Epitaxy, electrical transport and phase transitions in charge ordered Ruddlesden-Popper perovskite oxide layers

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In this project we want to investigate growth mechanisms and structure-property relationships of semi-doped epitaxial Pr-Ca-Mn-O (PCMO) Ruddlesden-Popper (RP) films of order n=1 and n=2. The RP phases of the manganates represent quasi two-dimensional highly correlated electron systems with polaron quasi-particles, which can crystallize to a charge-ordered phase. The charge order can be "melted" by external stimuli and has long-lived electron-hole-pair excitations. This enables fascinating new functions, especially in photovoltaics, sensor technology and microelectronics. The main goal of the project is to gain an understanding of the mechanisms that determine the growth orientation of epitaxial layers. These are not only determined by the mismatch to the substrate, but also by the chemistry of the surface / interface. Furthermore, the defects resulting from stress relaxation will be determined and their effect on the ordering temperature will be investigated. The synthesis as thin films offers the possibility to control the properties of the films by epitaxy, strain and defects in order to influence the functionalities in heterojunctions. The deposition is performed by pulsed laser deposition (PLD). The main focus of the characterization is the investigation of epitaxy and strains (X-ray diffraction, TEM), the growth process (AFM, SEM) and elastic stresses (beam bending method). This is done in combination with temperature dependent electrical transport measurements in magnetic and electric fields. The investigation of defects and charge order is performed by means of high-resolution transmission electron microscopy (HRTEM). Based on the first phase of the project, which is focused on Ruddlesden-Popper-Manganate films of order n=1 with a Tc=320 K, the second phase will be extended to the order n=2 and higher, which have higher transition temperatures of up to 370 K.