Nano phase separation in cuprates and nickelates

• Applicant: Dr. Alexander Komarek
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 320571839

Recently, we developed a novel charge and magnetic nano phase separation model for cobalt oxide materials which is able to explain the hour-glass magnetic excitation spectra in these materials.These famous hour-glass shaped magnetic spectra are important for the high-temperature superconducting cuprates in which they have been systematically observed by means of inelastic neutron scattering experiments. We think that nano phase separation also plays an important role for the excitation spectra in these cuprates and also for their physical properties in general. Indeed, first evidence of strucural nano phase separation was observed in cuprates with oxygen excess by the group of Prof. Bianconi using hard X-rays at the Cu K-edge. In these oxygen excess cuprates oxygen ordering reflections and also superstructure reflections arising from local lattice distortions that were interpreted as signatures of charge stripe order were studied. Finally, it was reported that structural nano phase separation appears and that charge stripe order and oxygen ordering are anticorrelated in these systems. But, especially no connection between nano phase separation in the charge sector and magnetic excitation spectra was made.Here, we propose to study the prototypical cuprate materials LBCO, LSCO and LNSCO. We are interested in the observation of nano phase separation regarding the charge stripe ordering reflections directly. For these purposes we propose to perform microdiffraction measurements at the Cu L-edge which will allow us to get detailed information about the microscopic charge distribution within the copper oxygen planes. We are especially interested in the study of any changes of nano phase separation at the superconducting transition temperature in order to discover the interplay of charge stripe order, nano phase separation and superconductivity.Moreover, the isostructural nickelates LSNO are an interesting reference system with well-known charge stripe order. Due to the robustness of the charge order in LSNO (i.e. due to stronger superstructure reflections), we will be able to perform microdiffraction measurements at the Ni K-edge for studying the microstructure of the charge stripes. For all studied samples the magnetic excitation spectra will be measured by means of inelastic neutron scattering and complementary spin wave simulations will be done in order to study the impact of nano phase separation on the magnetic excitation spectra - similar as we have done in our studies on hour-glass spectra in cobaltates.From all these findings we expect to obtain a deeper insight in the physical properties and high-temperature superconductivity in cuprates.

• DFG Programme: Research Grants