title: Cuprates: electrical resistivity under uniaxial pressure
Creators: name: Klebel, Benjamin
Publisher: name: TU Wien
Year: 2019
Subjects: subject: uniaxial pressure
subject: cuprates

description: The discovery of superconductivity in 1911 and the subsequent development of technologies allowing practical applications brought important scientific and technological advancements. In this regard, the ultimate goal is to find a superconductor which is easy to use at room-temperature. High temperature superconductivity (maximum T c 133K) as found in copper oxides (“cuprates”) was a big step in this direction. However, despite great efforts to understand cuprates, no theory which captures all defining characteristics of these high-transitiontemperature (high-T c ) superconductors has found wide acceptance in the scientific community. A deeper understanding of those materials, ultimately leading to a microscopic theory of high temperature superconductivity could substantially quicken technological advances, which would hopefully bring benefit to society. Recent experimental observations sparked a phenomenological model of superconductivity in cuprates. According to this model, one charge carrier being localised per unit cell mediates the pairing interaction required for superconductivity. As the localization of the one carrier emerges in a spatially inhomogeneous manner, and the Fermi surface evolution exhibits strongly anisotropic behaviour, the experimental observations and the suggested model imply an importance of certain details of the crystal structure. In particular, there appears to be a strong coupling between charge and lattice degrees of freedom. Therefore, the process which leads to pairing likely depends on the crystal structure symmetry (and related local distortions). More insight into the response of the electronic system on structural distortions in cuprates is therefore essential for an extension of the phenomenological model to a microscopic description of cuprate superconductivity. In this regard, the work conducted within the scope of this thesis addresses one of the most urgent questions of solid state physics. Cuprates are comprised of a variety of different compounds with complex structural differences. The effect of structure could be studied by a comparison between compounds. However, those variations are not systematic. Therefore, we applied uniaxial pressure to tune the crystal symmetry in a clean and continuous way, independently of other parameters. We have studied Nd 2x Ce x CuO 4 NCCO. The main challenge was the development of a uniaxial pressure cell. Within the framework of this thesis, the author contributed to the development of springand gas-driven cells. In particular the gas-driven cell is a major experimental achievement: It enables in-situ changes of the pressure and a temperature-independent measurement of the applied force. Furthermore, a cryostat was restored and software tools for automatic measurement control were developed, enabling measurements of electrical resistivity over extended ranges of temperature and magnetic field. The spring-driven cell was integrated into this setup, allowing measurements of the electrical resistivity under uniaxial pressure down to liquid helium temperatures. With these setups, we measured the electrical resistivity at room temperature as a function of uniaxial pressure. Furthermore, we measured the electrical resistivity under uniaxial pressure as a function of temperature in the vicinity of the transition temperature T c . The resistivity measured perpendicular to the direction of the compressive force is found to decrease at room temperature upon applying uniaxial pressure. Moreover, T c was observed to decrease with increased uniaxial pressure. The developed experimental technique is expected to enable further results and development of complementary techniques that will help to advance the understanding of cuprate superconductors and possibly other materials, in the future.