Crystal structure change accompanying insulator-metal phase transition in VO$_{2}$ field-effect transistor

The insulator-metal transition induced by the carrier accumulation in VO$_{2}$ field-effect transistor (FET) gated by electric double layers of ionic liquid has been extensively studied. To clarify the origin of this transition, we performed simultaneous measurements of in-situ synchrotron x-ray diffraction and resistivity on VO$_{2}$ FET at BL19LXU, SPring-8, Japan. By using micro-beam x-ray, the diffraction only on the carrier-accumulated channel of VO$_{2}$ FET can be measured. By applying a gate voltage, the VO$_{2}$ film becomes metallic. The $c$-lattice length estimated from the peak position of (0 0 2) diffraction on the channel of VO$_{2}$ film shows an increase of 1.4{\%} at 150 K. The $c$-lattice length in the metallic state hardly depends on the temperature, which is consistent with the temperature-independent-metallic resistivity. The changes of $c$-lattice length and resistivity by a gate voltage are reversible. This structural change is quite different with those of thermally-, x-ray-, and pressure-induced metallic phases. The crystal structure with elongated $c$-lattice length is realized only in the metallic state induced by the carrier accumulation.