Direct observation of the M2 phase with its Mott transition in a VO2 film

Abstract

In VO 2 , the explicit origin of the insulator-to-metal transition is still disputable between Peierls and Mott insulators. Along with the controversy, its second monoclinic (M2) phase has received considerable attention due to the presence of electron correlation in undimerized vanadium ions. However, the origin of the M2 phase is still obscure. Here, we study a granular VO 2 film using conductive atomic force microscopy and Raman scattering. Upon the structural transition from monoclinic to rutile, we observe directly an intermediate state showing the coexistence of monoclinic M1 and M2 phases. The conductivity near the grain boundary in this regime is six times larger than that of the grain core, producing a donut-like landscape. Our results reveal an intra-grain percolation process, indicating that VO 2 with the M2 phase is a Mott insulator.Among the Magnéli phases of vanadium oxides, VO 2 shows an insulator-to-metal transition (IMT) at 340 K, which is remarkably close to room temperature 1 . On top of it, the tunability of the transition involves various aspects of defects 2-4 , gate voltage 5 , and photon excitation 6 . In addition, its sensitivity to lattice modifications has triggered several investigations such as doping 7 , strain 8-12 , and pressure 13 . Owing to the applicability and flexibility of the transition, VO 2 has been widely examined to uncover its transition nature.From the electronic point of view, a Mott insulator due to on-site Coulomb interaction experiences an IMT, when the Hubbard gap is reduced and is overcome by the attractive electron-hole pair energy 14,15 . At the crossover, a large number of free charge carriers form discontinuously, meeting Mott's criteria of the critical carrier density 5,14-16 . On the other hand, the IMT in VO 2 is accompanied by a structural phase transition (SPT) from a monoclinic (M1) to a rutile (R) phase. The driving force initiating the IMT remains unclear since both occur concomitantly. Therefore, two mechanisms are competitively discussed at present: (1) a Peierls instability, i.e., a dimerization of vanadium atoms, and (2) a Mott IMT. The former is supported by band-structure calculations, considering a homopolar V-V bond to split d orbitals 17 , and by DFT-LDA calculations 18 . It is further corroborated by a simultaneous observation of both SPT and IMT within the internal thermalization time of a pump-probe method 19 and the measurement of a delayed structural response time upon a pulsed excitation 20 . The latter mechanism, a) Corresponding author: jeehoon@postech.ac.kr which induces a gap by repulsive electron-electron correlation, is mainly supported by experimental evidence. In the context of a Mott IMT, examples include a diverging effective mass at the crossover 21 , an estimation of the temperature difference between SPT and IMT 22 , a metallic monoclinic phase 6,22-25 , and an IMT triggered by electron injection 5,26 . The conflicting results also have implications on the cooperativity between the two mechanisms, but the conclusive origin ...

Mott transition in chain structure of strained VO2 films revealed by coherent phonons

“…The VO 2 below T IMT has the monoclinic structure (M1) and the second monoclinic structure (M2) with two kinds of substructures of V-atoms, which are a CDW substructure with lattice distortion and an antiferromagnetic equally spaced insulator-chain (IC) one. The M2 phase, as an intermediate between M1 and R, was discovered in Cr-doped 21 , 22 and strained 23 – 26 VO 2 samples. It enables the Mott IMT scenario implying transformation from the IC to a metallic chain (MC), which occurs by excitation of charges in the indirect impurity band formed by such as oxygen vacancies leading to the chain collapse of the main band 27 .…”

Impact of ion irradiation, elemental doping and coating cycles on structural characteristic parameters of nanocrystalline VO 2 thin films

“…This explanation moves away from both Mott and Peierls transitions in competition and, therefore, may be dubbed to some extent as Mott–Peierls transition [26–30]. Apart from R and M1 structural phases, the existence of triclinic (T) and monoclinic (M 2 ) structural phases further exacerbate the dilemma [14, 31–41].…”

“…This explanation moves away from both Mott and Peierls transitions in competition and, therefore, may be dubbed to some extent as Mott-Peierls transition [26][27][28][29][30]. Apart from R and M1 structural phases, the existence of triclinic (T) and monoclinic (M 2 ) structural phases further exacerbate the dilemma [14,[31][32][33][34][35][36][37][38][39][40][41]. In this Letter, nanocrystalline VO 2 thin films grown by sol-gel process and spin-coated on alumina/glass substrates were characterised by X-ray diffractometry for studying the structural characteristic parameters (SCP) of the thin films.…”

Persistent M2 phase in strongly strained (011)-oriented grains in VO2 films grown on sapphire (001) in reactive sputtering

“…19 However, the appearance of an intermediate phase in the SPT pathway between the monoclinic M1 phase and the tetragonal R phase, especially, in VO 2 thin films, remains unresolved. [20][21][22][23][24][25] Conditions for the appearance of the intermediate M2 phase with a monoclinic symmetry have been intensively investigated by many authors by using VO 2 singlecrystalline nanorods (usually called nanobeams). [26][27][28][29][30][31] Phase diagrams as functions of temperature and stress along the c R -axis have been displayed for the SPT of single-crystalline VO 2 .…”