Mott transition revealed by coherent phonons in a film of VO₂/AlN/Si

Vanadium dioxide VO₂ with a metallic electronic structure of 3d¹ is an insulator below T_c=340 K and undergoes the insulator-metal transition(IMT) and the structural phase transition(SPT) near T_c. It has two substructures of zigzag chain and the charge density wave (CDW) with repeatedly a long and a short distance between two atoms, induced by the structural distortion ; this is called M2 phase. The CDW undergoes the Peierls IMT and is accompanied with the SPT, while in the zigzag chain structure, the Mott transition caused by melting of electron correlation occurs. Therefore, it is so difficult to discover the Mott transition. We had successfully deposited the strained VO₂/AlN/Si films with a large difference between the IMT and the SPT[1] and observed the monoclinic metal phase without the SPT in the M2 phase through analyzing of the temperature dependence of the coherent phonons measured by a high speed femtosecond system [2]. We explain the monoclinic metal phase using the transport data of Hall effect and the impurity-driven IMT[3]. [1] APL Materials 4, 026101; [2] Scientific report 7, 16038; [3] J. Phys.: Condens. Matter. 28, 085602.