Non-equilibrium Phases in Jahn-Teller-distorted Perovskite Manganites
Dissertation
Datum der mündl. Prüfung:2023-09-25
Erschienen:2023-12-04
Betreuer:Prof. Dr. Vasily Moshnyaga
Gutachter:Prof. Dr. Stefan Mathias
Gutachter:Prof. Dr. Michael Seibt
Dateien
Name:K-Stroh_2023_PhD-thesis.pdf
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Zusammenfassung
Englisch
Optical control of condensed matter, being inherently fast and energy efficient, sparks hope for potential technical applications. However, these require a fundamental understanding of the physical mechanisms behind. In order to access timescales of femto- to nanoseconds relevant to the ultrafast dynamics of photoinduced phase transitions, pump-probe spectroscopy is an ideal tool. Further, strong correlations between electron, spin, and lattice degrees of freedom make materials like complex oxides promising candidates for such studies. The present thesis investigates the formation of photo-induced phases in two groups of mixed valence manganites, in lightly self-doped or A-site Sr-substituted LaMnO3 (LMO or LSMO) as well as in optimally doped (La{1-y}Pr{y}){1-x}Ca{x}MnO3 (LPCMO) thin films. After laser excitation, long-living non-thermal dynamics emerge in either of these perovskites on sub-picosecond timescales. Insulating La0.9Sr0.1MnO3 and the similar compound La0.9MnO{3+d}, on the one hand, exhibit transient ferromagnetism below and also above their Curie temperature. In (La0.6Pr0.4)0.7Ca0.3MnO3, on the other hand, a transient phase of increased conductivity is found in the polaronic regime below the charge-ordering temperature. Both of these phenomena can be interpreted as a collapse of Jahn-Teller distortions inherent to the oxygen octahedra around Mn3+ ions and their temporal conversion into Mn4+ like ions in a process that may be called photo-doping. This comes along with changes of the phase-separated domain structure. Partly differing observations in the LMO-like and LPCMO compounds are discussed, for example, in terms of doping level, the configuration of Jahn-Teller distortions, and the scale of phase separation. Whereas the possibility for optical control seems to rely on the proximity to an equilibrium phase boundary in case of LMO, it originates from the polaronic state underlying colossal magnetoresistance in LPCMO.
Keywords: correlated oxides; perovskites; manganites; ultrafast dynamics; photo-doping; transient phases; photo-induced phase transitions