|氏名： 渡邉 伊吹
指導教員名： 岩佐 義宏 教授
発表題目（英語）： Anomalous photovoltaic effect in van der Waals interface
要旨（英語）： Since the discovery of graphene, various two-dimensional van der Waals (vdW) crystals with diverse electronic properties has been studied. Each layer in vdW crystals is weakly bound to neighboring layers by van der Waals interaction, so they are feasible to isolate and stack on different crystal to form vdW heterostructures. vdW heterostructures has been studied from the viewpoint of the atomically flat substrates or the building blocks for functional devices with the heterostructure of layered material with different electronic properties. In addition, nowadays, symmetry engineering is focused on as the new research viewpoint, such as Moiré physics.
In this presentation, I will introduce the study about the anomalous photovoltaic effect in van der Waals interface with broken inversion symmetry.
|氏名： 元 志喜
指導教員名： 志村 努 教授
発表題目（英語）： Measurement of lateral optical force on gold nanorod pairs
要旨（英語）： Two gold nanorods of different lengths can produce highly directional scattering in the plane perpendicular to the direction of propagation of incident light(lateral scattering). As a recoil of this scattering, lateral force acts on a pair of gold nanorods in the opposite direction of the scattering. And I showed that not only the absolute value but also the sign (direction) of this force can be changed depending on the refractive index of surrounding materials by finite element method(FEM).
To verify this in experiment, it is necessary to keep the relative position between two gold nanorods during the measurement, which is impossible when nanorods are directly trapped by optical tweezers. But if we can hold the relative position of two rods by placing them inside the micro-sized solid materials, it may be possible to measure the lateral force by trapping the micro-sized solid materials instead of the rods.
In this presentation, I will explain the principle of the measurement method mentioned above and report the progress with some experimental results.
 Evlyukhin, A. B. et al., Nano Lett. 10, 4571-4577(2010).
 Won, J. et al., The 80th JSAP Autumn Meeting 2019
|氏名： SAIKA Bruno Kenichi
指導教員名： 石坂 香子 教授
発表題目（英語）： Electronic structure of CrxNbSe2 epitaxial thin films studied by angle-resolved photoemission spectroscopy
要旨（英語）： In van der Waals (vdW) layered materials, covalent bonds extend into two-dimensional (2D) structures which interact with other layers via weak vdW forces. Such materials have attracted attention due to the novel physical properties appearing in low dimensions. In addition, further modification to vdW layered materials can be performed via the intercalation of dopant atoms to the layered structures. The intercalation atoms sit at the vdW gaps between different layers, constructing superlattice structures which depend on the doping amount. In the layered transition metal dichalcogenide (TMD), intercalation of *3d*-transition metal atoms has been widely studied due to the presence of magnetic phases with varied spin-ordering structures.
By employing molecular-beam epitaxy, single crystalline CrδNbSe2 was successfully fabricated. In this work, we focus on the electronic structure of MBE-grown CrδNbSe2 determined via angle-resolved photoemission spectroscopy (ARPES). We also performed Fermi-surface mapping which indicated the presence of a modified Fermi-surface beyond the simplistic rigid-band shift model. In the presentation, I will interpret results and discuss possible scenarios.
|氏名： 董 禹
指導教員名： 岩佐 義宏 教授
発表題目（英語）： Symmetry engineering in van der Waals interfaces
要旨（英語）： Van der Waals interface is an ideal platform to observe exotic physical phenomena and realize unique functionalities, because of rich material combination and alignment order . Recently, moire' potential or strain are found to be new degrees of freedom to tune interface electronic state [2-5]. Correlated electronic state and superconductivity in twisted heterostructure is one example [2-5]. Another example is the graphene/black phosphorus heterostructure , in which electronic state is affected by the periodic strain. Sample-wide pseudo-magnetic field is characterized by quantum transport results.
