応用物理学輪講 I
5月27日
[注意事項]
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発表日
2022年5月27日(金)16:50~18:50

Aグループ

座長
村田 啓人
指導
教員名
長谷川 達生 教授
座長
森 春仁
指導
教員名
鹿野田 一司 教授
発表者名 飯塚 太一
指導教員名 吉岡 孝高 准教授
発表題目(英語) Studying growth mechanism and elastic characteristics of layered organic semiconductors; An attempt using in-situ capillary wave analysis
要旨(英語) Solution-processable organic semiconductors (OSCs) attract considerable recent attentions because of their potential applicability into flexible and wearable electronics. Among them, the OSCs composed of rod-like π-electron cores linked with long alkyl chains are shown to exhibit very high layered crystallinity, which is a critical feature for obtaining high performance organic thin-film transistors (OTFTs). More importantly for the device application, it was recently demonstrated that these OSCs allow non-epitaxial growth of extremely large-area single-crystalline thin films by solution processes, although the film formation mechanism has not yet been disclosed. In this talk, I will first discuss a proposed solution-based film-growth model in which the molecular layer formation should take place at around liquid-air interface and undergo stepwise ordering via liquid-crystalline-like layer [1]. This intriguing model obtained by molecular dynamics simulation analysis is essentially different from conventional crystal growth, though further experimental evidence is necessary to establish it. I would like to show our study to investigate experimentally the unique film-formation by using a technique of in-situ excited capillary wave observation [2]. I present our attempts and results to assess the elastic characteristics of the OSC layer, which is a crucial property for the flexible electronics application.

[1] Yoneya, M., Minemawari, H., Yamada, T., Hasegawa, T., J. Phys. Chem. C 2017, 121, 8796.
[2] Slavchov, Radomir I., Boyan Peychev, and A. Said Ismail. Phys. Fluids 2021, 33, 101303.
発表言語 日本語
発表者名 魚住 亮介
指導教員名 吉岡 孝高 准教授
発表題目(英語) Development of a high-resolution probing laser suited for cold positronium Doppler spectroscopy
要旨(英語) Cooling of positronium (Ps), an electron-positron bound state, down to a temperature below 10 K leads to important research such as precision measurements of energy intervals or realizing Bose-Einstein condensation. Laser cooling and other cooling methods are on intense research to achieve this goal. On this occasion, evaluating the temperature of cold Ps is an important task to be undertaken. Obtaining the Doppler broadening by laser spectroscopy is one of the preferred methods since it allows high-resolution measurements regarding the laser technology of today.
To conduct efficient and high-resolution laser spectroscopy, the probing laser should have unique frequency structures along with sufficient laser power. As for the frequency structure, the linewidth of the laser should be optimized between the balance of high resolution and quantity of Ps at resonance, while the longitudinal mode interval should be comparable to the natural width of Ps Lyman-??(50 MHz) to efficiently excite Ps inside the laser linewidth. Adequate laser power would also be a demand to excite enough Ps in a cloud whose typical size is around 10 mm to realize a high signal-to-noise ratio.
Since commercially available lasers do not satisfy all of the requirements at the same time, we have developed a frequency-tunable UV pulsed laser. Properties of this laser was measured to fulfill the demanding frequency structures. The output is also confirmed to be powerful enough through a numerical simulation, which is based on optical Bloch equations under laser field whose frequency structure are realistically modelled. We will also introduce preliminary results of experiments in which the excitation of Ps by irradiating the newly developed laser was observed.
発表言語 日本語
発表者名 石﨑 雄士
指導教員名 福谷 克之 教授
発表題目(英語) Lifting of the (√3× √3)R30° structure of Ag(111)-Bi by Adsorption of Deuterium
要旨(英語) The Ag(111)-Bi reveals a fascinating feature of giant Rashba-splitting with a (√3× √3)R30° superstructure [1], which is potentially applied to spintronics devices. To investigate the spin exchange at the surface, we aim to observe the spin-charge exchange corresponding to the inverse-Edelstein effect [2] with a spin-polarized atomic hydrogen beam [3]. As a first step towards this purpose, we have conducted LEED and TDS experiments on the deuterium adsorption on Ag(111)-Bi surfaces to clarify the interaction between hydrogen and the Ag(111)-Bi surface in the present study.
The Ag(111)(√3× √3)R30°-Bi was prepared by depositing Bi on Ag(111) in an ultra-high vacuum chamber, which was confirmed with LEED and AES. After formation of the superstructure, the sample surface was exposed to deuterium atoms at 85-90 K and the adsorption properties of deuterium atoms on Ag(111)-Bi were evaluated with thermal desorption spectroscopy (TDS). The results of TDS showed two peaks at 160 and 240 K. Since the D desorption on the Ag(111) occurs at 220 K, the peak at 240 K seems to be related to adsorption at Ag atoms, while the peak at 160 K is associated with adsorption at Bi atom. With increasing exposure, the peak at 160 K shifts to a higher temperature whereas the peak at 240 K shifts to a lower temperature. Furthermore, the (√3×√3)R30° was found to change to (1x1) after a prolonged exposure as observed by LEED. Note that the superstructure remained at 190 K even with a long exposure of D. Based on these results, we argue that the Bi atoms in the surface moved to subsurface sites by D adsorption forming a disordered state.

