Tuning Electronic Correlations in an Organic Mott Insulator via Uniaxial Strain

Based on their electrical conductivity, most materials can be divided into metals, insulators, or semiconductors, each of them having a particular temperature dependence of their resistivity. Even though the Coulomb interaction between conduction electrons is generally not small, it can be neglected in many cases according to Landau’s Fermi-liquid theory. In some material classes, the change of an external tuning parameter like temperature, pressure or magnetic field can lead to the sudden localization of formerly quasi-free electrons, causing a transition from metallic to insulating behavior. The electron-electron interaction can thus be influenced and must be taken into account, which leads to a complex many-body problem that is difficult to solve.In this thesis, a novel non-thermal method to fine-tune electronic correlations in a quasi-twodimensional organic conductor and its use in exploring the Mott metal-insulator transition (MIT) is described. The correlation strength of the charge transfer salt κ-[BEDTTTF]2Cu[N(CN)2]Br, which represents the single-band Hubbard model in its purest form, was tuned via a piezoelectric based strain cell while performing DC-transport measurements. This allowed a comprehensive and precise reconstruction of the rich phase diagram surrounding the MIT and opens a new possibility to study anisotropic materials by uniaxial pressure tuning. Apart from studying the direct influence of uniaxial strain on the correlation strength and achieving a fully bandwidth-controlled MIT, measurements in magnetic fields have been performed to examine the field dependence of the superconducting transition temperature.Low-temperature Fermi-liquid behavior, another property of many strongly correlated electron systems, has also been observed and was studied regarding its dependency on the applied strain.