To further study exotic physical properties originated from van der Waals interface, we designed Gr/GeS and WSe2/BP interface. By aligning van der Waals materials with different symmetries, we can engineer the interfacial symmetry to create a polarized interface. We performed transport and optical study on both Gr/GeS and WSe2/BP interface, and detailed results will be shown in the presentation.
A. K. Geim and I. V. Grigorieva, Nature. 499, 419-425 (2013).
 Y. Cao, et al., Nature. 556, 43-50 (2018).
 R. Ribeiro-Palau, et al., Science. 361, 690-693 (2018).
Y. Cao, et.al., Nature. 583, 215-220 (2020)
K. Tran et al., Nature. 567, 71-75 (2019)
Y. Liu, et al., Nature nanotech. 12, 823-834 (2018)
|氏名： 任 統
指導教員名： 為ヶ井 強 准教授
発表題目（英語）： Growth and Characterizations of Iron-based Superconductor (Ba1-xRbx)Fe2As2 Single Crystals
要旨（英語）： Spontaneous C4 symmetry breaking has been widely reported in iron-based superconductors. Such a novel phenomenon named "nematic superconductivity" draws a lot of attention. In particular, novel nematic states with variable direction of the principal axis has been reported in hole-doped 122-type (Ba1-xRbx)Fe2As2 . On the other hand, in contrast to (Ba1-xKx)Fe2As2, not much is known on the normal state and superconducting properties of (Ba1-xRbx)Fe2As2.
In this study, we synthesized high-quality single crystals of (Ba1-xRbx)Fe2As2 close to the optimal composition of x ≈ 0.4, and have characterized normal state and superconducting properties for pristine/proton irradiated samples. The highest Tc was found to be ~38 K for pristine samples. The critical current density (Jc) in (Ba0.56Rb0.44)Fe2As2 at 2 K under self-field (sf) is ~1.7 MA/cm2, which is comparable to that in (Ba0.6K0.4)Fe2As2. After the H+ irradiation, Jc(2 K, sf) reaches 12 MA/cm2 at a dose of 3 × 1016 H+/cm2, which is comparable to (Ba1-xKx)Fe2As2 (11 MA/cm2 at x = 0.4, 5.8 × 1016 H+/cm2 ), but larger than those in Ba(Fe1-xCox)2As2 (2.5 MA/cm2 at x = 0.07, 1.2 × 1016 H+/cm2 ), and BaFe2(As1-xPx)2 (4.5 MA/cm2 at x = 0.33, 1.0 × 1016 H+/cm2 ). Jc at low temperatures follows the power-law dependence on the magnetic field (Jc ∝ H-α) with α changed from ~ 1/2 to ~ 1/3 after the H+ irradiation. Unusual peak effects with their peaks at around the self-field were observed in H+ irradiated samples. We tentatively interpret this phenomenon as being caused by the one-dimensional correlation among introduced point defects.
In addition, magneto-optical imaging based on the Faraday effect of an indicator garnet film will be employed to image superconducting state at temperatures below Tc. This technique may also allow us to image nematic domains, which will help us further explore its novel nematic state. In preparation for this observation at low temperatures, the presence of twin-boundaries is confirmed optically in underdoped samples below the structural transition.
 K. Ishida et al., Proc. Natl. Acad. Sci. USA, 117, 6424 (2020).
 T. Taen et al., Supercond. Sci. Technol. 28, 085003 (2015).
 T. Taen et al., Phys. Rev. B 86, 094527 (2012).
 A. Park et al., Phys. Rev. B 101, 224507 (2020).
|氏名： 刘 彦慷
指導教員名： 渡辺 悠樹 准教授
発表題目（英語）： A pedagogical introduction to Bethe ansatz
要旨（英語）： Bethe ansatz is a series of methods to exactly solve systems with interaction, first developed by Hans Bethe in 1930s. In this talk, I would like to introduce this method in detail for the ferromagnetic Heisenberg model. With the basic knowledge on hand, we can proceed further to the goal of the calculation of physical quantities, say, optical conductivity.