Browsing by Author "Ogawa, N."

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Photocreating supercooled spiral-spin states in a multiferroic manganite
(2016-08-18)
Sheu, Y. M.
;
Ogawa, N.
;
Kaneko, Y.
;
Tokura, Y.
;
電子物理學系
We demonstrate that the dynamics of the ab-spiral-spin order in a magnetoelectric multiferroic Eu0.55Y0.45MnO3 can be unambiguously probed through optical second harmonic signals, generated via spin-induced ferroelectric polarization. In the case of weak excitation, the ferroelectric and the spiral-spin order remains interlocked, both relaxing through spin-lattice relaxation in the nonequilibrium state. When the additional optical pulse illuminating the sample is intense enough to induce a local phase transition thermally, the system creates ametastable state of the bc-spiral-spin order (with the electric polarization P parallel to c) via supercooling across the first-order phase transition between the ab and bc spiral. The supercooled state of the bc-spiral spin is formed in the thermodynamical ground state of the ab spiral (P parallel to a), displaying a prolonged lifetime with strong dependence on the magnetic field along the a axis. The observed phenomena provide a different paradigm for photoswitching between the two distinct multiferroic states, motivating further research into a direct observation of the photocreated supercooled bc-spiral spin in multiferroic manganites.
Selective probe of coherent polar phonon and quasiferromagnetic resonance modes in multiferroic GdFeO3
(2018-09-26)
Sheu, Y. M.
;
Ogawa, N.
;
Tokunaga, Y.
;
Chan, H. C.
;
Tokura, Y.
Using time-resolved second harmonic generation (TRSHG) and optical transmission, we demonstrate simultaneous optical access to both magnetic and ferroelectric order parameters in a magnetoelectric multiferroic GdFeO3 below the transition temperature 2.2 K. In magnetoelectric phase, the pump pulses impulsively generate polar phonon modes of B-2 and A(1) symmetry and a quasiferromagnetic resonance mode, creating coherent lattice and spin motions. While collective motions associated with the ferroelectric and antiferromagnetic ordering are simultaneously excited, only the polar motion is detected in TRSHG, resulting in a direct subpicosecond excitation of the improper ferroelectric order. Our result also experimentally indicates that the soft mode in improper ferroelectrics is IR inactive above the phase transition, unlike proper ferroelectrics. Furthermore, we investigate the crystal polar symmetry and potential coupling between the polar phonons and the quasiantiferromagnetic resonance mode, which can shed light on the studies of subpicosecond magnetoelectric coupling based on collective quasiparticle excitations.