Tuning electronic correlations in an organic mott insulator via uniaxial strain

Abstract

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.