Akitoshi Koreeda

Excitation of coupled spin–orbit dynamics in cobalt oxide by femtosecond laser pulses

Ultrafast control of magnets using femtosecond light pulses attracts interest regarding applications and fundamental physics of magnetism. Antiferromagnets are promising materials with magnon frequencies extending into the terahertz range. Visible or near-infrared light interacts mainly with the electronic orbital angular momentum. In many magnets, however, in particular with iron-group ions, the orbital momentum is almost quenched by the crystal field. Thus, the interaction of magnons with light is hampered, because it is only mediated by weak unquenching of the orbital momentum by spin–orbit interactions. Here we report all-optical excitation of magnons with frequencies up to 9 THz in antiferromagnetic CoO with an unquenched orbital momentum. In CoO, magnon modes are coupled oscillations of spin and orbital momenta with comparable amplitudes. We demonstrate excitations of magnon modes by directly coupling light with electronic orbital angular momentum, providing possibilities to develop magneto-optical devices operating at several terahertz with high output-to-input ratio.Light pulses can control magnetism in a material, and the effective creation of magnetic oscillations leads to spintronic devices with higher efficiency. Here, the authors increase the efficiency of magnon excitation by using a material in which orbital angular momenta are not quenched.

Correlation between valence state of tin and elastic modulus of Sn-doped Li2O-B2O3-SiO2 glasses

The correlation between the longitudinal elastic modulus c11 and tin valence state in Li2O–B2O3–SiO2 ternary glasses is investigated. Substitution of B2O3 for SiO2 increases the glass-transition temperature and c11 but decreases the melting temperature. 119Sn Mossbauer spectra show that the valence state of tin increased with increasing molten temperature, Tmolten, in air, and that it also affects the c11 value even though the 1.0 mol. % addition. The c11 values, whose error bars are governed by those of density, suggest that the addition of SnO2 increases c11 whereas SnO decreases. The relationship between Tmolten and the amount of Sn2+ suggests that the main factor affecting the tin valence state is oxygen, whose reactivity may correlate with the glass melt viscosity.

kHz stimulated Brillouin spectroscopy

We have developed a high precision stimulated Brillouin spectrometer. Since this spectrometer is based on the stimulated light scattering, Brillouin measurements are possible without any signal reduction even at extremely low temperatures. The use of stable lasers for the pump and probe waves and a real-time frequency-monitor gives a high frequency resolution reaching 20 kHz, which is, to the best of authors knowledge, the highest resolution so far reported for Brillouin spectrometers. The frequency-monitor has been achieved through the measurement of the beat frequency between the two lasers by using a microwave frequency counter. With this spectrometer, we have succeeded to measure Brillouin spectra in single crystals of TeO2 from room temperature to liquid helium temperature. Furthermore, in a sample with [110] surfaces, we have observed a spectral splitting due to the multireflection of phonons at crystal surfaces.

Note: Higher resolution Brillouin spectroscopy by offset stabilization of a tandem Fabry-Pérot interferometer

A simple modification to a Sandercock-type tandem Fabry-Pérot interferometer is demonstrated. By adding an independent reference laser with temperature tunability, narrow Brillouin lines that are tens GHz shifted from the Rayleigh line can be recorded with much higher frequency resolution than in the original system.

Formation of metallic cation-oxygen network for anomalous thermal expansion coefficients in binary phosphate glass

Understanding glass structure is still challenging due to the result of disorder, although novel materials design on the basis of atomistic structure has been strongly demanded. Here we report on the atomic structures of the zinc phosphate glass determined by reverse Monte Carlo modelling based on diffraction and spectroscopic data. The zinc-rich glass exhibits the network formed by ZnOx (averaged x<4) polyhedra. Although the elastic modulus, refractive index and glass transition temperature of the zinc phosphate glass monotonically increase with the amount of ZnO, we find for the first time that the thermal expansion coefficient is very sensitive to the substitution of the phosphate chain network by a network consisting of Zn-O units in zinc-rich glass. Our results imply that the control of the structure of intermediate groups may enable new functionalities in the design of oxide glass materials.

Photoluminescence of Sn 2+ -centre as probe of transient state of supercooled liquid

Physical properties of oxide glasses depend on the cooling process from the super-cooled liquid state. It is demonstrated that the Sn2+ species possessing electrons in the outermost shell can function as a qualitative probe cation to evaluate the randomness of the host glass network. With slower cooling, the formation of a more dense glass network is observed as confirmed by the heat capacity, elastic modulus, 11B NMR, first sharp diffraction peak of X-ray diffraction, and Boson peak results. The optical absorption, photoluminescence (PL), and Sn K-edge X-ray absorption fine structure data strongly suggest that the aggregation of Sn2+ results from the slow-cooling of the melt, and that PL properties of Sn2+ can be affected by the ordering of the transient state of super-cooled liquid. The results highlight that the structure of the transient state of glass is important for the functionalization of optical glasses.

Boson peak dynamics of glassy glucose studied by integrated terahertz-band spectroscopy

We performed terahertz time-domain spectroscopy, low-frequency Raman scattering, and Brillouin light scattering on vitreous glucose to investigate the boson peak (BP) dynamics. In the spectra of

Broadband light scattering spectroscopy utilizing an ultra-narrowband holographic notch filter

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Quasielastic Light Scattering and Second Sound in SrTiO3

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