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Symmetry analysis of cross-circular and parallel-circular Raman optical activity
Authors:
Hikaru Watanabe,
Rikuto Oiwa,
Gakuto Kusuno,
Takuya Satoh,
Ryotaro Arita
Abstract:
The Raman scattering regarding the circularly-polarized incident and scattered lights is closely related to the circular activity of a given system. We investigate the symmetry of its activity, called the cross-circular and parallel-circular Raman optical activity. The analysis is systematically performed with the magnetic point groups and indicates that the response allows for a useful diagnosis…
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The Raman scattering regarding the circularly-polarized incident and scattered lights is closely related to the circular activity of a given system. We investigate the symmetry of its activity, called the cross-circular and parallel-circular Raman optical activity. The analysis is systematically performed with the magnetic point groups and indicates that the response allows for a useful diagnosis of the symmetry of materials like chirality and (magneto-)axiality. It is also shown that the Stokes and anti-Stokes processes are related to each other by the conserved antiunitary symmetry for the time-reversal operation and that combined with the mirror reflection.
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Submitted 2 September, 2025; v1 submitted 26 June, 2025;
originally announced June 2025.
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Raman Optical Activity Induced by Ferroaxial Order in $\textrm{NiTiO}_3$
Authors:
Gakuto Kusuno,
Takeshi Hayashida,
Takayuki Nagai,
Hikaru Watanabe,
Rikuto Oiwa,
Tsuyoshi Kimura,
Takuya Satoh
Abstract:
Raman optical activity (ROA) -- the dependence of Raman scattered light intensity on the circular polarization of incident and scattered light -- has traditionally been associated with chiral molecules and magnetic materials. In this study, we demonstrate that ROA can also arise in ferroaxial materials that possess spatial inversion and time-reversal symmetries. Using circularly polarized Raman sp…
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Raman optical activity (ROA) -- the dependence of Raman scattered light intensity on the circular polarization of incident and scattered light -- has traditionally been associated with chiral molecules and magnetic materials. In this study, we demonstrate that ROA can also arise in ferroaxial materials that possess spatial inversion and time-reversal symmetries. Using circularly polarized Raman spectroscopy on single-crystalline $\textrm{NiTiO}_3$, we observed a pronounced ROA signal in the cross-circular polarization configuration, which correlates with the ferroaxial domain structure. Our symmetry analysis and tight-binding model calculations reveal that the natural ROA (NROA) originates from the ferroaxial order and persists even within the electric dipole approximation. These results establish ROA as a powerful probe of ferroaxial order in centrosymmetric systems.
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Submitted 28 May, 2025;
originally announced May 2025.
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Enhancing the Dynamic Range of Quantum Sensing via Quantum Circuit Learning
Authors:
Hideaki Kawaguchi,
Yuichiro Mori,
Takahiko Satoh,
Yuichiro Matsuzaki
Abstract:
Quantum metrology is a promising application of quantum technologies, enabling the precise measurement of weak external fields at a local scale. In typical quantum sensing protocols, a qubit interacts with an external field, and the amplitude of the field is estimated by analyzing the expectation value of a measured observable. Sensitivity can, in principle, be enhanced by increasing the number of…
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Quantum metrology is a promising application of quantum technologies, enabling the precise measurement of weak external fields at a local scale. In typical quantum sensing protocols, a qubit interacts with an external field, and the amplitude of the field is estimated by analyzing the expectation value of a measured observable. Sensitivity can, in principle, be enhanced by increasing the number of qubits within a fixed volume, thereby maintaining spatial resolution. However, at high qubit densities, inter-qubit interactions induce complex many-body dynamics, resulting in multiple oscillations in the expectation value of the observable even for small field amplitudes. This ambiguity reduces the dynamic range of the sensing protocol. We propose a method to overcome the limitation in quantum metrology by adopting a quantum circuit learning framework using a parameterized quantum circuit to approximate a target function by optimizing the circuit parameters. In our method, after the qubits interact with the external field, we apply a sequence of parameterized quantum gates and measure a suitable observable. By optimizing the gate parameters, the expectation value is trained to exhibit a monotonic response within a target range of field amplitudes, thereby eliminating multiple oscillations and enhancing the dynamic range. This method offers a strategy for improving quantum sensing performance in dense qubit systems.
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Submitted 8 May, 2025;
originally announced May 2025.
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Polarization-dependent photocurrent in a quadrilateral-shaped bulk crystalline tellurium chip with near-infrared light excitation
Authors:
Hiro Munekata,
Gakuto Kusuno,
Kohei Miyazaki,
Takuya Satoh
Abstract:
We report the detection of zero-bias photocurrents induced by interband excitation using oblique near-infrared light (λ=0.780 μm) along the directions parallel and perpendicular to the helical axis in a millimeter-sized Te crystal. The photocurrent parallel to the helical axis exhibits a circular-polarization-dependent component, namely circular photogalvanic effect. We create a framework for esti…
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We report the detection of zero-bias photocurrents induced by interband excitation using oblique near-infrared light (λ=0.780 μm) along the directions parallel and perpendicular to the helical axis in a millimeter-sized Te crystal. The photocurrent parallel to the helical axis exhibits a circular-polarization-dependent component, namely circular photogalvanic effect. We create a framework for estimating the gyrotropic photoconductivity tensor \b{eta}, the ratio between the circular-polarization-dependent photocurrent density and intensity of light, through which extrinsic \b{eta} values ranging from a few to a few tens of nA/W are obtained. Searching the \b{eta} values through existing literatures reveals that these values vary significantly with sample conditions, excitation wavelengths, and forms of crystallite samples. A particular interesting point found in our study is that the \b{eta} values for the interband excitation may be greater than those for intraband excitation as far as bulk Te is concerned.
