「メゾスコピック超伝導体におけるボルテックダイナミクス」
宇治 進也 氏

Oct 23, 2013

北大理学部物理学部門主催、日本物理学会北海道支部講演会・ 第194回エンレイソウの会共催
講演題目：　メゾスコピック超伝導体におけるボルテックダイナミクス
講　師　：　宇治 進也 氏
　　　　　物質・材料研究機構　超伝導物性ユニット　ユニットリーダー
日　時　：　平成25年10月23日 (水) 16:30-17:30
場　所　：　北海道大学理学部２号館211室（2-2-11）

要　旨　：
超伝導性を特徴づけるコヒーレンス長や磁場侵入長が、試料サイズと同程度になるメゾスコピック超伝導体では、試料の形状・境界条件が超伝導状態に大きく影響を与え（サイズ効果）、バルクの試料とは異なったメゾスコピック系特有の振る舞いが観測される。金属アルミニウムはバルクでは第一種の超伝導体であるが、 微小構造体では、そのサイズ効果により、第二種の超伝導体のように振る舞い、磁場中でボルテックス状態が形成される。電流を流すと、ボルテックスが駆動され、そこでエネルギーが散逸する。バルクでは、この散逸による抵抗は常伝導状態での抵抗を超えることはあり得ない。しかしメゾスコピック試料では、常伝導状態での抵抗の８倍もの大きさになることもあり、これには特異的なボルテックスのダイナミクスが関係している。この実験的な検証とメカニズムについて議論したい。

世話人　 松永悟明

北海道大学大学院理学研究院物理学部門

"Magnetic and transport properties of MnSi thin films"
Ms. Josefin Engelke

Sep 19, 2013

トポロジー理工学教育研究センター第192回エンレイソウの会 ・ 物理学会北海道支部講演共催
講演題目：　Magnetic and transport properties of MnSi thin films
講　師　：　Ms. Josefin Engelke
　　　　　Technische Universitat Braunschweig, Institute for Condensed Matter Physics, 38106 Braunschweig, Germany
日　時　：　平成25年9月19日 (木) 15:00-16:00
場　所　：　北海道大学理学部２号館211室（2-2-11）

要　旨　：
MnSi offers a variety of different magnetic phases. The competition of ferromagnetic exchange with the Dzyaloshinskii-Moriya interaction causes a helical spin structure as the ground state of the system. In applied magnetic fields a conical structure and ferromagnetic spin alignment can be observed. In addition, a skyrmion lattice phase occurs in a small field and temperature region close to the ordering temperature, which is known as the A-phase. In thin films a larger skyrmionic phase is expected in the magnetic phase diagram due to surface effects and strain. Using molecular beam epitaxy MnSi thin films have been grown on Si(111) substrates. Magnetization and magnetoresistivity measurements have been carried out and related to the magnetic phase diagram. The magnetic ordering temperature T_ord is enhanced and shows a thickness dependence. In comparison to bulk MnSi also the critical magnetic fields of the thin films are enhanced. Furthermore several features can be identified occuring in the magnetization as well as in the magnetoresistivity measurements, which are not present in bulk material. Potentially these are indications of an enlarged skyrmionic phase.
Reference: "Spin-spin correlation length in MnSi thin film", J. Engelke, T. Reimann, L. Hoffmann, S. Gas, S. Sullow, D. Menzel, J. Phys. Soc. Jpn. 81 (2012) 124709.