References:
[1] C. R. Ast, et. al., Phys. Rev. Lett. 98, 186807 (2007).
[2] V. M. Edelstein, Solid State Commun. 73, 233 (1990).
[3] Y. Nagaya, et. al., J. Chem. Phys. 155, 194201 (2021).
発表言語 日本語
発表者名 宇都 隆宏
指導教員名 志村 努 教授
発表題目(英語) Helical dichroism in plasmonic nanostructures
要旨(英語) Besides spin angular momentum(SAM), as the new degree of freedom of Light, orbital angular momentum(OAM) has been attracting attention in recent years due to its physical properties and applications[1]. In particular, people have been actively studying the optical response depending on OAM, called helical dichroism or optical vortex dichroism, caused by the interaction with chiral matters[2].
In this presentation, I will talk about the microspectroscopy imaging system with a Laguerre-Gaussian beam, one of the lights with OAM. I will also discuss the measurement results with chiral plasmonic nanostructures, which have simple plasmonic modes and are relatively easy to analyze.

[1]: M. J. Padgett, Orbital angular momentum 25 years on [Invited]. Opt. Express 25, 11265–11274 (2017).
[2]: K. A. Forbes and D. L. Andrews, Orbital angular momentum of twisted light: chirality and optical activity, J. Phys. Photon. 3, 022007 (2021).
発表言語 日本語

Bグループ

座長
安波 貴広
指導
教員名
川﨑 雅司 教授
発表者名 池田 直樹
指導教員名 関 真一郎 准教授
発表題目(英語) Skyrmion in itinerant magnet
要旨(英語) Magnetic skyrmion is a vortex-like swirling magnetic structure with particle nature, which is recently attracting much attention as a potential information carrier. Magnetic skyrmions were originally discovered in a series of materials with broken space-inversion symmetry, where they are stabilized by DM interaction. On the other hand, the formation of nanometric skyrmions has recently been identified in a few itinerant
magnets, where RKKY interaction plays an important role for their stabilization.
In this study, we newly investigate some Eu/Gd-based itinerant magnets with and without space inversion symmetry, in which either RKKY or DM interaction can potentially stabilize magnetic skyrmion. I will talk about their basic physics properties and associated spin textures in these systems.
発表言語 日本語
発表者名 池田 侑哉
指導教員名 森本 高裕 准教授
発表題目(英語) Photocurrent generation by illumination of bicircular light
要旨(英語) Nonlinear optical responses in solids, including photovoltaic effect and harmonic generation, are physical phenomena triggered by strong light irradiation to materials. These are of great importance in studying the physics of excited states of the system as well as for applications to optical devices and solar cells. In particular, bulk photovoltaic effect (BPVE), namely DC photocurrents in response to uniform AC external fields, have attracted great attention as related to the Berry phase and the band topology of materials [1]. Here we propose the mechanism of a novel photocurrent induced by two-frequency drive with bicircular light (BCL), which can control the rotational symmetry of the system [2]. While ordinary second-order photocurrents can exist only in systems lacking inversion symmetry, BCL driving can induce photocurrents in materials with inversion or rotational symmetry. We show theoretically, using Keldysh-Floquet formalism and Feynman diagram, that BCL irradiation can produce third-order photocurrent and dynamically control the direction of the current.

[1] T. Morimoto and N. Nagaosa, Sci. Adv. 2, e1501524 (2017)
[2] Y. Ikeda et. al., Prog. Theor. Exp. Phys. ptab127 (2021)
発表言語 日本語
発表者名 梅村 洸介
指導教員名 齊藤 英治 教授
発表題目(英語) Time resolved magneto-optical imaging of nonlinear excited spin waves
要旨(英語) Spin waves are magnetization dynamics in which the precession of magnetization propagates as a wave. Spin waves have strong nonlinearities due to dipole and exchange interactions, and there are abundant nonlinear processes between spin waves with different wavenumbers. An example of a linear excitation process is ferromagnetic resonance (FMR), which can excite a uniform precession of magnetization, i.e., a spin-wave with wavenumber zero. On the other hand, there is a second-order Suhl process, in which two wavenumber-zero magnons transform into two magnons with finite wavenumbers. These excitation dynamics have been observed by microwave spin-wave spectroscopy [1] and inelastic scattering [2], but magnetization dynamics in real space have not been observed due to measurement method limitations.
In this study, second-order Suhl process is observed by spin-wave spectroscopy based on time-resolved magneto-optical imaging[3].
Time-resolved magneto-optical imaging is a method to observe magnetization in magnetic materials in real space by measuring the change in polarization of light u ing the Faraday effect. In the experiment, FMR was induced in a magnetic garnet (Bi1Lu2Fe3.4Ga1.6O12) thin film with a relatively large Faraday rotation angle, and the distribution of each spin-wave mode was obtained by Fourier transforming the captured magnetization dynamics to analyze spin waves with finite wave numbers. As a result, it was confirmed that the amplitude of the spin-wave with finite wavenumber behaves in a threshold manner with respect to the intensity of the pump microwave. In the presentation, we will discuss the measured threshold behavior and the correlation between different spin waves.

[References]
1. N. Bloembergen and R. W. Damon Phys. Rev. 85, 699 (1952).
2. P. Kabos, et al., J. Appl. Phys. 80, 3962 (1996).
3. Y. Hashimoto, et al, Nat. Commun. 8, 15859 (2017).
発表言語 日本語