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Register a new userPhotoemission evidence for a Mott-Hubbard metal-insulator transition in VO$_2$
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The temperature ($T$) dependent metal-insulator transition (MIT) in VO$_2$ is investigated using bulk sensitive hard x-ray ($sim$ 8 keV) valence band, core level, and V 2$p-3d$ resonant photoemission spectroscopy (PES). The valence band and core level spectra are compared with full-multiplet cluster model calculations including a coherent screening channel. Across the MIT, V 3$d$ spectral weight transfer from the coherent ($d^1underbar{it {C}}$ final) states at Fermi level to the incoherent ($d^{0}+d^1underbar{it {L}}$ final) states, corresponding to the lower Hubbard band, lead to gap-formation. The spectral shape changes in V 1$s$ and V 2$p$ core levels as well as the valence band are nicely reproduced from a cluster model calculations, providing electronic structure parameters. Resonant-PES finds that the $d^1underbar{it{L}}$ states resonate across the V 2$p-3d$ threshold in addition to the $d^{0}$ and $d^1underbar{it {C}}$ states. The results support a Mott-Hubbard transition picture for the first order MIT in VO$_2$.
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Here we present experimental and computational evidences to support that rock-salt cubic VO is a strongly correlated metal with Non-Fermi-Liquid thermodynamics and an unusually strong spin-lattice coupling. An unexpected change of sign of metallic thermopower with composition is tentatively ascribed to the presence of a pseudogap in the density of states. These properties are discussed as signatures of the proximity to a magnetic quantum phase transition. The results are summarized in a new electronic phase diagram for the 3d monoxides, which resembles that of other strongly correlated systems. The structural and electronic simplicity of 3d monoxides make them ideal candidates to progress in the understanding of highly correlated electron systems.
We present a spectroscopic study that reveals that the metal-insulator transition of strained VO$_2$ thin films may be driven towards a purely electronic transition, which does not rely on the Peierls dimerization, by the application of mechanical strain. Comparison with a moderately strained system, which does involve the lattice, demonstrates the crossover from Peierls- to Mott-like transitions.
The insulator-to-metal transition (IMT) of the simple binary compound of vanadium dioxide VO$_2$ at $sim 340$ K has been puzzling since its discovery more than five decades ago. A wide variety of photon and electron probes have been applied in search of a satisfactory microscopic mechanistic explanation. However, many of the conclusions drawn have implicitly assumed a {em homogeneous} material response. Here, we reveal inherently {em inhomogeneous} behavior in the study of the dynamics of individual VO$_2$ micro-crystals using a combination of femtosecond pump-probe microscopy with nano-IR imaging. The time scales of the photoinduced bandgap reorganization in the ultrafast IMT vary from $simeq 40 pm 8$ fs, i.e., shorter than a suggested phonon bottleneck, to $sim 200pm20$ fs, with an average value of $80 pm 25$ fs, similar to results from previous studies on polycrystalline thin films. The variation is uncorrelated with crystal size, orientation, transition temperature, and initial insulating phase. This together with details of the nano-domain behavior during the thermally-induced IMT suggests a significant sensitivity to local variations in, e.g., doping, defects, and strain of the microcrystals. The combination of results points to an electronic mechanism dominating the photoinduced IMT in VO$_2$, but also highlights the difficulty of deducing mechanistic information where the intrinsic response in correlated matter may not yet have been reached.
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