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Abstract:
The development of new phases of matter at oxide interfaces and surfaces
by extrinsic electric fields is of considerable significance both
scientifically and technologically. Vanadium dioxide (VO2), a strongly
correlated material, exhibits a temperature-driven metal-to-insulator
transition, which is accompanied by a structural transformation from
rutile (high-temperature metallic phase) to monoclinic (low-temperature
insulator phase). Recently, it was discovered that a low-temperature
conducting state emerges in VO2 thin films upon gating with a liquid
electrolyte. Using photoemission spectroscopy measurements of the core
and valence band states of electrolyte-gated VO2 thin films, we show
that electronic features in the gate-induced conducting phase are
distinct from those of the temperature-induced rutile metallic phase.
Moreover, polarization-dependent measurements reveal that the V 3d
orbital ordering, which is characteristic of the monoclinic insulating
phase, is partially preserved in the gate-induced metallic phase,
whereas the thermally induced metallic phase displays no such orbital
ordering. Angle-dependent measurements show that the electronic
structure of the gate-induced metallic phase persists to a depth of at
least similar to 40 angstrom, the escape depth of the high-energy
photoexcited electrons used here. The distinct electronic structures of
the gate-induced and thermally induced metallic phases in VO2 thin films
reflect the distinct mechanisms by which these states originate. The
electronic characteristics of the gate-induced metallic state are
consistent with the formation of oxygen vacancies from electrolyte
gating.
