Two Routes from Mott Insulators to Metals: Dynamics of Correlated Charge Carriers

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The project aims at the understanding of the metallic quantum fluctuations in the vicinity of the first-order Mott insulator-to-metal transition. For a fully frustrated model, theoretical calculations yield a coexistence region of metallic and insulating solutions; only very recently we could reveal the first indications of these metallic fluctuations. In the insulating state of quantum spin liquids, the dynamical conductivity becomes significantly enhanced as the temperature is reduced T = 0 K. In this project we suggest to investigate quantum spin liquids as the primary model system with no magnetic order observed down to lowest temperatures. These Mott insulators can be tuned across the phase boundary from the insulator to the metal by varying the effective correlations U/W via increase of the bandwidth W. Here we want to utilize two routes: chemical substitution and hydrostatic pressure.• We want to study the temperature dependence of the (semi-)metallic properties. We want to characterize the metallic fluctuations and see a crossover from quantum fluctuations to thermally driven fluctuations as temperature increases. We have to separate classical percolation effects from quantum-mechanical correlation effects in this coexistence regime.• We want to study the dynamics of these metallic fluctuations by looking at the frequency-dependent conductivity. We search for power laws and their temperature and correlation dependence, as well as for scaling laws.• We want to measure the enhancement of the effective mass in the Fermi liquid regime as the metal-to-insulator transition is approached and compare it with Brinkman-Rice picture.• The bandwidth tuning across the Mott transition will be done by chemical substitution and physical pressure. This implies targeted synthesis and crystal growth on the one hand, pressure-dependent transport and optical experiments on the other hand. • The focus will be on organic charge-transfer salts which constitute quantum spin liquids due to strong frustration on a triangular lattice; supplemented by inorganic systems on kagome and honeycomb lattices.