Exploring the transport properties of quantum systems on different lenght scales

Abstract

In this thesis, after introducing some theoretical aspects of Mott insulators, pure single crystals of the Mott insulator Ca2RuO4 was investigated from an experimental point of view. The surprisingly low current density required to induce the insulator to metal transition has made this material an attractive candidate for developing Mott based electronics devices. However, the mechanism driving the resistive switching remains a controversial topic in the field of strongly correlated electron systems. Here, electrical transport measurements combined with X-ray diffraction have been performed to refine the crystallographic structural phases. A non-equilibrium state which is the precursor of the fully metallic phase was found and adequately characterised. Furthermore, electric transport measurements of the eutectic system constituted by Ca2RuO4 with embedded metallic Ru inclusions have been performed. In this system, the same properties of the pure Ca2RuO4 were found, although the Ru inclusions make it structurally stiffer. Moreover, using the sample size as a principal tuning parameter, an uncovered region of the phase space of pure Ca2RuO4 single crystal has been probed. To decrease the dimensions of the crystals down to the microscopic scale, an innovative technique that involves a Ga+ focused ion beam has been employed. Upon reducing the crystal size, an increase of four orders of magnitude in the current density is required for driving Ca2RuO4 out of the insulating state into the non-equilibrium phase. This effect was observed for the very first time. By fabricating a microscopic platinum thermometer directly on top of selected samples and performing thermal simulations, it was gained insight into the local temperature during simultaneous application of current, and it was established that the observed “size effect” is not a result of Joule heating. The findings suggest an inhomogeneous current distribution in the nominally homogeneous crystal. This study paves the way to a new approach to investigate the physical properties of Mott insulators and calls for a re-examination of the interplay between sample size, charge current, and temperature in driving Ca2RuO4 towards the insulator to metal transition. [edited by Author]