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Submitted 23 November, 2025; v1 submitted 1 April, 2025;
originally announced April 2025.
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Anomalous Nernst effect in Co thin films under laser irradiation
Authors:
Soichiro Mochizuki,
Itaru Sugiura,
Tetsuya Narushima,
Teruo Ono,
Takuya Satoh,
Kihiro T. Yamada
Abstract:
The anomalous Nernst effect (ANE) generates electromotive forces transverse to temperature gradients and has attracted much attention for potential applications into alternative thermoelectric power generators. ANE efficiency is generally characterized by uniform temperature gradients in a steady state prepared by heaters. However, although focusing laser beams on a magnetic film can form much lar…
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The anomalous Nernst effect (ANE) generates electromotive forces transverse to temperature gradients and has attracted much attention for potential applications into alternative thermoelectric power generators. ANE efficiency is generally characterized by uniform temperature gradients in a steady state prepared by heaters. However, although focusing laser beams on a magnetic film can form much larger temperature gradients, the laser-irradiation method has not been sufficiently considered for quantifying the ANE coefficient due to the difficulty in estimating the localized in-homogeneous temperature gradients. In this study, we present a quantitative study of ANE in Ru(5 nm)/Co($t_{\mathrm{Co}}$) ($t_{\mathrm{Co}}$ = 3, 5, 7, 10, 20, 40, and 60 nm) bilayers on sapphire (0001) substrates by combining a laser irradiation approach with finite-element analysis of temperature gradients under laser excitation. We find that the estimated ANE coefficients are consistent with previously reported values and one independently characterized using a heater. Our results also reveal the advantages of the laser irradiation method over the conventional method using heaters. Intensity-modulated laser beams can create ac temperature gradients as large as approximately 10$^3$ K/mm at a frequency of tens of kilohertz in a micrometer-scale region.
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Submitted 2 November, 2025; v1 submitted 28 January, 2025;
originally announced January 2025.
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Magnetic resonance frequency of two-sublattice ferrimagnet with magnetic compensation temperature
Authors:
Kouki Mikuni,
Toshiki Hiraoka,
Takumi Kuramoto,
Yasuhiro Fujii,
Akitoshi Koreeda,
Sergii Parchenko,
Andrzej Stupakiewicz,
Takuya Satoh
Abstract:
Ferrimagnetic materials with a compensation temperature have recently attracted interest because of their unique combination of ferromagnetic and antiferromagnetic properties. However, their magnetization dynamics near the compensation temperature are complex and cannot be fully explained by conventional ferromagnetic resonance (FMR) or exchange resonance modes. Therefore, practical models are nec…
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Ferrimagnetic materials with a compensation temperature have recently attracted interest because of their unique combination of ferromagnetic and antiferromagnetic properties. However, their magnetization dynamics near the compensation temperature are complex and cannot be fully explained by conventional ferromagnetic resonance (FMR) or exchange resonance modes. Therefore, practical models are necessary to capture these dynamics accurately. In this study, we derived the analytical solutions for the magnetic resonance frequencies of compensated ferrimagnets over all temperature ranges, considering both the in-plane and out-of-plane orientations of the magnetization. Our solutions successfully reproduce the experimental data obtained from time-resolved magneto-optical Faraday rotation and Brillouin light scattering measurements for the in-plane and out-of-plane cases, respectively. This reproduction is achieved by incorporating the exchange stiffness and temperature dependence of the magnetic anisotropy into the free energy density. Additionally, at temperatures sufficiently far from the compensation temperature, our analytical solutions converge with the conventional FMR and exchange resonance models.
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Submitted 22 November, 2024;
originally announced November 2024.
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Evidence of relativistic field-derivative torque in nonlinear THz response of magnetization dynamics
Authors:
Arpita Dutta,
Christian Tzschaschel,
Debankit Priyadarshi,
Kouki Mikuni,
Takuya Satoh,
Ritwik Mondal,
Shovon Pal
Abstract:
Understanding the complete light-spin interactions in magnetic systems is the key to manipulating the magnetization using optical means at ultrafast timescales. The selective addressing of spins by terahertz (THz) electromagnetic fields via Zeeman torque is one of the most successful ultrafast means of controlling magnetic excitations. Here we show that this traditional Zeeman torque on the spins…
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Understanding the complete light-spin interactions in magnetic systems is the key to manipulating the magnetization using optical means at ultrafast timescales. The selective addressing of spins by terahertz (THz) electromagnetic fields via Zeeman torque is one of the most successful ultrafast means of controlling magnetic excitations. Here we show that this traditional Zeeman torque on the spins is not sufficient, rather an additional relativistic field-derivative torque is essential to realize the observed magnetization dynamics. We accomplish this by exploring the ultrafast nonlinear magnetization dynamics of rare-earth, Bi-doped iron garnet when excited by two co-propagating THz pulses. First, by exciting the sample with an intense THz pulse and probing the magnetization dynamics using magneto-optical Faraday effect, we find the collective exchange resonance mode between rare-earth and transition metal sublattices at 0.48 THz. We further explore the magnetization dynamics via the THz time-domain spectroscopic means. We find that the observed nonlinear trace of the magnetic response cannot be mapped to the magnetization precession induced by the Zeeman torque, while the Zeeman torque supplemented by an additional field-derivative torque follows the experimental evidences. This breakthrough enhances our comprehension of ultra-relativistic effects and paves the way towards novel technologies harnessing light-induced control over magnetic systems.
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Submitted 30 December, 2024; v1 submitted 10 August, 2024;
originally announced August 2024.
