Jai Seok Ahn

Ultrafast IR spectroscopic study of coherent phonons and dynamic spin–lattice coupling in multiferroic LuMnO3

The concurrent existence of ferroelectricity and magnetism within a single crystalline system characterizes the multiferroic materials discovered in recent years. To understand and develop the multiferroic phenomenon, we need to investigate the unusual coupling between spin and lattice degrees of freedom. Spins in multiferroics are expected to be elastically coupled to phonons. Therefore, the time-dependent study can be a crucial factor in understanding the coupled dynamics. Here, we report the observations of strong dynamic spin–lattice coupling in multiferroic LuMnO3. A coherent optical phonon of 3.6 THz and its temperature dependence is measured for the first time from our femtosecond IR pump and probe spectroscopy. Also, we observed a coherent acoustic phonon of 47 GHz similar to a previous report (Lim et al 2003 Appl. Phys. Lett. 83 4800). Temperature-dependent measurements show that both optical and acoustic phonons become significantly underdamped as temperature decreases to TN, and they disappear below TN. These observations reveal that phonons are coupled to spins by magneto-elastic coupling, and the disappearance of phonon modes at TN is consistent with the isostructural coupling scheme suggested by Lee et al (2008 Nature 451 805).

Strong spin-lattice coupling in multiferroic hexagonal manganite YMnO3 probed by ultrafast optical spectroscopy

We report the observation of spin-lattice coupling in multiferroic YMnO3 by femtosecond near-infrared pump and probe spectroscopy. A coherent 31 GHz acoustic phonon was detected above the magnetic ordering temperature, and a higher frequency coherent mode was observed in the antiferromagnetic phase. This temperature-dependent measurement demonstrates that the acoustic phonon excitation is coupled to spin ordering.

Coherent optical and acoustic phonons coupled with the charge-ordering phase transition in La 1/4 Pr 3/8 Ca 3/8 MnO 3

Ultrafast pump-probe spectroscopy is used to investigate La<inf>1/4</inf>Pr<inf>3/8</inf>Ca<inf>3/8</inf>MnO<inf>3</inf>. Coherent optical and acoustic phonons are observed strongly coupled with the charge-ordering phase transition. Phonon characteristics are well understood in a hypothesis related with charge-ordering gap opening.

IR pump-probe study of multiferroic LuMnO 3

We observe coherent phonons of 3.6 THz and 47 GHz oscillation frequencies of multiferroic LuMnO3 using femtosecond IR spectroscopy. Both coherent phonons disappear below TN as antiferromagnetically aligned spins strongly coupled with lattice vibration. This disappearance of phonons is related with the dynamic spin-phonon coupling and the giant magneto-elastic coupling near TN.

IR pump-probe study of phase separated hole-doped manganite, La 1/4 Pr 3/8 Ca 3/8 MnO 3

Ultrafast IR pump-probe spectroscopy is used to investigate La1/4Pr3/8Ca3/8MnO3 (LPCMO). Two types of coherent phonons strongly coupled with charge-ordering (CO) phase are observed. Temperature dependences of coherent phonons are explained by a hypothesis related with CO gap opening. According to the hypothesis, photo-induced electrons relax to acoustic phonon in the paramagnetic phase, but to optical phonons in the CO phase.