応用物理学輪講 I
10月14日
[注意事項]
発表の10日前までに　office[at]ap.t.u-tokyo.ac.jp　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
2022年10月14日（金）16：50～18：50

Aグループ

座長
梅村　洸介
指導
教員名
齊藤　英治　教授
発表者名 祖　宇航
指導教員名 為ヶ井　強　准教授
発表題目（英語） Abnormal current path in SrxBi2Se3 single crystals in the normal state
要旨（英語） Bi2Se3 is one of the representative topological insulators that show unique metallic surface state despite its insulating nature of the bulk. By doping Cu or Sr, Bi2Se3 can become a topological superconductor, which show novel properties such as the nematicity [1]. In this study, we prepared high-quality single crystals of SrxBi2Se3 and investigated its in-plane magnetic-field-angle dependence of resistivity. Resistivity (ρ) was measured at 2 K by applying the in-plane magnetic field along the direction with an angle φ from a-axis. We have measured the 4 percent sample (x = 0.04) and the 17 percent sample (x = 0.17) and only the 17 percent sample is superconductor. We show the in-plane magnetic-field-angle dependence of resistivity result as a function of φ at 8.5 T, which is defined by Δρ ≡ ρ - ρ0. Δρ(φ) mainly shows two-fold symmetry, or in other words nematicity. We fit the angular dependence of 17 percent sample by Δρ(φ) = ρ1cos(φ + α1) + ρ2cos(2φ + α2), where the 2nd term originates from the nematicity, magnetoresistance, or misalignment of the field from the in-plane direction. The fact that the minimum value of the 2φ component is at φ = 0o suggests that it is caused either by nematicity or by magnetoresistance. In the case of magnetoresistance, its magnitude should be proportional to the square of the transverse component of the field with respect to the current, (Bsin(φ))2, which means that it is proportional to cos(2φ). One of the possible origins of the 1st term is the Hall effect for currents flowing along the c-axis. If this interpretation is true, the Hall voltage should be zero at φ = 0 and p. However, the zero-value angle is offset by ~-35o. Such a shift can be explained by the shift of effective locations of the two voltage terminals, V+ and V-. Namely, if locations of the effective voltage terminals are not parallel to a-axis, the zero-value position of the Hall effect should change. We also fitted 4 percent sample and the result is Δρ(φ) = ρ1cos(φ + α1) + ρ2cos(2φ + α2) +ρ3cos(3φ + α1) + ρ4cos(4φ + α2). The 3rd and 4th terms may have physics meanning but it needs further research. The abnormal current may not have relationship with the superconductivity of SrxBi2Se3, because even low-doped SrxBi2Se3 shows it. With the x decrease, the amplitude of cos(φ) term decrease (Sr0.17Bi2Se3: 8%, Sr0.04Bi2Se3: 0.3%), which implies that there can be a critical value of x above which cos(φ) term appears due to inhomogeneous current flow. Different percent sample will be measured to find the critical x value when the cos(φ) term appears and we will try to find out the possible physical origins of the 3rd and 4th terms by analyzing more measurement results.
発表言語 英語
発表者名 ＷＵ　Wenxi
指導教員名 為ヶ井　強　准教授
発表題目（英語） Magnetic-field trapping in (Ba,Na)Fe2As2 superconducting bulk synthesized under high pressure
要旨（英語） Superconductors with a large critical current density (Jc) can trap a high magnetic field when their powders are pressed into bulk and sintered, which could be used as superconducting magnets. In this work, we attempted to improve the Jc characteristics and trapped field of bulk iron-based superconductors by increasing the pressure used in the manufacturing process. In this work, we attempted to improve the Jc and the trapped field of bulk iron-based superconductors by increasing the pressure used in the manufacturing process. In the present study, we prepared a cylindrical bulk polycrystalline Ba0.6Na0.4Fe2As2 superconductor with a diameter of 3.9 mm and 2.06 mm in thickness. Polycrystalline powder was synthesized by the mechanochemical method as described in Ref. [1] and was sealed in a silver tube with an inner diameter of 4 mm. A cold press process was performed in the glove box to texture the grains along c-axis. The sample was then put into a cubic press and sintered under a pressure of ~2.2 GPa at ~850 ℃ for 1 hour. The critical temperature of this bulk is ~33 K. A Hall probe was set at the position z ~0.38 mm from the bottom surface of the bulk to measure the trapped field. In the remanent state, measured magnetic induction is ~3,718 G. Jc under the self-field is estimated by Biot-Savart approximation [2] to be ~65.9 kA/cm2, which is larger than that in the previous work [3] but lower than the value in high-quality wires [1]. The trapped field on the surface of the bulk was finally calculated by the same approximation to be ~7,200 G. From XRD result measured on the top surface of the bulk, it can be seen that the ratio between the intensity of (002) peak and that of (103) peak increased from ~1/5 in random powder to ~1/3 in the bulk, suggesting that grains are textured along c-axis. However, this ratio is still much lower compared with the result in well-textured tapes [4]. We will next try to use higher pressure for the cold pressing to enhance the texturing and to further improve Jc and trapped field.
　
Reference
[1] S. Pyon et al., Supercond. Sci. Technol. 33, 065001 (2020).
[2] T. Naito et al., Supercond. Sci. Technol. 25, 095012 (2012).
[3] J. D. Weiss et al., Supercond. Sci. Technol. 28, 112001 (2015).
[4] Z. Gao et al., Sci. Rep. 4, 4065 (2014).
発表言語 英語
発表者名 万　秋明
指導教員名 芦原　聡　教授
発表題目（英語） Ultrafast vibrational spectroscopy and vibrational control of electrochemical reactions.
要旨（英語） Mid-infrared (mid-IR) laser pulses can directly excite a specific molecular vibration enabling us bond-selective control of chemical reactions. So far, we have achieved vibrationally-mediated bond dissociation of liquid-phase W(CO)6 molecules by using down-chirped mid-IR laser pulses enhanced by surface plasmon resonance. This study aims to apply our method to electrochemical reactions on metal surfaces. We are developing a pump-probe spectroscopy system in the attenuated total reflectance (ATR) geometry for vibrational excitation of the reactants and monitoring the following electrochemical reaction processes. In this presentation, we will talk about the scheme to achieve our goal and present recent progress.
発表言語 英語