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Sublattice-selective inverse Faraday effect in ferrimagnetic rare-earth iron garnet
Authors:
Toshiki Hiraoka,
Ryo Kainuma,
Keita Matsumoto,
Kihiro T. Yamada,
Takuya Satoh
Abstract:
We performed time-resolved pump--probe measurements using rare-earth iron garnet \ce{Gd3/2Yb1/2BiFe5O12} as a two-sublattice ferrimagnet. We measured the initial phases of the magnetic resonance modes below and above the magnetization compensation temperature to clarify the sublattice selectivity of the inverse Faraday effect in ferrimagnets. A comparison of the time evolution of magnetization est…
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We performed time-resolved pump--probe measurements using rare-earth iron garnet \ce{Gd3/2Yb1/2BiFe5O12} as a two-sublattice ferrimagnet. We measured the initial phases of the magnetic resonance modes below and above the magnetization compensation temperature to clarify the sublattice selectivity of the inverse Faraday effect in ferrimagnets. A comparison of the time evolution of magnetization estimated using the equations of motion revealed that the inverse Faraday effect occurring in ferrimagnetic materials has sublattice selectivity. This is in striking contrast to antiferromagnets, in which the inverse Faraday effect acts on each sublattice identically. The initial phase analysis can be applied to other ferrimagnets with compensation temperatures.
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Submitted 27 December, 2023;
originally announced December 2023.
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Generation of third-harmonic spin oscillation from strong spin precession induced by terahertz magnetic near fields
Authors:
Zhenya Zhang,
Fumiya Sekiguchi,
Takahiro Moriyama,
Shunsuke C. Furuya,
Masahiro Sato,
Takuya Satoh,
Yu Mukai,
Koichiro Tanaka,
Takafumi Yamamoto,
Hiroshi Kageyama,
Yoshihiko Kanemitsu,
Hideki Hirori
Abstract:
The ability to drive a spin system to state far from the equilibrium is indispensable for investigating spin structures of antiferromagnets and their functional nonlinearities for spintronics. While optical methods have been considered for spin excitation, terahertz (THz) pulses appear to be a more convenient means of direct spin excitation without requiring coupling between spins and orbitals or…
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The ability to drive a spin system to state far from the equilibrium is indispensable for investigating spin structures of antiferromagnets and their functional nonlinearities for spintronics. While optical methods have been considered for spin excitation, terahertz (THz) pulses appear to be a more convenient means of direct spin excitation without requiring coupling between spins and orbitals or phonons. However, room-temperature responses are usually limited to small deviations from the equilibrium state because of the relatively weak THz magnetic fields in common approaches. Here, we studied the magnetization dynamics in a HoFeO3 crystal at room temperature. A custom-made spiral-shaped microstructure was used to locally generate a strong multicycle THz magnetic near field perpendicular to the crystal surface; the maximum magnetic field amplitude of about 2 T was achieved. The observed time-resolved change in the Faraday ellipticity clearly showed second- and third-order harmonics of the magnetization oscillation and an asymmetric oscillation behaviour. Not only the ferromagnetic vector M but also the antiferromagnetic vector L plays an important role in the nonlinear dynamics of spin systems far from equilibrium.
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Submitted 28 March, 2023;
originally announced March 2023.
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Sub-millimeter propagation of antiferromagnetic magnons via magnon-photon coupling
Authors:
Ryo Kainuma,
Keita Matsumoto,
Toshimitsu Ito,
Takuya Satoh
Abstract:
For the realization of magnon-based current-free technologies, referred to as magnonics, all-optical control of magnons is an important technique for both fundamental research and practical applications. Magnon-polariton is a coupled state of magnon and photon in a magnetic medium, expected to exhibit magnon-like controllability and photon-like high-speed propagation. While recent studies have obs…
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For the realization of magnon-based current-free technologies, referred to as magnonics, all-optical control of magnons is an important technique for both fundamental research and practical applications. Magnon-polariton is a coupled state of magnon and photon in a magnetic medium, expected to exhibit magnon-like controllability and photon-like high-speed propagation. While recent studies have observed magnon-polaritons as modulation of incident terahertz waves, the influence of magnon-photon coupling on magnon propagation properties remains unexplored. This study aimed to observe the spatiotemporal dynamics of coherent magnon-polaritons through time-resolved imaging measurements. BiFeO$_3$ was selected as the sample due to its anticipated strong coupling between magnons and photons. The observed dynamics suggest that antiferromagnetic magnons can propagate over long distances, up to hundreds of micrometers, through strong coupling with photons. These results enhance our understanding of the optical control of magnonic systems, thereby paving the way for terahertz opto-magnonics.
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Submitted 10 June, 2024; v1 submitted 3 February, 2023;
originally announced February 2023.
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Chiral phonons: circularly polarized Raman spectroscopy and $\textit{ab initio}$ calculations in a chiral crystal tellurium
Authors:
Kyosuke Ishito,
Huiling Mao,
Kaya Kobayashi,
Yusuke Kousaka,
Yoshihiko Togawa,
Hiroaki Kusunose,
Jun-ichiro Kishine,
Takuya Satoh
Abstract:
Recently, phonons with chirality (chiral phonons) have attracted significant attention. Chiral phonons exhibit angular and pseudo-angular momenta. In circularly polarized Raman spectroscopy, the peak split of the $Γ_3$ mode is detectable along the principal axis of the chiral crystal in the backscattering configuration. In addition, peak splitting occurs when the pseudo-angular momenta of the inci…
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Recently, phonons with chirality (chiral phonons) have attracted significant attention. Chiral phonons exhibit angular and pseudo-angular momenta. In circularly polarized Raman spectroscopy, the peak split of the $Γ_3$ mode is detectable along the principal axis of the chiral crystal in the backscattering configuration. In addition, peak splitting occurs when the pseudo-angular momenta of the incident and scattered circularly polarized light are reversed. Until now, chiral phonons in binary crystals have been observed, whereas those in unary crystals have not been observed. Here, we observe chiral phonons in a chiral unary crystal Te. The pseudo-angular momentum of the phonon is obtained in Te by an $\textit{ab initio}$ calculation. From this calculation, we verified the conservation law of pseudo-angular momentum in Raman scattering. From this conservation law, we determined the handedness of the chiral crystals. We also evaluated the true chirality of the phonons using a measure with symmetry similar to that of an electric toroidal monopole.
