Electronic and structural transformations near the insulator-to-metal transition in vanadium dioxide (original) (raw)
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Nanoscale imaging of the electronic and structural transitions in vanadium dioxide
Physical Review B, 2011
We investigate the electronic and structural changes at the nanoscale in vanadium dioxide (VO 2) in the vicinity of its thermally driven phase transition. Both electronic and structural changes exhibit phase coexistence leading to percolation. In addition, we observe a dichotomy between the local electronic and structural transitions. Nanoscale x-ray diffraction reveals local, non-monotonic switching of the lattice structure, a phenomenon that is not seen in the electronic insulator-to-metal transition mapped by near-field infrared microscopy.
Infrared spectroscopy and nano-imaging of the insulator-to-metal transition in vanadium dioxide
Physical Review B, 2009
We present a detailed infrared study of the insulator-to-metal transition ͑IMT͒ in vanadium dioxide ͑VO 2 ͒ thin films. Conventional infrared spectroscopy was employed to investigate the IMT in the far field. Scanning near-field infrared microscopy directly revealed the percolative IMT with increasing temperature. We confirmed that the phase transition is also percolative with cooling across the IMT. We present extensive near-field infrared images of phase coexistence in the IMT regime in VO 2. We find that the coexisting insulating and metallic regions at a fixed temperature are static on the time scale of our measurements. A distinctive approach for analyzing the far-field and near-field infrared data within the Bruggeman effective medium theory was employed to extract the optical constants of the incipient metallic puddles at the onset of the IMT. We found divergent effective carrier mass in the metallic puddles that demonstrates the importance of electronic correlations to the IMT in VO 2. We employ the extended dipole model for a quantitative analysis of the observed near-field infrared amplitude contrast and compare the results with those obtained with the basic dipole model.
Mott transition in vanadium dioxide (VO $ _2 $) observed by infrared spectroscopy and nano-imaging
2008
The driving mechanism for the temperature-induced insulator-to-metal transition (IMT) in vanadium dioxide (VO 2) has been debated for the past five decades. Central to this debate is the relative importance of electron-electron correlations and chargeordering to the IMT. We report near-field infrared images of VO 2 films that directly show the percolative IMT. In combination with far-field infrared spectroscopy, the new data reveal the Mott transition with divergent optical mass in the metallic puddles that emerge at the onset of the IMT.
Inhomogeneous electronic state near the insulator-to-metal transition in the correlated oxide VO2
Physical Review B, 2009
We investigate the percolative insulator-to-metal transition ͑IMT͒ in films of the correlated material vanadium dioxide ͑VO 2 ͒. Scattering-type scanning near-field infrared microscopy and atomic force microscopy were used to explore the relationship between the nucleation of metallic regions and the topography in insulating VO 2 . We demonstrate that the IMT begins within 10 nm from grain boundaries and crevices by using mean curvature and statistical analysis. We also observe coexistence of insulating and metallic domains in a single crystalline grain that points to intrinsic inhomogeneity in VO 2 due to competing electronic phases in the IMT regime.
Condensed Matter
Among transition metal oxides, VO 2 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 L 23-edge using X-ray Absorption Near-Edge Structure (XANES). The vanadium L 23-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 L 3-and at the L 2-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.
The metal-insulator transition of M1 vanadium dioxide
arXiv: Materials Science, 2016
Materials that undergo reversible metal-insulator transitions are obvious candidates for new generations of devices. For such potential to be realised, the underlying microscopic mechanisms of such transitions must be fully determined. In this work we probe the correlation between the energy landscape and electronic structure of the metal-insulator transition of vanadium dioxide and the atomic motions occurring using first principles calculations and high resolution X-ray diffraction. Calculations find an energy barrier between the high and low temperature phases corresponding to contraction followed by expansion of the distances between vanadium atoms on neighbouring sublattices. X-ray diffraction reveals anisotropic strain broadening in the low temperature structure's crystal planes, however only for those with spacings affected by this compression/expansion. GW calculations reveal that traversing this barrier destabilises the bonding/anti-bonding splitting of the low temperature phase. This precise atomic description of the origin of the energy barrier separating the two structures will facilitate more precise control over the transition characteristics for new applications and devices.
Nature of the Metal Insulator Transition in Ultrathin Epitaxial Vanadium Dioxide
Nano Letters, 2013
We have combined hard X-ray photoelectron spectroscopy with angular dependent O K-edge and V Ledge X-ray absorption spectroscopy to study the electronic structure of metallic and insulating end point phases in 4.1 nm thick (14 units cells along the c-axis of VO 2) films on TiO 2 (001) substrates, each displaying an abrupt MIT centered at ∼300 K with width <20 K and a resistance change of ΔR/R > 10 3. The dimensions, quality of the films, and stoichiometry were confirmed by a combination of scanning transmission electron microscopy with electron energy loss spectroscopy, X-ray spectroscopy, and resistivity measurements. The measured end point phases agree with their bulk counterparts. This clearly shows that, apart from the strain induced change in transition temperature, the underlying mechanism of the MIT for technologically relevant dimensions must be the same as the bulk for this orientation.
Insulator-to-metal transition in ultrathin rutile VO2/TiO2(001)
npj Quantum Materials
An insulator-to-metal transition (IMT) is an emergent characteristic of quantum materials. When the IMT occurs in materials with interacting electronic and lattice degrees of freedom, it is often difficult to determine if the energy gap in the insulating state is formed by Mott electron–electron correlation or by Peierls charge-density wave (CDW) ordering. To solve this problem, we investigate a representative material, vanadium dioxide (VO2), which exhibits both strong electron–electron interaction and CDW ordering. For this research, VO2 films of different thicknesses on rutile (001) TiO2 substrates have been fabricated. X-ray diffraction (XRD) data show that ultrathin VO2 films with thickness below 7.5 nm undergo the IMT between rutile insulator below Tc and rutile metal above Tc, while an ultrathin VO2 film with a thickness of 8 nm experiences the structural phase transition from the monoclinic structure below Tc to the rutile structure above Tc. Infrared and optical measurement...
Physical Review B, 2015
We have systematically studied a variety of vanadium dioxide (VO 2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO 2 with sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in VO 2 thin films not only triggers spontaneous local phase separations but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain and symmetry dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches.