Ellipsometric investigations of metal-insulator transitions

• Applicants: Professor Dr. Martin Dressel; Dr. Bruno Gompf
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 444655392

This project brings together two previously distinct areas: We employ the novel experimental methods of spectroscopic ellipsometry and Müller matrix metrology to investigate the correlation-driven phase transition in charge-ordered and genuine Mott insulators. Charge-order transitions are supposed to be of second order, but strong involvement of the underlying lattice challenge this description. For Mott insulators a first-order phase transition is proposed when the electronic interactions decrease, inferring a range in which spatially separated regions of metallic and insulating states coexist. Hence, the electrodynamics across the transition resembles percolation. Here we utilize a novel low-temperature spectroscopic ellipsometric technique, which we developed in the field of percolating gold particles at surfaces. The Müller matrix contains the entire optical information accessible by a macroscopic measurement, including spatial fluctuations hidden in the depolarization. Comparing decomposed measured and simulated Müller matrices gives access to these fluctuations. Müller matrix metrology is an advanced method to identify the shape of the metallic regions and their statistical size distribution from the depolarization of the scattered light. We apply Müller matrix spectroscopy to gain information on the microscopic nature of the coexisting phases and their hysteresis. The aim of the project is to elucidate and understand the coexistence regime at the metal-insulator transition in strongly correlated electron systems. The selected materials for this study range from vanadium oxides to organic charge-transfer salts.

• DFG Programme: Research Grants