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Submitted 28 January, 2023; v1 submitted 5 December, 2022;
originally announced December 2022.
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Observation of quasi-elastic light scattering in BiFeO$_3$
Authors:
Eiichi Oishi,
Yasuhiro Fujii,
Akitoshi Koreeda,
Takuya Satoh,
Toshimitsu Ito
Abstract:
We observed quasi-elastic light scattering (QELS) in BiFeO$_3$ using Raman spectroscopy over a temperature range of 300-860 K. The QELS has two components: a narrow and broad component. The temperature dependence of the intensity and linewidth of the broad component differed below and beyond the Néel point, and the broad QELS may have a magnetic origin.
We observed quasi-elastic light scattering (QELS) in BiFeO$_3$ using Raman spectroscopy over a temperature range of 300-860 K. The QELS has two components: a narrow and broad component. The temperature dependence of the intensity and linewidth of the broad component differed below and beyond the Néel point, and the broad QELS may have a magnetic origin.
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Submitted 11 August, 2022;
originally announced August 2022.
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Truly chiral phonons in α-HgS
Authors:
Kyosuke Ishito,
Huiling Mao,
Yusuke Kousaka,
Yoshihiko Togawa,
Satoshi Iwasaki,
Tiantian Zhang,
Shuichi Murakami,
Jun-ichiro Kishine,
Takuya Satoh
Abstract:
Chirality is a manifestation of the asymmetry inherent in nature. It has been defined as the symmetry breaking of the parity of static objects, and the definition was extended to dynamic motion such that true and false chiralities were distinguished. Recently, rotating, yet not propagating, atomic motions were predicted and observed in two-dimensional materials, and they were referred to as "chira…
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Chirality is a manifestation of the asymmetry inherent in nature. It has been defined as the symmetry breaking of the parity of static objects, and the definition was extended to dynamic motion such that true and false chiralities were distinguished. Recently, rotating, yet not propagating, atomic motions were predicted and observed in two-dimensional materials, and they were referred to as "chiral phonons" . A natural development would be the discovery of truly chiral phonons that propagate while rotating in three-dimensional materials. Here, we used circularly polarised Raman scattering and first-principles calculations to identify truly chiral phonons in chiral bulk crystals. This approach enabled us to determine the chirality of a crystal in a non-contact and non-destructive manner. In addition, we demonstrated that the law of the conservation of pseudo-angular momentum holds between circularly polarised photons and chiral phonons. These findings are expected to help develop ways for transferring the pseudo-angular momentum from photons to electron spins via the propagating chiral phonons in opto-phononic-spintronic devices.
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Submitted 13 October, 2022; v1 submitted 22 October, 2021;
originally announced October 2021.
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Ultrafast Optomagnonics in Ferrimagnetic Multi-Sublattice Garnets
Authors:
Andrzej Stupakiewicz,
Takuya Satoh
Abstract:
This review discusses the ultrafast magnetization dynamics within the gigahertz to terahertz frequency range in ferrimagnetic rare-earth iron garnets with different substitutions. In these garnets, the roles of spin-orbit and exchange interactions have been detected using femtosecond laser pulses via the inverse Faraday effect. The all-optical control of spin-wave and Kaplan-Kittel exchange resona…
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This review discusses the ultrafast magnetization dynamics within the gigahertz to terahertz frequency range in ferrimagnetic rare-earth iron garnets with different substitutions. In these garnets, the roles of spin-orbit and exchange interactions have been detected using femtosecond laser pulses via the inverse Faraday effect. The all-optical control of spin-wave and Kaplan-Kittel exchange resonance modes in different frequency ranges is shown. Generation and localization of the electric field distribution inside the garnet through the metal-bound surface plasmon-polariton strongly enhance the amplitude of the exchange resonance modes. The exchange resonance mode in yttrium iron garnets was observed using circularly polarized Raman spectroscopy. The results of this study may be utilized in the development of a wide class of optomagnonic devices in the gigahertz to terahertz frequency range.
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Submitted 14 July, 2021;
originally announced July 2021.
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Ultrafast amplification and non-linear magneto-elastic coupling of coherent magnon modes in an antiferromagnet
Authors:
D. Bossini,
M. Pancaldi,
L. Soumah,
M. Basini,
F. Mertens,
M. Cinchetti,
T. Satoh,
O. Gomonay,
S. Bonetti
Abstract:
We investigate the role of domain walls in the ultrafast magnon dynamics of an antiferromagnetic NiO single crystal in a pump-probe experiment with variable pump photon energy. Analysing the amplitude of the energy-dependent photo-induced ultrafast spin dynamics, we detect a yet unreported coupling between the material's characteristic THz- and a GHz-magnon modes. We explain this unexpected coupli…
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We investigate the role of domain walls in the ultrafast magnon dynamics of an antiferromagnetic NiO single crystal in a pump-probe experiment with variable pump photon energy. Analysing the amplitude of the energy-dependent photo-induced ultrafast spin dynamics, we detect a yet unreported coupling between the material's characteristic THz- and a GHz-magnon modes. We explain this unexpected coupling between two orthogonal eigenstates of the corresponding Hamiltonian by modelling the magneto-elastic interaction between spins in different domains. We find that such interaction, in the non-linear regime, couples the two different magnon modes via the domain walls and it can be optically exploited via the exciton-magnon resonance.
