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Register a new userElectron Correlation driven Metal-Insulator transition in Strained and Disordered VO2 films
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The Metal-Insulator transition (MIT) in VO2 is characterized by the complex interplay among lattice, electronic and orbital degrees of freedom. In this contribution we investigated the strain-modulation of the orbital hierarchy and the influence over macroscopic properties of the metallic phase of VO2 such as Fermi Level (FL) population and metallicity, i.e., the material ability to screen an electric field, by means of temperature-dependent X-ray Absorption Near Edge Structure (XANES) and Resonant Photoemission spectroscopy (ResPES). We demonstrate that the MIT in strained VO2 is of the Filling Control type, hence it is generated by electron correlation effects. In addition, we show that the MIT in Nanostructured (NS) disordered VO2, where the structural phase transition is quenched, is driven by electron correlation. Therefore a fine tuning of the correlation could lead to a precise control and tuning of the transition features.
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In this paper we used Raman spectroscopy to investigate the optical properties of vanadium dioxide (VO2) thin films during the thermally induced insulating to metallic phase transition. We observed a significant difference in transition temperature in similar VO2 films grown on quartz and sapphire substrates: the film grown on quartz displayed the phase transition at a lower temperature (Tc=50C) compared a film grown on sapphire (Tc=68C). We also investigated differences in the detected Raman signal for different wavelengths and polarizations of the excitation laser. We found that for either substrate, a longer wavelength (in our case 785 nm) yielded the clearest VO2 Raman spectra, with no polarization dependence.
We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO2. We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO2 films are preceded by the purely-electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K (+/- 0.3 K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically-promising material.
We present experimental data and a theoretical interpretation on the conductance near the metal-insulator transition in thin ferromagnetic Gd films of thickness b approximately 2-10 nm. A large phase relaxation rate caused by scattering of quasiparticles off spin wave excitations renders the dephasing length L_phi < b in the range of sheet resistances considered, so that the effective dimension is d = 3. The observed approximate fractional temperature power law of the conductivity is ascribed to the scaling regime near the transition. The conductivity data as a function of temperature and disorder strength collapse on to two scaling curves for the metallic and insulating regimes. The best fit is obtained for a dynamical exponent z approximately 2.5 and a correlation length critical exponent u approximately 1.4 on the metallic side and a localization length exponent u approximately 0.8 on the insulating side.
The correlated oxide SmNiO3 (SNO) exhibits an insulator to metal transition (MIT) at 130 {deg}C in bulk form. We report on synthesis and electron transport in SNO films deposited on LaAlO3 (LAO) and Si single crystals. X-ray diffraction studies show that compressively strained single-phase SNO grows epitaxially on LAO while on Si, mixed oxide phases are observed. MIT is observed in resistance-temperature measurements in films grown on both substrates, with charge transport in-plane for LAO/SNO films and out-of-plane for Si/SNO films. Electrically-driven memristive behavior is realized in LAO/SNO films, suggesting that SNO may be relevant for neuromorphic devices.
Transport in ultrathin films of LaNiO3 evolves from a metallic to a strongly localized character as the films thickness is reduced and the sheet resistance reaches a value close to h/e2, the quantum of resistance in two dimensions. In the intermediate regime, quantum corrections to the Drude low- temperature conductivity are observed; they are accurately described by weak localization theory. Remarkably, the negative magnetoresistance in this regime is isotropic, which points to magnetic scattering associated with the proximity of the system to either a spin glass state or the charge ordered antiferromagnetic state observed in other rare earth nickelates.
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