| 氏名： 井口
指導教員名： 高橋 陽太郎 准教授
発表題目（英語）： Nonreciprocal birefringence and natural optical activity on electromagnon resonance in helical magnet CuFe1-xGaxO2 (x=0.035)
要旨（英語）： The interplay between magnetism and ferroelectricity, i.e., magnetoelectric effect, in multiferroics is of great interest because of the possible applications as well as the novel phenomena. In optical region, the magnetoelectric effect causes nonreciprocal directional dichroism and nonreciprocal birefringence. Since the particular spin structures including helical spin one produce strong magnetoelectric coupling in matter, the collective excitation of spins possess the electrical activity, which is referred to as electromagnon. The inherent magnetoelectric coupling for the electromagnon resonance always gives rise to the optical magnetoelectric effects in terahertz region.
Here we demonstrate gigantic nonreciprocal birefringence on the electromagnon resonance by using time-domain terahertz spectroscopy under the magnetic field. The multiferroics helimagnet CuFe1-xGaxO2 (x=0.035) provides ferroelectric polarization parallel to the magnetization, whose symmetry ensures the nonreciprocal birefringence. Furthermore, we found that the electromagnon resonance exhibits gigantic natural optical activity derived from the chirality of spin structure irrespective of underlying lattice symmetry. We will discuss the correlation between these optical effects and helical magnetism with a possible origin of them.
| 氏名： 稲垣
指導教員名： 鹿野田 一司 教授
発表題目（英語）： Magnetoresistance of Dirac electrons in organic conductor under pressure control.
要旨（英語）： Since the discovery of massless Dirac fermions (MDFs) in graphehe, MDFs have been of particular interest in solid-state physics and found in several materials. The quasi-two-dimensional organic conductor, α-(ET)2I3 is among them. At ambient pressure, α-(ET)2I3 undergoes a transition from a conducting state to a charge-ordered insulating state at 135K due to strong electron correlation. The transition is suppressed by pressure above ~12kbar. The existence of the charge-ordered phase implies that the electrons in α-(ET)2I3 are strongly correlated. When free electrons are in a strong magetic field , discrete energy levels called Landau levels (LLs), which linearly vary with the applied filed, are formed. In case of MDFs, however, a particular LL called the zero-mode LL resides at the zero energy irrespectively of the magnitude of applied field. My interest is the effect of electron correlation to the zero-mode-LL and the observation of it by means of charge transport and NMR.
As the intensity of the electron correlation in α-(ET)2I3 is expected to be changed by pressure, I have investigated the magnetoresistance of α-I3 under magnetic fields perpendicular to the conductive plane at various pressures. The results indicate the interplay between the Zeeman effect and the Coulomb effect, which may induce the so-called valley splitting.
| 氏名： 石田
指導教員名： 三尾 典克 特任教授
発表題目（英語）： Measurement of nonlinear absorption at 532 nm in periodically poled Mg:SLT
要旨（英語）： High power and high quality green laser light can be obtained by second harmonic generation (SHG) from infrared laser light. Periodically poled Mg doped stoichiometric LiTaO3 (PPMg:SLT) has high SHG efficiency and high thermal conductivity among a variety of nonlinear crystals. However, the optical breakdown occurs in high power output operation. To clarify the cause of optical breakdown, I investigate the behavior of absorption before optical breakdown.
In this presentation, I will introduce about the small absorption measurement system using Michelson interferometer and improvement of it. Then, I will introduce the measurement nonlinear absorption system and progress of it.
| 氏名： 上田
指導教員名： 樽茶 清悟 教授
発表題目（英語）： Approach to observe the non-locality of Majorana fermion in hybrid Superconductor-semiconductor nanowire device
要旨（英語）： Majorana fermion is the new particle which is its own antiparticle. It is proposed by Ettore Majorana in 1937. Majorana fermions can be bounded to observe as a zero-bias conductance peak, which is called Majorana zero mode or Majorana bound state. In 2012, Majorana fermion is observed indirectly in the stage of hybrid superconductor-semiconductor device. In this time, I propose observing the non-locality of Majorana in the hybrid device by Cooper pair splitting, which is separating Cooper pairs dimensionally. The current condition of my experiment is on the way of fabricating devices, so I also mention the fabrication of the devices.
| 氏名： 伊藤
指導教員名： 為ヶ井 強 准教授
発表題目（英語）： An Anomalous Peak in Critical Current Density for the Iron-based Superconductor Ba(1-x)KxFe2As2 with Splayed Columnar Defects
要旨（英語）： Columnar defects in type-II superconductors serve as artificial pinning centers, which lead to enhancement of critical current density (Jc). Enhancement of Jc by dispersing the direction of the columnar defects is proposed theoretically and confirmed experimentally. However, vortex motion in superconductors with columnar defects is not thoroughly understood.
To clarify the vortex motion in superconductors with splayed columnar defects, Ba0.6K0.4Fe2As2 single crystals were synthesized, and columnar defects were installed into the crystals by heavy ion irradiation. Magnetization measurements were performed, and Jc was calculated from the results of the magnetization measurements. We found an anomalous peak in Jc-H for Ba0.6K0.4Fe2As2 single crystals with splayed columnar defects.
In this presentation, I will report results of detailed magnetization measurements in Ba0.6K0.4Fe2As2 single crystals with splayed columnar defects, and discuss possible origins of the anomalous peak effect.
指導教員名： 長谷川 達生 教授
発表題目（英語）： Field modulation imaging of polarization domain walls in organic ferroelectrics thin films
要旨（英語）： Switchable electric polarization of ferroelectrics is widely used in many types of devices such as nonvolatile memories. These days, proton transfer type organic ferroelectrics are expected to be key materials for the realization of low voltage driving devices by printing process because they have small coercive electric field and they can be solved in many kinds of solvents. In order to apply these materials to devices, it is indispensable to understand the mechanism of polarization reversal in thin films.
In this research, we developed a new optical method “Ferroelectrics field modulation imaging (FFMI)” which visualize the polarization domains of proton transfer type organic ferroelectrics, and observed the dynamics of domain motion in thin films. We focused on the driving domain walls, and analyzed the signals of FFMI which have the information of domain structure in the thickness direction. As a result, it was revealed that the motion of polarization domains are hindered at the interface between film and substrate. Details of measurement method and the analysis of domain walls are discussed on the day.