Bグループ

座長
大竹　雄太郎
指導
教員名
求　幸年　教授
発表者名 相原　孝広
指導教員名 高橋　陽太郎　准教授
発表題目（英語） Observation of shift current by electromagnon excitations in multiferroics RMn2O5
要旨（英語） Shift current is the photocurrent which arises when the real-space center of charge shifts by light excitation and has attracted much recent attention in terms of the quantum-mechanical geometric phase of electronic states. This shift current was thought to arise with interband transition, but it is now known that they are also induced by elementary excitations, such as phonon. In this presentation, we focus on the shift current induced by electromagnon, an elementary excitation specific to multiferroics in the terahertz region. It was theoretically proposed in 2019 [1] and successfully observed for the first time in our lab.
In this session, after explaining the shift current mechanism, the basic physical properties of the target material multiferroics RMn2O5, measurement methods, and experimental results will be presented.

[1] T. Morimoto et al., Phys. Rev. B 100, 235138 (2019).
発表言語 日本語
発表者名 青木　俊太
指導教員名 岩佐　義宏　教授
発表題目（英語） Photovoltaic effect in a van der Waals antiferromagnet
要旨（英語） Since the discovery of van der Waals 2D ferromagnets [1], a variety of novel 2D magnets, which can be exfoliated down to mono- or few-layer crystals, have been explored. Among them, 2D antiferromagnets are attracting much attention due to their versatile magnetic structures and resultant complex symmetries. Especially, characteristic properties originating from magnetic parity breaking, i.e., inversion symmetry breaking induced by the magnetic ordering, have been reported in several 2D antiferromagnets [2]. Here we studied the photovoltaic response reflecting the symmetry breaking in a 2D antiferromagnet. Finite zero-bias photocurrent has been observed in graphene-sandwiched few-layer 2D antiferromagnet devices. In the presentation, detailed temperature dependence of the photocurrent and possible relationship with the antiferromagnetic ordering will be discussed.

[1] B. Huang, *et al., Nature, **546* 270-273(2017)
[2] Z. Ni, *et al., Nat. Nanotech., **16* 782-787(2021)
発表言語 日本語
発表者名 赤塚　俊輔
指導教員名 石坂　香子　教授
発表題目（英語） Breaking of spatial inversion symmetry in 180-degree-twisted bilayer ReSe2
要旨（英語） The development of mechanical exfoliation and dry-transfer techniques has made it possible to stack two-dimensional flakes dynamically and fabricate new materials that cannot be synthesized through thermodynamic processes. In such composite flakes, changes in the symmetry of the crystal structures can lead to emergent physical properties that would not appear in each component flake alone.
Here, we fabricated a 180-degree-twisted bilayer (180-BL) ReSe2 with a spatial-inversion-symmetry broken crystal structure by stacking monolayer ReSe2 flakes with a centrosymmetric 1T-distorted crystal structure. By angle-resolved photoemission spectroscopy and second harmonic generation, we observed the emergent band dispersions and the artificially induced spatial-inversion-symmetry breaking in the 180-BL ReSe2. Our result fully demonstrates the potential of creating new materials that can exhibit spintronic functions and Berry-curvature-related physical phenomena by controlling the presence or absence of spatial inversion symmetry.
発表言語 日本語
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