|氏名： 早野 陽紀
指導教員名： 古川 亮 准教授
発表題目（英語）： Rheology of active suspensions
要旨（英語）： Active matter is a class of materials that consume energy and turn it into mechanical work. Examples of active matter include all forms of living matter ranging from self organization of cells and bacteria to schools of fish and flocks of birds. In our study, we especially focus on swimming microorganisms (microswimmers) that live in a fluid environment, such as E. coli and Chlamydomonas: they are self-propelled particles on the order of micro meters, moving through a fluid medium by non-reciprocal cyclic flagellar motion. Accompanying an active propulsion force, due to momentum conservation, momentum exchanges between a microswimmer and a suspending fluid occur, which inevitably induce a flow field around it. Such a self-propulsion induced flow determines hydrodynamic interactions among microswimmers, which are of purely dynamic origin, and thus results in novel collective behaviors and transport properties that are quite different from those observed in "passive" matter systems.
One of the most fascinating examples is that the viscosity of a dilute E. coli suspension is significantly smaller than that of a pure solvent, which indicates a violation of the well-known Einstein viscosity describing the viscosity of dilute passive particle suspensions.
While it has been generally thought that this anomalous rheological behavior is attributable to the anisotropic changes in the mean swimming directions under an external flow field, its detailed mechanism, especially the role of the hydrodynamic effect, is still debated today. We are aiming to elucidate this problem through a systematic numerical study of minimal microorganism models using a hybrid simulation method. In this talk, as well as presenting an overview of the active matter physics, I will briefly explain our research approach.
|氏名： 福田 光
指導教員名： 十倉 好紀 卓越教授
発表題目（英語）： Topological transport phenomena in non-coplanar magnets
要旨（英語）： In recent years, there has been a lot of research that has brought a topological perspective to physics, including the quantum Hall effect. Topological materials are considered to be candidates for the next generation of electronic devices because of their non-dissipative nature and robustness to fluctuations. In particular, according to the Berry phase theory, electrons in topological materials are subjected to a huge effective magnetic field (Berry curvature). It indicates that controlling Berry curvature is equivalent to controlling a huge effective magnetic field and non-dissipative current. However, it is difficult to control the Berry curvature of electronic bands with an external field because its energy scale is much larger than that of the external field. On the other hand, Non-coplanar magnetic structures endow itinerant electrons with a huge effective magnetic field. Therefore, in non-coplanar magnetic materials, the Berry curvature can be easily controlled by a magnetic field through the manipulation of spins. In this presentation, I will talk about topological transport phenomena coupled with non-coplanar magnetic moments.
|氏名： 半田 光
指導教員名： 高橋 陽太郎 准教授
発表題目（英語）： Nonlinear terahertz response of (Pb, Sn)Te thin film
要旨（英語）： In terahertz region, dynamics of elementary excitation and electron, for example Drude response, phonon and antiferromagnetic resonance of spin, can be observed, so terahertz region recently attracts a great deal of attention.
It has been reported that in SrTiO3, by using intense terahertz pulses, soft mode resonance frequency hardens at high field, so nonlinearity is observed . And it has also been reported that by using intense terahertz pulses, ferroelectric soft mode in SrTiO3 drives strongly and, displacement of ferroelectric soft mode is comparable to ferroelectric phase induced by pressure . These studies suggest that transient ferroelectric phase is induced by intense terahertz pulses.
(Pb, Sn)Te is good candidate for large nonlinearity by intense terahertz pulses.
(Pb, Sn)Te is narrow gap semiconductor, and PbTe is paraelectric and SnTe is ferroelectric.
In this presentation, I will talk about composition ratio dependence of soft mode in (Pb, Sn)Te and nonlinearity induced by intense terahertz pulses. I will explain the nonlinearity by using anharmonic potential model.
 I. Katayama *et al*., PRL(2012)
 X. Li *et al*., Science (2019).
|氏名： 藤本 健
指導教員名： 齊藤 英治 教授
発表題目（英語）： Braiding of Majorana-like state in electric circuits
要旨（英語）： Topological quantum computer is one of the promising fault tolerant means to perform quantum computation. In this computation, particles with exotic statistics called non-Abelian anyones are needed and research on how to realize such kind of quantum states is now under intense debate.
This presentation provides a prospective candidate of physics systems experimentally testable for its realization in a classically known electric circuit.
First, we’ll give some introduction to concepts of equality between quantum states and electric circuits. And then discuss the realization of Majorana-like edge state and its braiding operation in an electric circuit with simulation results.
|氏名： 福田 大地
指導教員名： 鹿野田 一司 教授
発表題目（英語）： NMR Study of Uniaxial-Strain-Induced Metal-Insulator Transition in the Charge-Frustrated System θ-(ET)2I3
要旨（英語）： The θ-(ET)2X family of layered organic conductors, composed of conducting ET layers with triangular lattices and insulating layers of anion molecules X, is the representative charge-frustrated system. The strength of charge frustration is varied by X and determines ground states; for weak frustration, the system exhibits a charge ordering (CO) transition upon cooling, whereas the long-range CO is prevented in a strongly frustrated system, yielding a charge glass (CG) state.
θ-(ET)2I3, which is the most charge-frustrated system with a nearly isotropic triangular lattice, however, does not show the CG or CO transition and remains in the metallic state down to very low temperature, invoking a quantum melting of charges caused by charge frustration. Remarkably, under a uniaxial strain of 2 kbar applied in a direction to reduce the frustration, the electrical resistivity abruptly increases around 140 K, possibly suggesting that the insulating state is induced by the uniaxial strain. However, the microscopic electronic state of this phase has not been uncovered.
Thus, in the present study, we examine the microscopic electronic state of θ-(ET)2I3 under uniaxial strains using the 13C NMR spectroscopy. Below a uniaxial strain of 4 kbar, the NMR spectra and the spin-lattice relaxation rates are little changed from those at ambient pressure, which is inconsistent with the previous resistivity result. At a uniaxial strain of 6 kbar, although the minor part with an extremely long relaxation time emerges, implying the insulating state, the major part seems almost unchanged. We consider that the uniaxial strain is not applied homogeneously to the sample for the present experimental condition, and thus are currently trying another pressurization method.
|氏名： 古澤 千晶
指導教員名： 賀川 史敬 准教授
発表題目（英語）： Current-induced motion of metastable skyrmions in MnSi
要旨（英語）： Magnetic skyrmions, vortex-like swirling spin textures, have attracted attention with their unique physical properties rooted by their topological nature.
Characteristics of magnetic skyrmions discovered so far, particularly their stability and controllability with low current owing to their topological nature in addition to their nanometric size, suggest that magnetic skyrmions have potential application to information carriers for high-density, highly efficient magnetic storage.
On the other hand, even very fundamental requirement for practical application such as writing practical size skyrmions locally, driving them without damage and observing them in a same device is a remaining challenge. We focus on this problem and aim to investigate behaviors of locally-generated metastable skyrmions with electric current application in B-20 type MnSi, a typical skyrmion-hosting material.
In this talk, I will briefly introduce basic properties of magnetic skyrmions, motions of them under current application and status of our experiment.