Nanoscale Phase Separation and Lattice Complexity in VO2: The Metal–Insulator Transition Investigated by XANES via Auger Electron Yield at the Vanadium L23-Edge and Resonant Photoemission

Augusto Marcelli,Antonio Bianconi,Alessandro Ricci,Chongwen Zou,Wei Xu,Marcello Coreno,Matus Stredansky,Alessandro D’Elia,Wangsheng Chu,Albano Cossaro,Shiqiang Wei,Lele Fan

doi:10.3390/condmat2040038

Abstract

Among transition metal oxides, VO2 is a particularly interesting and challenging correlated electron material where an insulator to metal transition (MIT) occurs near room temperature. Here we investigate a 16 nm thick strained vanadium dioxide film, trying to clarify the dynamic behavior of the insulator/metal transition. We measured (resonant) photoemission below and above the MIT transition temperature, focusing on heating and cooling effects at the vanadium L23-edge using X-ray Absorption Near-Edge Structure (XANES). The vanadium L23-edges probe the transitions from the 2p core level to final unoccupied states with 3d orbital symmetry above the Fermi level. The dynamics of the 3d unoccupied states both at the L3- and at the L2-edge are in agreement with the hysteretic behavior of this thin film. In the first stage of the cooling, the 3d unoccupied states do not change while the transition in the insulating phase appears below 60 °C. Finally, Resonant Photoemission Spectra (ResPES) point out a shift of the Fermi level of ~0.75 eV, which can be correlated to the dynamics of the 3d// orbitals, the electron–electron correlation, and the stability of the metallic state.

Highlights

Transition metal (TM) oxides offer a wide spectrum of phase-separated systems with characteristic anomalies in different properties such as the electrical resistivity and/or the optical transmission
With temperature-dependent synchrotron radiation high-resolution X-ray diffraction data and Raman spectroscopy, the authors suggested that the structural phase transition in the temperature range near the metal transition (MIT) is suppressed by epitaxial strain, and the electronic transition triggers the MIT in strained films [9]
Many studies report the growth of high quality VO2 films on TiO2 substrates, and the phase transition temperature depends on the involved interfacial strain/stress

Summary

Introduction

Transition metal (TM) oxides offer a wide spectrum of phase-separated systems with characteristic anomalies in different properties such as the electrical resistivity and/or the optical transmission. These phenomena originate from electronic correlation and interactions involving spin, lattice, and charge degrees of freedom. Separated regions with distinct structural, magnetic, and electronic properties occur in these systems, which can be described as a multiscale phase separation between two (or more) phases that have a comparable free energy This heterogeneity of the material may extend from the atomic scale to the mesoscale domain, indicating arrested phase separations, typical of TM oxides where complex textures emerge from the coexisting phases. With temperature-dependent synchrotron radiation high-resolution X-ray diffraction data and Raman spectroscopy, the authors suggested that the structural phase transition in the temperature range near the MIT is suppressed by epitaxial strain, and the electronic transition triggers the MIT in strained films [9]

Methods

Results

Conclusion