Kim Myung-Whun

Charge states and magnetic ordering in LaMnO3/SrTiO3 superlattices

We investigated the magnetic and optical properties of [(LaMnO{sub 3}){sub n}/(SrTiO{sub 3}){sub 8}]{sub 20} (n = 1, 2, and 8) superlattices grown by pulsed-laser deposition. We found that a weak ferromagnetic and semiconducting state developed in all superlattices. An analysis of the optical conductivity showed that the LaMnO{sub 3} layers in the superlattices were slightly doped. The amount of doping was almost identical regardless of the LaMnO{sub 3} layer thickness up to eight unit cells, suggesting that the effect is not limited to the interface. On the other hand, the magnetic ordering became less stable as the LaMnO{sub 3} layer thickness decreased, probably due to a dimensional effect.

Effects of oxygen-reducing atmosphere annealing on LaMnO3 epitaxial thin films

We investigated the effects of annealing on LaMnO3 epitaxial thin films grown by pulsed laser deposition (PLD) and propose an efficient method of characterizing their stoichiometry. Structural, magnetic and optical properties coherently indicate non-stoichiometric ferromagnetic and semiconducting phases for as-grown LaMnO3 films. By annealing in an oxygen-reducing atmosphere, we recovered the antiferromagnetic and insulating phases of bulk-like stoichiometric LaMnO3. We show that non-destructive optical spectroscopy at room temperature is one of the most convenient tools for identifying the phases of LaMnO3 films. Our results serve as a prerequisite for studying LaMnO3 based heterostructures grown by PLD.

Photo-induced optical anisotropy and coherent oscillations in a quasi-two-dimensional two-orbital model for LaMnO3

Ultrafast transient optical conductivity of spin-orbital ordered LaMnO3 was investigated theoretically. A quasi-two-dimensional Hubbard Hamiltonian based on a four-site Mn cluster was established to simulate the LaMnO3 system. The time evolution of the transient optical conductivity was numerically calculated. Intensity oscillations of the optical conductivity were observed, which were interpreted as the result of the quantum interference between phonon-coupled orbiton excited states. Also, the photo-induced anisotropy was observed, which revealed the broken orbital symmetry of the optical excited state.

Room temperature optical anisotropy of a LaMnO3 thin-film induced by ultra-short pulse laser

We observed ultra-short laser pulse-induced transient optical anisotropy in a LaMnO3 thin film. The anisotropy was induced by laser pulse irradiation with a fluence of less than 0.1 mJ/cm2 at room temperature. The transmittance and reflectance showed strong dependence on the polarization states of the pulses. For parallel and perpendicular polarization states, there exists a difference of approximately 0.2% for transmittance and 0.05% for reflectance at 0.3 ps after the irradiation with a pump pulse, respectively. The theoretical values for optical transmittance and reflectance with an assumption of an orbital ordering of 3d eg electrons in Mn3+ ions showed good agreement with the experimental results, demonstrating that the transient optical anisotropy in LaMnO3 thin film is due to the photo-induced symmetry-breaking of orbital ordering in excited states.

Phase change measurement of birefringent optical devices with white light interferometry

This paper describes a method to determine the phase retardation of birefringent optical components by combining spectral interferometry and the Fourier transform method. The retardation of each orthogonal polarization component was resolved by using two rotatable linear polarizers in the interferometer. The phase retardation measured by using suggested method was compared to that measured using the conventional polarimetric method. The results of independent methods were well matched, which confirms the validity of the proposed method.