regular-triangle trimer and charge order preserving the Anderson condition in the pyrochlore structure of CsW2O6

“…As shown in Fig. 1B, the electrical resistivity, , of a single crystal strongly increases below Tt = 215 K with decreasing temperature, as in the cases of a polycrystalline sample and a thin film 11,13 . This increase is accompanied by a small but obvious temperature hysteresis, indicating that a first-order phase transition occurs at Tt.…”

“…The crystal structure of the insulating phase was reported to have orthorhombic Pnma space group 11 ; however, this space group was suggested to be incorrect via electronic structure calculations 12 . Recent photoemission experiments of thin films suggested that the valence of W in the insulating phase disproportionates into 5+ and 6+ 13 .…”

Perfect flat band with chirality and charge ordering out of strong spin-orbit interaction

“…Before the recent discovery of trimerized charge ordering that keeps the Anderson condition 31 , CsW 2 O 6 was considered to undergo a Peierls-type of metal-insulator transition 46 . This was partially because the DFT calculation showed a large enhancement of the density of states near the Fermi level 47 , which was ascribed to the electronically driven structural-metal-insulator transition to a zig-zag-like one-dimensional structure.…”