応用物理学輪講 I
7月9日
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発表の10日前までに宛てに「氏名」「指導 教員」「発表題目(英語)」「要旨(英語)」「発表言語(英語または日本語)」を送付して下さい。
発表日
2021年7月9日(金)16:50〜

Aグループ

座長
荻原 琢磨
指導
教員名
求 幸年 教授
発表者名 松前 拓実
指導教員名 為ヶ井 強 准教授
発表題目(英語) Growth of topological superconductor SrxBi2Se3 single crystals and annealing effect on the superconducting transition
要旨(英語) Introduction:
Topological insulator Bi2Se3 becomes a superconductor by doping Cu, Sr. This material attracts much interest because majorana particles, whose application in topological quantum computing is proposed, are expected to appear on the surface. However, it is known that obtaining high quality samples AxBi2Se3 (A=Cu, Sr) is difficult.

Goals:
To get optimal samples, which is significant to develop research in topological matters, I focused on growing crystals (a) with high Tc (~ 3.0 K), (b) which shows a sharp superconducting transition (ΔTc < 1 K), and (c) have a large amount of volume fraction (~ 100 %).

Method:
Single crystals SrxBi2Se3 were synthesized with Sr composition 0.1 < x < 0.3 by the self-flux method in different temperature conditions. To improve the sample quality, obtained crystals were annealed in different ways. Superconducting properties were evaluated by measuring temperature dependence of magnetization and magnetic field dependence of magnetization.

Results:
It was indicated that the most significant factor to make samples SrxBi2Se3 high quality is temperature. Conventionally, quenching is thought to be essential for superconductivity to emerge, which has been shown in Cu-doped Bi2Se3, but the results revealed this was not true in Sr-doped one because samples obtained by furnace cooling showed the highest Tc. Annealing also played a significant role particularly in improving sharpness of the superconducting transition. As a result, a recipe for good samples SrxBi2Se3 was achieved based on these experiments.

Prospects:
Using the best sample obtained, further experiments are planned to determine lower critical filed Hc1 and upper critical field Hc2, from which penetration depth λ and coherence length ξ are calculated by Ginzburg-Landau theory. Magnetic-field-angle dependence of Hc2 is also planned to be measured.
発表言語 日本語
発表者名 松山 直史
指導教員名 小濱 芳允 准教授
発表題目(英語) Quantum oscillations in a centrosymmetric skyrmion-hosting material
要旨(英語) Magnetic skyrmions, topologically stable spin swirls with a particle-like character, have been intensively investigated as a candidate for high-density information bits. Previously, skyrmions have been discovered in non-centrosymmetric materials. This lack of symmetry causes Dzyaloshinskii-Moriya (DM) interaction, which favors a twisted spin texture, and have been thought to be essential for the skyrmion formation. However, recent theories predicted that skyrmions can be stabilized even in centrosymmetric materials. Indeed, very recently, skyrmions have been observed in several centrosymmetric Gd compounds[1,2].
 GdRu2Si2 is one of such centrosymmetric skyrmion-hosting materials[2]. Its magnetic behavior is well explained by the model where the magnetism is mediated by itinerant electrons[3], though the first principle calculation predicted that the itinerant-electron-mediated interaction in this material is negligibly small[4]. Thus, the actual mechanism of skyrmion formation is still under debate.
 In this research, to observe quantum oscillations, we performed magnetic-torque measurements. Based on the observed quantum oscillations, I will discuss the electronic properties and the relationship to the magnetism in GdRu2Si2.
[1] T. Kurumaji, et al., Science 365, 914 (2019).
[2] N. D. Khanh, et al., Nat. Nanotechnol. 15, 444 (2020).
[3] S. Hayami, et al., Phys. Rev. B 95, 224424 (2017).
[4] T. Nomoto, et al., Phys. Rev. Lett. 125, 117204 (2020).
発表言語 日本語
発表者名 宮田 稜
指導教員名 長谷川 達生 教授・中嶋 健 教授
発表題目(英語) Improved Performance and Its Origin in Organic TFT Fabricated from Small Molecule Organic Semiconductor / Insulating Polymer Blended Solution
要旨(英語) Organic semiconductors (OSC) have the advantage of being soluble, so they are expected to application to printed electronics, where electronic devices are fabricated by printing. Among organic devices, we focus on polycrystalline TFT (thin film transistor), which have polycrystalline OSC thin film as active layer. Polycrystalline thin film is so easy to fabricate that polycrystalline TFT are expected to the future application. However, in solution process, it is difficult to obtain high-crystallinity, uniform, and large-area thin film due to strong intermolecular interaction, leading to poor TFT performances and its reproducibility. Therefore, it is important to control microstructure of polycrystalline thin films to realize high-performance TFT. One of the effective ways to control thin film morphology and improving device performance is fabricating thin film from OSC and insulating polymer blended solution. Despite only adding an insulating polymer to the solution, its effectiveness has been reported in TFT fabricated from various OSC/polymer matrix. However, its origin of such significant improvement remained unclear in terms of how polymer blend solution affects the mechanism of thin film formation and carrier transport.
 We have succeeded in producing TFTs with the highest-level performance and processability in the world, such as high carrier mobility exceeding 4 cm²/Vs and high reproducibility by spin-coating from 2-C8-BTNT and PMMA blended solution. In this presentation, I will discuss the improvement and its origin of TFT properties fabricated from OSC/polymer blended solution by decoding thin film structure from mechanical properties measured by atomic force microscopy.
発表言語 日本語