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Submitted 12 August, 2021; v1 submitted 20 March, 2021;
originally announced March 2021.
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Observation of terahertz magnon of Kaplan-Kittel exchange resonance in yttrium-iron garnet by Raman spectroscopy
Authors:
Wei-Hung Hsu,
Ka Shen,
Yasuhiro Fujii,
Akitoshi Koreeda,
Takuya Satoh
Abstract:
Backscattering Raman spectroscopic investigations were performed on an yttrium-iron garnet single crystal using linearly and circularly polarized light. A terahertz (THz) magnon of the Kaplan-Kittel (KK) exchange resonance was discovered, which had been regarded as unobservable via optical methods. The KK exchange resonance had a 7.8-THz frequency at 80 K, and the polarization selection rule led t…
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Backscattering Raman spectroscopic investigations were performed on an yttrium-iron garnet single crystal using linearly and circularly polarized light. A terahertz (THz) magnon of the Kaplan-Kittel (KK) exchange resonance was discovered, which had been regarded as unobservable via optical methods. The KK exchange resonance had a 7.8-THz frequency at 80 K, and the polarization selection rule led to an antisymmetric Raman tensor of the $A_{2}$ mode. Moreover, the assignment of all the Raman-active phonon modes, $3A_{1g}$, $8E_{g}$, and $14T_{2g}$, was proposed. This study will stimulate further investigation of the coupling of THz magnons and phonons and pave the way toward THz optomagnonics.
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Submitted 25 November, 2020; v1 submitted 8 May, 2020;
originally announced May 2020.
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Observation of evanescent spin waves in the magnetic dipole regime
Authors:
Keita Matsumoto,
Isao Yoshimine,
Kosei Himeno,
Tsutomu Shimura,
Takuya Satoh
Abstract:
We observed spin-wave transmission through an air gap that works as a prohibited region. The spin waves were excited by circularly polarized pump pulses via the inverse Faraday effect, and their spatial propagation was detected through the Faraday effect of probe pulses using a pump-probe imaging technique. The experimentally observed spin-wave transmission was reproduced using numerical calculati…
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We observed spin-wave transmission through an air gap that works as a prohibited region. The spin waves were excited by circularly polarized pump pulses via the inverse Faraday effect, and their spatial propagation was detected through the Faraday effect of probe pulses using a pump-probe imaging technique. The experimentally observed spin-wave transmission was reproduced using numerical calculations with a Green's function method and micromagnetic simulation. We found that the amplitude of the spin waves decays exponentially in the air gap, which indicates the existence of evanescent spin waves in the magnetic dipole regime. This finding will pave the way for controllable amplitudes and phases of spin waves propagating through an artificial magnonic crystal.
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Submitted 5 May, 2020;
originally announced May 2020.
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Efficient spin excitation via ultrafast damping-like torques in antiferromagnets
Authors:
Christian Tzschaschel,
Takuya Satoh,
Manfred Fiebig
Abstract:
Damping effects form the core of many emerging concepts for high-speed spintronic applications. Important characteristics such as device switching times and magnetic domain-wall velocities depend critically on the damping rate. While the implications of spin damping for relaxation processes are intensively studied, damping effects during impulsive spin excitations are assumed to be negligible beca…
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Damping effects form the core of many emerging concepts for high-speed spintronic applications. Important characteristics such as device switching times and magnetic domain-wall velocities depend critically on the damping rate. While the implications of spin damping for relaxation processes are intensively studied, damping effects during impulsive spin excitations are assumed to be negligible because of the shortness of the excitation process. Herein, we show that, unlike in ferromagnets, ultrafast damping plays a crucial role in antiferromagnets because of their strongly elliptical spin precession. In time-resolved measurements, we find that ultrafast damping results in an immediate spin canting along the short precession axis. The interplay between antiferromagnetic exchange and magnetic anisotropy amplifies this canting by several orders of magnitude towards large-amplitude modulations of the antiferromagnetic order parameter. This leverage effect discloses a highly efficient route towards the ultrafast manipulation of magnetism in antiferromagnetic spintronics.
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Submitted 5 December, 2020; v1 submitted 4 August, 2019;
originally announced August 2019.
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Tracking the ultrafast motion of an antiferromagnetic order parameter
Authors:
Christian Tzschaschel,
Takuya Satoh,
Manfred Fiebig
Abstract:
The unique functionalities of antiferromagnets offer promising routes to advance information technology. Their compensated magnetic order leads to spin resonances in the THz-regime, which suggest the possibility to coherently control antiferromagnetic (AFM) devices orders of magnitude faster than traditional electronics. However, the required time resolution, complex sublattice interations and the…
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The unique functionalities of antiferromagnets offer promising routes to advance information technology. Their compensated magnetic order leads to spin resonances in the THz-regime, which suggest the possibility to coherently control antiferromagnetic (AFM) devices orders of magnitude faster than traditional electronics. However, the required time resolution, complex sublattice interations and the relative inaccessibility of the AFM order parameter pose serious challenges to studying AFM spin dynamics. Here, we reveal the temporal evolution of an AFM order parameter directly in the time domain. We modulate the AFM order in hexagonal YMnO$_\mathrm{3}$ by coherent magnon excitation and track the ensuing motion of the AFM order parameter using time-resolved optical second-harmonic generation (SHG). The dynamic symmetry reduction by the moving order parameter allows us to separate electron dynamics from spin dynamics. As transient symmetry reductions are common to coherent excitations, we have a general tool for tracking the ultrafast motion of an AFM order parameter.
