IVC17 / ICSS13 and ICN+T 2007

Temperature dependent resonant soft X-ray emission study of the metal-insulator transition in VO₂
Schmitt, Thorsten¹; Duda, Laurent²; Mattesini, Maurizio³; Augustsson, Andreas²; Dong, C.L.⁴; Guo, Jinghua⁴; Klemm, Matthias⁵; Eyert, Volker⁵; Horn, Siegfried⁵; Ahuja, Rajeev²; Nordgren, Joseph² ¹Switzerland; ²Sweden; ³Spain; ⁴United States; ⁵Germany

Vanadium oxides possess interesting technological applications, for instance in catalysis, optoelectronics and as smart thermochromic windows. In particular, most vanadium oxides display metal-to-insulator transitions (MIT) as a function of the temperature, which are closely linked to their potential value in applications. Vanadium dioxide, VO₂, undergoes a MIT manifested in a sudden conductivity drop of several orders of magnitude from a paramagnetic metal with tetragonal rutile structure to a nonmagnetic semiconductor with monoclinic structure at T=340K. Despite decades of studies the nature of the MIT between these phases in VO₂ is not fully understood. Particularly, the role of electron correlation effects is still under debate.

In this work, we present a combined experimental and theoretical investigation of the electronic structure changes between the semiconducting and metallic phase of single crystalline VO₂. Resonant soft x-ray emission (RSXE) spectroscopy at V 2p and O 1s resonances is employed to study changes in the electronic structure of the occupied valence bands in VO₂. Soft X-ray absorption was used to probe conduction band states and to determine specific excitation energies for the RSXE investigation. We find that the MIT of VO₂ gives rise to dramatic spectral changes in the bulk sensitive RSXE spectra. The RSXE spectra, reflecting the atomic-site specific occupied partial-density of states, show that the metallic phase exhibits both bandwidth broadening of the O 2p - V 3d hybridized states as well as a shift in spectral weight to V 3d-like states. Large changes in the spectral weight of the O 2p – V 3d hybridized states highlight a Peierls-like character of the MIT in VO₂.

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