Role of the charge correlations in the mechanism of high temperature superconductivity

Abstract

High-temperature superconductivity in cuprates is one of the most important conceptual problems in contemporary condensed matter physics. However, the mechanism of superconductivity, that is presumably universal in all cuprate compounds, has yet to be understood. In this thesis, an atypical approach of studying it is proposed. Instead of focusing on the factors that cause the increase of the critical temperature, we were attracted by the phenomena that decrease it. In particular, the main interest was in studying the symmetry breaking fields that affect the main building block of the cuprates - the CuO2 plane. This symmetry breaking can be achieved by uniaxial pressure, or it emerges naturally in the form of the charge density wave (CDW) order that coexists, yet competes with superconductivity. Therefore, the general aim of this Thesis is to first thoroughly characterize the CDW order, and then describe its interplay with superconductivity. In order to achieve these goals, selected experiments using synchrotron radiation were performed. The standard synchrotron techniques were furthermore extended by the application of the in-situ uniaxial pressure.Altogether, the experimental results provide evidence that the short-range CDW order is a universal phenomenon in cuprates, which exists in a limited range of temperatures and doping, and, although very weak, has a detrimental influence on superconductivity. The local distortion of the lattice caused by uniaxial pressure application does not influence the strength of these charge correlations. Moreover, CDW does not significantly impact the lattice dynamics observed through the optical phonons. However, under magnetic field and at sufficiently low temperatures, it reconstructs the Fermi surface into a small electron pocket