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Submitted 20 March, 2019;
originally announced March 2019.
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Optical determination of the exchange stiffness constant in an iron garnet
Authors:
Keita Matsumoto,
Thomas Brächer,
Philipp Pirro,
Dmytro Bozhko,
Tobias Fischer,
Moritz Geilen,
Frank Heussner,
Thomas Meyer,
Burkard Hillebrands,
Takuya Satoh
Abstract:
Brillouin light scattering measurements were performed in the backscattering geometry on a Bi-substituted rare earth iron garnet. We observed two different peaks, one attributed to a surface spin wave in the dipole-exchange regime. The other is referred to as a backscattering magnon mode, because the incident light in this case is scattered backward by exchange-dominated spin wave inside the mater…
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Brillouin light scattering measurements were performed in the backscattering geometry on a Bi-substituted rare earth iron garnet. We observed two different peaks, one attributed to a surface spin wave in the dipole-exchange regime. The other is referred to as a backscattering magnon mode, because the incident light in this case is scattered backward by exchange-dominated spin wave inside the material. We propose a method to estimate the exchange stiffness constant from the frequency of the backscattering magnon mode. The obtained value is comparable with the previously reported values for Y$ _3 $Fe$ _5 $O$ _{12} $.
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Submitted 15 June, 2018; v1 submitted 1 June, 2018;
originally announced June 2018.
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Excitation of multiple phonon modes in copper metaborate CuB$_2$O$_4$ via non-resonant impulsive stimulated Raman scattering
Authors:
Kotaro Imasaka,
Roman V. Pisarev,
Leonard N. Bezmaternykh,
Tsutomu Shimura,
Alexandra M. Kalashnikova,
Takuya Satoh
Abstract:
Excitation of four coherent phonon modes of different symmetries has been realized in copper metaborate CuB$_2$O$_4$ via impulsive stimulated Raman scattering (ISRS). Phonons were detected by monitoring changes in the linear optical birefringence using the balanced-detection (BD) technique. We compare the results of BD-ISRS experiment to the polarized spontaneous Raman scattering spectra. We show…
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Excitation of four coherent phonon modes of different symmetries has been realized in copper metaborate CuB$_2$O$_4$ via impulsive stimulated Raman scattering (ISRS). Phonons were detected by monitoring changes in the linear optical birefringence using the balanced-detection (BD) technique. We compare the results of BD-ISRS experiment to the polarized spontaneous Raman scattering spectra. We show that the agreement between the two sets of data obtained by these allied techniques in a wide phonon frequencies range of 4-14 THz can be achieved by rigorously taking into account the symmetry of the phonon modes, and the corresponding excitation and detection selection rules. It is also important to account for the difference between incoherent and coherent phonons in terms of their contributions to the Raman scattering process. This comparative analysis highlights the importance of the ratio between the frequency of a particular mode, and the pump and probe spectral widths. We demonstrate analytically that the pump and probe pulse durations of 90 and 50 fs, respectively, used in our experiments, limit the highest frequency of the excited and detected coherent phonon modes to 12 THz, and define their relative amplitudes.
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Submitted 11 July, 2018; v1 submitted 17 April, 2018;
originally announced April 2018.
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Excitation of coherent optical phonons in iron garnet by femtosecond laser pulses
Authors:
Pritam Khan,
Masataka Kanamaru,
Wei-Hung Hsu,
Minori Kichise,
Yasuhiro Fujii,
Akitoshi Koreeda,
Takuya Satoh
Abstract:
We employed femtosecond pump probe technique to investigate the dynamics of coherent optical phonons in iron garnet. A phenomenological symmetry-based consideration reveals that oscillations of the terahertz T2g mode are excited. Selective excitation by a linearly polarized pump and detection by a circularly polarized probe confirm that impulsive stimulated Raman scattering (ISRS) is the driving f…
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We employed femtosecond pump probe technique to investigate the dynamics of coherent optical phonons in iron garnet. A phenomenological symmetry-based consideration reveals that oscillations of the terahertz T2g mode are excited. Selective excitation by a linearly polarized pump and detection by a circularly polarized probe confirm that impulsive stimulated Raman scattering (ISRS) is the driving force for the coherent phonons. Experimental results obtained from ISRS measurements reveal excellent agreement with spontaneous Raman spectroscopy data, analyzed by considering the symmetry of the phonon modes and corresponding excitation and detection selection rules.
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Submitted 5 April, 2019; v1 submitted 13 February, 2018;
originally announced February 2018.
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Surface plasmon-mediated nanoscale localization of laser-driven sub-THz spin dynamics in magnetic dielectrics
Authors:
Alexander L. Chekhov,
Alexander I. Stognij,
Takuya Satoh,
Tatiana V. Murzina,
Ilya Razdolski,
Andrzej Stupakiewicz
Abstract:
Ultrafast all-optical control of spins with femtosecond laser pulses is one of the hot topics at the crossroads of photonics and magnetism with a direct impact on future magnetic recording. Unveiling light-assisted recording mechanisms for an increase of the bit density beyond the diffraction limit without excessive heating of the recording medium is an open challenge. Here we show that surface pl…
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Ultrafast all-optical control of spins with femtosecond laser pulses is one of the hot topics at the crossroads of photonics and magnetism with a direct impact on future magnetic recording. Unveiling light-assisted recording mechanisms for an increase of the bit density beyond the diffraction limit without excessive heating of the recording medium is an open challenge. Here we show that surface plasmon-polaritons in hybrid metal-dielectric structures can provide spatial confinement of the inverse Faraday effect, mediating the excitation of localized coherent spin precession with 0.41 THz frequency. We demonstrate a two orders of magnitude enhancement of the excitation efficiency at the surface plasmon resonance within the 100 nm layer in dielectric garnet. Our findings broaden the horizons of ultrafast spin-plasmonics and open pathways towards non-thermal opto-magnetic recording at the nano-scale.
