|氏名： WOLSKI Samuel
指導教員名： 中村 泰信 教授
発表題目（英語）： Investigating two-level systems in yttrium-iron garnet to improve magnon lifetimes
要旨（英語）： Magnons are the quanta of collective spin excitations in magnetically-ordered materials, and present a promising platform for developments in quantum information processing and quantum sensing. In the context of hybrid quantum systems, magnons in materials such as yttrium-iron garnet (YIG) have many desirable properties, but suffer from short lifetimes of around 100 ns. This limits their use for working with coherent quantum information alongside longer-lived systems such as superconducting qubits. I will present recent experiments suggesting evidence for a bath of two-level systems (TLSs) coupled to the magnetostatic modes in a YIG sphere. When the TLS bath is saturated with a strong microwave pump, its contribution to the decay of magnons in the magnetostatic modes of the YIG is suppressed, and the magnon lifetime is increased. Such a technique could enable carrying out experiments in quantum magnonics that are currently impractical due to short magnon lifetimes, such as the creation and manipulation of quantum states of magnons. Furthermore, the TLSs themselves appear to have a lifetime on the order of 10 s, suggesting an exciting avenue for further exploration with potential applications as a long-lived quantum memory.
|氏名： 余 同桦
指導教員名： 有田 亮太郎 教授
発表題目（英語）： First-Principles Materials Design of Magnetic Electrides with Nontrivial Band Topology
要旨（英語）： Electrides are ionic solids where the excess electrons distributed within the cationic framework serve as anions. Magnetized anionic electrons could offer a variety of applications such as spin injection. Nevertheless, explorations of properties and functionalities related to magnetism are limited due to the scarcity of stable magnetic electrides. Meanwhile, electrides are shown to be a promising platform for topological phases, owing to the interstitial states near the Fermi level. Here, we propose a systematic scheme of designing magnetic electrides using ab initio crystal structure prediction. As each natural electride has a possible oxidized form, we reverse this direction and exclude strong anions from a potential host compound, expecting a stable framework and excess electrons are left after global geometry optimization. Spin polarized anionic states are obtained by chemical substitution with magnetic atoms. In addition, topological nature is explored for these predicted electrides.
In this presentation, I will focus on the Y2Br2C2 system. By removing the bromine, we reach a stable two-dimensional electride Y2C2, demonstrated to be a nodal line semimetal (NLSM) in the absence of spin orbit coupling. A series of magnetic eletrides would be acquired when the yttrium is replaced by other lanthanides with large magnetic moments. Here I will take the ferromagnetic Gd2C2 as the main example for further analysis, which is also an NLSM.
|氏名： XU Tianyu
指導教員名： 小林 洋平 教授
発表題目（英語）： Laser process for superhydrophobic surface formation
要旨（英語）： Superhydrophobicity is the tendency of a surface to repel water drops. A surface can be qualified as a superhydrophobic surface if it has a high contact angle over than 150°. Due to their various use in industrial applications, such as water-resistant, antifogging, anti-icing , anticorrosion etc., many methods have been developed to create them. One of the keys of fabricating superhydroohobic surfaces is micro-nano structures, so that laser ablation, as an efficient way to create rough microstructures, has become one important techniques in these methods. However, it is difficult to find out the most suitable laser parameter for different materials and objects as a result of the complex mechanism of laser ablation.
In this presentation, I will share several experiments of superhydrophobic surfaces formation with different materials by laser process and simple chemical surface modification, and discuss the relation between laser process parameter and performance of superhydrophobic surfaces.
|氏名： 石塚 悠也
指導教員名： 小林 洋平 教授
発表題目（英語）： Development of mid-infrared comb using femtosecond mode-locked laser
要旨（英語）： Femtosecond laser has wide applications such as laser processing, ultrafast dynamics observation, and spectroscopy and is obtained by passive mode-locking using Kerr lensing and nonlinear polarization rotation.
MIR spectral region is called the fingerprint region and has various absorption lines of molecules. Therefore, the mid-infrared (MIR) spectroscopy using femtosecond laser is expected to realize breath diagnosis. We have already developed high sensitivity MIR spectroscopy system between 3-5um. To cover long-wavelength side enables spectroscopy of aromatic compounds. For this spectroscopy system, a MIR comb between 8-12um is needed and is constructed with femtosecond laser and nonlinear processes such as supercontinuum generation and difference frequency generation.
In this presentation, I will introduce the development of femtosecond laser based on Yb-doped Y_2 O_3 ceramic and the setup of MIR comb between 8-12um.
|氏名： 天野 元揮
指導教員名： 山地 洋平 特任准教授
発表題目（英語）： Concavity: a touchstone of the second law of thermodynamics
要旨（英語）： In the study of the second law of thermodynamics in quantum systems, unitary operators for time evolution are often used to represent a quasi-static process. Although unitary time evolution is
very useful in theoretical studies, it is difficult to construct the unitary representation of quasi-static time evolution. Even though the quasi-static process is a typical thermodynamic process, the quantum-mechanical representation of the quasi-static process has remained nontrivial.
These days, a new criterion of the second law of thermodynamics is proposed, which is called concavity. This criterion states that the principle of maximum work for adiabatic processes in the case of a
quench process, this is a special case of the second law of thermodynamics, is valid if and only if the corresponding energy eigenvalue is a concave function of the system’s coupling constant. In this criterion, the time evolution of the system is controlled by the coupling constant, instead of unitary operators. Then, in this framework, the quantum-mechanical representation of the quasi-static process becomes well-defined. Moreover, we can examine whether a specific eigenstate follows the second law of thermodynamics or not by numerically calculating the coupling constant dependence of the corresponding eigenvalue.
In this presentation, we will introduce this new criterion, concavity, and how to numerically examine the concavity of a given eigenstate.
|氏名： 伊藤 宏陽
指導教員名： 川﨑 雅司 教授
発表題目（英語）： Synthesis of thin films of pyrochlore oxides with possible flat band structure.
要旨（英語）： Flat band structure is one of the strongly correlated electron systems, which leads to some unusual physical properties such as emergence of ferromagnetism without magnetic elements and superconductivity. In spite of such fascinating properties, only theoretical studies have been reported due to the lack of actual materials possessing the flat band structure. However, since the first experimental realization in a twisted bilayer graphene in 2018, materials with the flat band structure have been of great interest in the field of condensed matter physics.
Here, we focus on pyrochlore oxides (A2B2O7) which are theoretically predicted to be another candidate of the host of flat band structure. Among them, Sn2Nb2O7, Sn2Ta2O7 and Pb2Ta2O7 are specifically expected to be promising compounds according to some first principle calculations. We aim to synthesize high-quality single crystalline thin films of them by pulsed laser deposition method in order to materialize the flat band structure.
In this presentation, I will explain why ferromagnetism emerges in the flat band system, and why pyrochlore oxides can be candidates of it. Besides, I will introduce my experimental results so far.