The behavior of electrons in materials induces two generally distinct states, either metallic or insulating depending on whether they are conducting an electrical current. Some materials, however, can change from one state to the other under the influence of external parameters (temperature, chemical doping, pressure, etc.) Understanding this metal-insulator transition (MIT) is essential to mastering the electronic properties of materials, with major technological implications in the search for faster and more efficient devices. This phenomenon remains, however, both poorly understood and highly complex in nature: the electrons have an intermediate behavior between metal and insulator that is poorly described by theory and difficult to characterize experimentally.
The most advanced spectroscopies using synchrotron radiation can provide answers to these open questions. As part of an international collaboration, researchers of CRISTAL, GALAXIES and PSICHE at SOLEIL have studied the metal-insulator transition in Cr doped V2O3, a model material of strongly correlated electron systems, using different techniques, from infrared to X-rays. The results of X-ray spectromicroscopy at ELETTRA reveal a coexistence of insulating and metallic phases at the microscopic level during temperature-induced MIT (blue and red regions in Figure 1). Surprisingly, the system remembers the pattern formed by the phase mixture, after a complete MIT cycle, probably due to Cr impurities that act as nucleation centers. During pressure-induced MIT, the coexistence of phases observed by X-ray diffraction (Figure 2) on the CRISTAL beamline at SOLEIL disappears at high pressure, leaving a more homogeneous metallic phase. More generally, this study highlights the complexity underlying the phase diagrams of correlated materials.
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| Figure 1 : Coexistence of metallic (red) and insulating (blue) phases in V2O3 observed by spectromicroscopy. The image covers a zone of 50 x 50 µm2. | Figure 2 : Metal insulator transition observed by X-ray diffraction in V2O3. While both the insulator phase (PI, in blue) and the metallic phase (PM, in red) are seen at low pressure, only the metallic phase persists at high pressure. |
Reference:
Lupi, S., Baldassarre, L., Mansart, B., Perucchi, A., Barinov, A., Dudin, P., Papalazarou, E., Rodolakis, F., Rueff, J. P., Itié, J. P., Ravy, S., Nicoletti, D., Postorino, P., Hansmann, P., Parragh, N., Toschi, A., Saha-Dasgupta, T., Andersen, O. K., Sangiovanni, G., Held, K., & Marsi, M.
A microscopic view on the Mott transition in chromium-doped V2O3.
Nature Communications, 2010, 1: art.n° 105