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Submitted 5 December, 2017;
originally announced December 2017.
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Unidirectional control of optically induced spin waves
Authors:
Isao Yoshimine,
Yoshito Y. Tanaka,
Tsutomu Shimura,
Takuya Satoh
Abstract:
Unidirectional control of optically induced spin waves in a rare-earth iron garnet crystal is demonstrated. We observed the interference of two spin-wave packets with different initial phases generated by circularly polarized light pulses. This interference results in unidirectional propagation if the spin-wave sources are spaced apart at 1/4 of the wavelength of the spin waves and the initial pha…
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Unidirectional control of optically induced spin waves in a rare-earth iron garnet crystal is demonstrated. We observed the interference of two spin-wave packets with different initial phases generated by circularly polarized light pulses. This interference results in unidirectional propagation if the spin-wave sources are spaced apart at 1/4 of the wavelength of the spin waves and the initial phase difference is set to pi/2. The propagating direction of the spin wave is switched by the polarization helicity of the light pulses. Moreover, in a numerical simulation, applying more than two spin-wave sources with a suitable polarization and spot shape, arbitrary manipulation of the spin wave by the phased array method was replicated.
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Submitted 9 May, 2017;
originally announced May 2017.
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Ultrafast optical excitation of coherent magnons in antiferromagnetic NiO
Authors:
Christian Tzschaschel,
Kensuke Otani,
Ryugo Iida,
Tsutomu Shimura,
Hiroaki Ueda,
Stefan Günther,
Manfred Fiebig,
Takuya Satoh
Abstract:
In experiment and theory, we resolve the mechanism of ultrafast optical magnon excitation in antiferromagnetic NiO. We employ time-resolved optical two-color pump-probe measurements to study the coherent non-thermal spin dynamics. Optical pumping and probing with linearly and circularly polarized light along the optic axis of the NiO crystal scrutinizes the mechanism behind the ultrafast optical m…
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In experiment and theory, we resolve the mechanism of ultrafast optical magnon excitation in antiferromagnetic NiO. We employ time-resolved optical two-color pump-probe measurements to study the coherent non-thermal spin dynamics. Optical pumping and probing with linearly and circularly polarized light along the optic axis of the NiO crystal scrutinizes the mechanism behind the ultrafast optical magnon excitation. A phenomenological symmetry-based theory links these experimental results to expressions for the optically induced magnetization via the inverse Faraday effect and the inverse Cotton-Mouton effect. We obtain striking agreement between experiment and theory that, furthermore, allows us to extract information about the spin domain distribution. We also find that in NiO the energy transfer into the magnon mode via the inverse Cotton-Mouton effect is about three orders of magnitude more efficient than via the inverse Faraday effect.
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Submitted 30 January, 2017;
originally announced February 2017.
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Two-Dimensional Propagation of a Photoinduced Spin Wave Packet
Authors:
Yuki Terui,
Takuya Satoh,
Rai Moriya,
B. A. Ivanov,
Kazuya Ando,
Eiji Saitoh,
Tsutomu Shimura,
Kazuo Kuroda
Abstract:
We report the two-dimensional propagation of photoinduced spin wave packets in Bi-doped rare-earth iron garnet. Spin waves were excited nonthermally and impulsively by a circularly polarized light pulse via the inverse Faraday effect. Space- and time resolved spin waves were detected with a magneto-optical pump-probe technique. We investigated propagation in two directions, parallel and perpendicu…
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We report the two-dimensional propagation of photoinduced spin wave packets in Bi-doped rare-earth iron garnet. Spin waves were excited nonthermally and impulsively by a circularly polarized light pulse via the inverse Faraday effect. Space- and time resolved spin waves were detected with a magneto-optical pump-probe technique. We investigated propagation in two directions, parallel and perpendicular to the magnetic field. Backward volume magnetostatic waves (BVMSWs) were detected in both directions. The frequency of BVMSWs depends on the propagation direction. The experimental results agreed well with the dispersion relation of BVMSWs.
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Submitted 12 July, 2011;
originally announced July 2011.
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Spectral dependence of photoinduced spin precession in DyFeO3
Authors:
Ryugo Iida,
Takuya Satoh,
Tsutomu Shimura,
Kazuo Kuroda,
B. A. Ivanov,
Yusuke Tokunaga,
Yoshinori Tokura
Abstract:
Spin precession was nonthermally induced by an ultrashort laser pulse in orthoferrite DyFeO3 with a pump-probe technique. Both circularly and linearly polarized pulses led to spin precessions; these phenomena are interpreted as the inverse Faraday effect and the inverse Cotton-Mouton effect, respectively. For both cases, the same mode of spin precession was excited; the precession frequencies and…
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Spin precession was nonthermally induced by an ultrashort laser pulse in orthoferrite DyFeO3 with a pump-probe technique. Both circularly and linearly polarized pulses led to spin precessions; these phenomena are interpreted as the inverse Faraday effect and the inverse Cotton-Mouton effect, respectively. For both cases, the same mode of spin precession was excited; the precession frequencies and polarization were the same, but the phases of oscillations were different. We have shown theoretically and experimentally that the analysis of phases can distinguish between these two mechanisms. We have demonstrated experimentally that in the visible region, the inverse Faraday effect was dominant, whereas the inverse Cotton-Mouton effect became relatively prominent in the near-infrared region.
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Submitted 7 March, 2011; v1 submitted 23 September, 2010;
originally announced September 2010.
