Transparent conductors as solar energy materials: A panoramic review

Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.

Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging

We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

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Electrodynamics of correlated electron materials

The complex interplay among several active degrees of freedom (charge, lattice, orbital, and spin) is thought to determine the electronic properties of many oxides. We report on combined ultrafast electron diffraction and infrared transmissivity experiments in which we directly monitored and separated the lattice and charge density reorganizations that are associated with the optically induced semiconductor-metal transition in vanadium dioxide (VO2). By photoexciting the monoclinic semiconducting phase, we were able to induce a transition to a metastable state that retained the periodic lattice distortion characteristic of the semiconductor but also acquired metal-like mid-infrared optical properties. Our results demonstrate that ultrafast electron diffraction is capable of following details of both lattice and electronic structural dynamics on the ultrafast time scale.

A photoinduced metal-like phase of monoclinic VO2 revealed by ultrafast electron diffraction

Complex systems in condensed phases involve a multidimensional energy landscape, and knowledge of transitional structures and separation of time scales for atomic movements is critical to understanding their dynamical behavior. Here, we report, using four-dimensional (4D) femtosecond electron diffraction, the visualization of transitional structures from the initial monoclinic to the final tetragonal phase in crystalline vanadium dioxide; the change was initiated by a near-infrared excitation. By revealing the spatiotemporal behavior from all observed Bragg diffractions in 3D, the femtosecond primary vanadium–vanadium bond dilation, the displacements of atoms in picoseconds, and the sound wave shear motion on hundreds of picoseconds were resolved, elucidating the nature of the structural pathways and the nonconcerted mechanism of the transformation.

https://science.sciencemag.org/content/sci/318/5851/788.full.pdf

4D visualization of transitional structures in phase transformations by electron diffraction.

We found direct experimental evidence for an orbital switching in the V $3d$ states across the metal-insulator transition in ${\mathrm{VO}}_{2}$. We have used soft-x-ray absorption spectroscopy at the V ${L}_{2,3}$ edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

/pdf/orbital-assisted-metal-insulator-transition-in-vo2-570ehyz0zy.pdf

Orbital-Assisted Metal-Insulator Transition in VO2

We present a detailed study of the valence and conduction bands of VO2 across the metal-insulator transition using bulk-sensitive photoelectron and O K x-ray absorption spectroscopies. We observe a giant transfer of spectral weight with distinct features that require an explanation which goes beyond the Peierls transition model as well as the standard single-band Hubbard model. Analysis of the symmetry and energies of the bands reveals the decisive role of the V 3d orbital degrees of freedom. Comparison to recent realistic many body calculations shows that much of the k dependence of the self-energy correction can be cast within a dimer model.

Transfer of spectral weight and symmetry across the metal-insulator transition in VO2

Orbital-assisted metal-insulator transition in VO$_{2}$

We present resonant inelastic x-ray scattering (RIXS) results on insulating monoclinic $\mathrm{V}{\mathrm{O}}_{2}$ with excitation across the ${L}_{2,3}$ edges of vanadium. The spectra measured near threshold in the ${L}_{3}$ region and in the ${L}_{2}$ region show a Raman behavior with a clear peak around $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ transferred energy. The analysis of the data allows the neutral excitation spectrum of the system up to about $2--2.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to be determined. These excitations are due to $dd$ transitions, whose main contributions are discussed. The possibility of ${{d}^{1};{d}^{1}\ensuremath{\rightarrow}{d}^{2};{d}^{0}}$ charge fluctuations within a dimer in RIXS is also discussed. The results are compared with available information on optical constants and on electron energy loss spectroscopy. Due to the low symmetry of the system, there is a similarity between the Raman spectrum obtained with RIXS and the optical absorption spectrum.

W. Reichelt

Papers

Orbital-Assisted Metal-Insulator Transition in VO2

Transfer of spectral weight and symmetry across the metal-insulator transition in VO2

Orbital-assisted metal-insulator transition in VO$_{2}$

Neutral 3d excitations in insulating VO2 as seen with resonant inelastic x-ray scattering at the VL3,2 edges