Anisotropic Electronic State via Spontaneous Phase Separation in Strained Vanadium Dioxide Films

Abstract

We resolved the enigma of anisotropic electronic transport in strained vanadium dioxide (VO2) films by inquiring into the role that strain plays in the nanoscale phase separation in the vicinity of the insulator-to-metal transition. The root source of the anisotropy was visualized as the formation of a peculiar unidirectional stripe state which accompanies the phase transition. Furthermore, nanoscale infrared spectroscopy unveils distinct facets of electron-lattice interplay at three different stages of the phase transition. These stages include the initial formation of sparse nonpercolating metallic domains without noticeable involvement of the lattice followed by an electron-lattice coupled anisotropic stripe state close to percolation which ultimately evolves into a nearly isotropic rutile metallic phase. Our results provide a unique mesoscopic perspective for the tunable macroscopic phenomena in strained metal oxide films.

Large epitaxial bi-axial strain induces a Mott-like phase transition in VO2

“…Peierls transition. [6] In highly strained epitaxial films (with the thickness much larger than what were reported here), an intriguing unidirectional strip state was observed with mixed metallic and insulating phases via scattering-type scanning nearfield optical microscopy, [9] which serve as strong evidence for the coexistence of rutile insulating and metallic phases.…”

“…Epitaxial films under bi-axial strain also demonstrated a very pronounced anisotropy in optical and transport properties, [7,8] as a result of the formation of unidirectional stripe states in which the semiconducting and metallic states coexisted. [9] In this study, we grew epitaxial VO 2 films with thickness from 5~ 17 nm on various single crystal substrates to obtain very high epitaxial strains. As a result, the maximum T MIT reached in this study is > 430 K, which is higher than previous reports, with 4 orders of magnitude change in the resistivity.…”

Phase transition in bulk single crystals and thin films ofVO2by nanoscale infrared spectroscopy and imaging

“…From the various samples investigated in this and previous studies [14,35,36 (T MIT < 340K perpendicular to c R ). The thicknesses of all the thin films we investigated were greater than 30nm (range from 30nm to 300nm).…”

“…highly oriented domains) of these films. VO 2 films on [110] R and [100] R TiO 2 substrates were grown by reactive-biased target ion beam deposition [21] and have been previously studied and reported [35,36,55,56,63].…”

“…Their electronic and lattice properties also show distinct temperature dependences and anisotropic behaviors [20,26,[33][34][35][36][37].…”