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Spin Oscillations in Antiferromagnetic NiO Triggered by Circularly Polarized Light
Authors:
Takuya Satoh,
Sung-Jin Cho,
Ryugo Iida,
Tsutomu Shimura,
Kazuo Kuroda,
Hiroaki Ueda,
Yutaka Ueda,
B. A. Ivanov,
Franco Nori,
Manfred Fiebig
Abstract:
Coherent spin oscillations were non-thermally induced by circularly polarized pulses in fully compensated antiferromagnetic NiO. This effect is attributed to an entirely new mechanism of the action, on the spins, of the effective magnetic field generated by an inverse Faraday effect. The novelty of this mechanism is that spin oscillations are driven by the time derivative of the effective magnet…
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Coherent spin oscillations were non-thermally induced by circularly polarized pulses in fully compensated antiferromagnetic NiO. This effect is attributed to an entirely new mechanism of the action, on the spins, of the effective magnetic field generated by an inverse Faraday effect. The novelty of this mechanism is that spin oscillations are driven by the time derivative of the effective magnetic field acting even on "pure" antiferromagnets with zero net magnetic moment in the ground state. The measured frequencies (1.07 THz and 140 GHz) of the spin oscillations correspond to the out-of-plane and in-plane modes of antiferromagnetic magnons.
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Submitted 3 March, 2010;
originally announced March 2010.
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X-ray Anomalous Scattering of Diluted Magnetic Oxide Semiconductors: Possible Evidence of Lattice Deformation for High Temperature Ferromagnetism
Authors:
Takeshi Matsumura,
Daisuke Okuyama,
Shinya Niioka,
Hideaki Ishida,
Tadashi Satoh,
Youichi Murakami,
Hidemi Toyosaki,
Yasuhiro Yamada,
Tomoteru Fukumura,
Masashi Kawasaki
Abstract:
We have examined whether the Co ions crystallographically substitute on the Ti sites in rutile and anatase Ti_{1-x}$Co$_{x}$O$_{2-delta}$ thin films that exhibit room-temperature ferromagnetism. Intensities of the x-ray Bragg reflection from the films were measured around the $K…
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We have examined whether the Co ions crystallographically substitute on the Ti sites in rutile and anatase Ti_{1-x}$Co$_{x}$O$_{2-delta}$ thin films that exhibit room-temperature ferromagnetism. Intensities of the x-ray Bragg reflection from the films were measured around the $K$-absorption-edge of Co. If the Co ions randomly substitute on the Ti sites, the intensity should exhibit an anomaly due to the anomalous dispersion of the atomic scattering factor of Co. However, none of the anatase and rutile samples did exhibit an anomaly, unambiguously showing that the Co ions in Ti$_{1-x}$Co$_{x}$O$_{2-delta}$ are not exactly located at the Ti sites of TiO$_2$. The absence of the anomaly is probably caused by a significant deformation of the local structure around Co due to the oxygen vacancy. We have applied the same method to paramagnetic Zn$_{1-x}$Co$_{x}$O thin films and obtained direct evidence that the Co ions are indeed substituted on the Zn sites.
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Submitted 28 August, 2007;
originally announced August 2007.
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Spin Fluctuation and Crystal Field Excitation of a Heavy Fermion Compound YbAgGe studied by Inelastic Neutron Scattering
Authors:
T. Matsumura,
H. Ishida,
T. J. Satoh,
K. Katoh,
Y. Niide,
A. Ochiai
Abstract:
Inelastic neutron scattering experiment has been performed on a new heavy fermion compound YbAgGe for a polycrystalline sample at zero magnetic field. A quasielastic scattering and a crystal field excitation at 12 meV was observed. Both of them are broadened with an intrinsic width of 0.9 meV at 1.5 K which increases to about 3 meV at 300 K. The temperature dependence follows Gamma_0+AT^(1/2) ty…
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Inelastic neutron scattering experiment has been performed on a new heavy fermion compound YbAgGe for a polycrystalline sample at zero magnetic field. A quasielastic scattering and a crystal field excitation at 12 meV was observed. Both of them are broadened with an intrinsic width of 0.9 meV at 1.5 K which increases to about 3 meV at 300 K. The temperature dependence follows Gamma_0+AT^(1/2) typically observed in Kondo systems. Crystal field parameters were also deduced. To explain the result that only one excitation isobserved at 12 meV, it is concluded that the B22 term should be the main component.
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Submitted 21 July, 2004;
originally announced July 2004.
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Vibrational and acoustical properties of a liquid drop in the phase-separated fluid with a highly mobile interface
Authors:
S. N. Burmistrov,
T. Satoh
Abstract:
We study the oscillation spectrum and acoustic properties of a liquid drop in the phase-separated fluid when the interfacial dynamics of phase conversion can be described in terms of the kinetic growth coefficient. For a readily mobile interface, i.e., as the growth coefficient becomes comparable with a reciprocal of the acoustic impedance, anomalous behavior is found in the oscillation spectrum…
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We study the oscillation spectrum and acoustic properties of a liquid drop in the phase-separated fluid when the interfacial dynamics of phase conversion can be described in terms of the kinetic growth coefficient. For a readily mobile interface, i.e., as the growth coefficient becomes comparable with a reciprocal of the acoustic impedance, anomalous behavior is found in the oscillation spectrum of a drop as well as in the velocity and absorption of a sound wave propagating through a suspension of drops in the two-phase system. Compared with the known case of two immiscible fluids, the high interface mobility leads to an anomalous softening of the radial drop pulsations and to the frequency- and temperature-dependent behavior for the sound velocity and absorption coefficient in a two-phase suspension.
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Submitted 29 June, 2001;
originally announced June 2001.