Terahertz spectroscopy of collective excitations in ordered phases of overdoped manganites

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In perovskite oxide manganites R1-xAxMnO3 (R – rare earth, A – alkaline element) the fineinterplay of nearly equal-strength spin, charge, lattice, and orbital interactions leads to a varietyof electronically correlated ground states. It provides the possibility to tune them by moderatevariations of system parameters, such as temperature, composition, particle size, internal strain,magnetic field, etc., which has attracted a lot of attention during past decades. However, theorigin of the ground states and the mechanisms of the correspondent phase transitions are notcompletely resolved. Presently, clarification of physical models of charge, orbital, and magneticorders in the systems is of broader interest in the context of electronically correlated transitionmetal oxides, in particular of the overdoped regime, x > 0.5, a sector of the phase diagram that isless explored compared to the underdoped one, where colossal magnetoresistance is observed.We plan to study the microscopic origin and collective modes of various charge, magneticallyand orbitally ordered phases of the family R1-xAxMnO3 (0.5≤x≤1) by investigating the nano-sizeeffects on magnetic and charge order, and thin-film, A-site cation ordering and strain effects onthe electronic transitions, the energy gaps and the phonon modes using optical spectroscopy withthe focus on low energies, of few millielectronvolts and below, where collective electroniceffects in solids are expected to show up. A set of spectrometers will be employed, includingoptical ellipsometer, infrared Fourier-transform spectrometer and terahertz and microwavesetups for low energies, complemented with magnetic measurements. A series of bulk (polycrystalline)and thin-film (epitaxially grown) compounds R1-xAxMnO3 (R = La, Pr, A = Ca) willbe studied with 0.5 < x < 1, respectively.