Bグループ

座長
大島 敦史
指導
教員名
石坂 香子 教授
座長
大西 由吾
指導
教員名
永長 直人 教授
発表者名 武藏 摩紀
指導教員名 中野 匡規 特任准教授
発表題目(英語) Nonreciprocal transport in noncentrosymmetric superconductor 3R-TaSe2
要旨(英語) Inversion symmetry breaking has a great effect on superconductors. Its characteristics differ depending on whether the polarity exists in the in-plane direction or the out-of-plane direction. When inversion symmetry is broken in the in-plane direction, it is called a Rashba superconductor, and when it is broken in the out-of-plane direction, it is called an Ising superconductor. These superconductors show peculiar physical properties by locking the spins in a specific direction due to spin-orbit interaction. For example, Ising superconductors have an extremely large in-plane critical magnetic field because the spins are locked in the direction perpendicular to the plane.
Recently, we have succeeded in making 3R-TaSe2 which is one of the transition metal dichalcogenide. This material shows superconducting at 3 K, and its structure is peculiar in that inversion symmetry is broken in both in-plane and out-of-plane direction. In this presentation, I will present the experimental results obtained by trying to observe the symmetry breaking in both directions in one sample of 3R-TaSe2. In the future, it is expected that the superconducting characteristics will be further specified by manufacturing various devices by taking advantage of the large size of the sample.
発表言語 日本語
発表者名 山野 新一郎
指導教員名 小芦 雅斗 教授
発表題目(英語) Continuous Variable Quantum key distribution using only heterodyne measurement
要旨(英語) Quantum Key Distribution (QKD), a kind of quantum cryptography, enable remote parties to share a secret key through quantum and classical communication. The security is guaranteed by quantum mechanics and QKD has been actively studied in recent years due to engineering and scientific interests. The type of QKD is divided into two categories; discrete variable (DV) and continuous variable (CV). In DV-QKD, polarization or time bin is taken as the information career. this type has been well studied in the past but various challenges are left for the implementation. On the other hand, In the CV-QKD, the information is encoded on the quadrature of coherent light, and decoded by homodyne or heterodyne measurement. CV has infinite dimensions and It is not easy to prove the security of the protocol, but It is higher efficiency and more compatible with existing communication technologies.
 In this talk, I will introduce the concept of general QKD and our recent research about the CV-QKD protocol. In the previous research [1], Two measurements are used for signal and eavesdropping detection. Homodyne measurement is used to obtain the bit-string signal and heterodyne measurement is used to estimate the amount of eavesdropping. In our research, we propose the way that both processes are handled by heterodyne measurement. This protocol eliminates the complex processing between Alice and Bob and can make it easier to incorporate into the communication systems. If there is more time, I explain another protocol using the symmetry of the phase space that we are currently working on.
[1] T. Matsuura et al., Nature Comm. 12, 252 (2020)
発表言語 日本語
発表者名 吉井 真央
指導教員名 森本 高裕 准教授
発表題目(英語) Fractal charge transport in one-dimensional quasi-periodic model
要旨(英語) Quasi-periodic system is a system which follows certain rules without periodicity. Among these systems, systems made up from periodic systems, such as the systems of multi-layer thin films and incommensurate magnetic field systems are well-known and understood. In contrast, dealing with quasi-periodic lattices is in general quite arduous.
Motivated by this, we study charge transport in quasi-periodic lattice systems[1]. Although it has already been investigated in quasi-periodic systems of incommensurate potential with k-space approximation, such approach is not applicable to all quasi-periodic systems. Therefore, we demonstrated a way to trace charge transport from real space. In order to investigate charge transport in a one-dimensional quasi-periodic system, we made two models based on the celebrated Fibonacci lattice and Rice Mele model. In this presentation, I will talk about the method to analyze the transport phenomena and the result of them.
[1] M. Yoshii et al., arXiv 2105.05654
発表言語 日本語