世話人　 網塚　浩
(amiami@phys.sci.hokudai.ac.jp)
北海道大学大学院理学研究院物理学部門

"Stable localized structures in one and two spatial dimensions: a review and a perspective"
Prof. Helmut R. Brand

Aug 19, 2013

講演題目：　 Stable localized structures in one and two spatial dimensions: a review and a perspective
講　師　：　Prof. Helmut R. Brand
　　　　　Technische Universitat Braunschweig, Institute for Condensed Matter Physics, 38106 Braunschweig, Germany
日　時　：　平成25年8月19日 (木) 13:00-14:00
場　所　：　北海道大学理学部２号館211室（2-2-11）

要　旨　：
We give an overview over stable localized solutions in nonlinear driven non-equilibrium systems, often denoted as dissipative solitons [1]. We cover different mechanisms to generate stable localized structures in various types of prototype equations, namely, envelope equations, order parameter equations and phase equations.For envelope equations nonvariational effects are a key ingredient as emphasized first by Thual and Fauve [2]. This mechanism turns out to be very robust and allows for many different stable particle- and hole-like solutions in one and two spatial dimensions. The interaction of localized solutions can give rise to many different types of outcomes including propagating holes as a result of a collision of propagating particles of fixed shape [3]. For order parameter equations a trapping mechanism can generate localized solutions of arbitrary lengths [4]. A third type of localized solutions involves nonlinear phase dynamics giving rise to stable localized patterns in the wavelength for a nonlinear phase equation [5] or in coupled envelope and phase equations [6]. We also cover briefly recent work on the effects of noise on localized solutions in envelope equations [7,8] and close with an overview of possible applications to systems as diverse as surface reactions under UHV conditions [9], binary fluid convection [10] or holes observed in corn and potato starch suspensions [11].

[1] N. Akhmediev and A. Ankiewicz, Eds., Dissipative Solitons, Springer, Heidelberg (2005).
[2] O. Thual and S. Fauve, J.Phys. France 49, 1829 (1988).
[3] O. Descalzi, J. Cisternas, and H.R. Brand, Phys. Rev. E 74, 065201 (2006).
[4] H. Sakaguchi and H.R. Brand, Physica D 97, 274 (1996).
[5] H.R. Brand and R.J. Deissler, Phys. Rev. Lett. 63, 508 (1989).
[6] H. Sakaguchi, Prog. Theor. Phys. 87, 1049 (1992).
[7] O. Descalzi, J. Cisternas, D. Escaff, and H.R. Brand, Phys. Rev. Lett. 102, 188302 (2009).
[8] C. Cartes, J. Cisternas, O. Descalzi, and H.R. Brand, Phys. Rev. Lett. 109, 178303 (2012).
[9] H.H. Rotermund, S. Jakubith, A. von Oertzen, and G. Ertl, Phys. Rev. Lett. 66, 3083 (1991).
[10] P. Kolodner, Phys. Rev. A 44, 6448 (1991); Phys. Rev. A 44, 6466 (1991).
[11] H. Ebata and M. Sano, Phys. Rev. Lett. 107, 088301 (2011).

世話人　 北　孝文
(kita@phys.sci.hokudai.ac.jp)
北海道大学大学院理学研究院物理学部門

"Real-time observations of the quasi-particle relaxation dynamics in various complex systems by means of femtosecond laser spectroscopy "
Ljupka Stojcevska

Jul 22, 2013

講演題目：　 Real-time observations of the quasi-particle relaxation dynamics in various complex systems by means of femtosecond laser spectroscopy
講　師　：　Ljupka Stojcevska
　　　　　Complex Matter Department, Jozef Stefan Institute, Slovenia
日　時　：　平成25年7月22日 16:30-
場　所　：　北海道大学 工学部大会議室(A1-17)

要　旨　：
We have performed systematic study of the quasiparticle relaxation dynamics in various complex systems such as cuprates, iron-based pnictides superconductors and charge-density wave systems. In undoped iron-based pnictides, exhibiting a spin-density wave (SDW) ordering, we observe a bottleneck associated with a partial charge-gap opening. Similarly as in the previous reported studies, a single relaxation process is observed, showing a remarkable critical slowing down of the quasiparticle (QP) relaxation dynamics at the SDW transition temperature. On the other hand, in the optimally doped crystals, a multiple relaxation processes are present with distinct SC-state quasiparticle recombination dynamics exhibiting a BCS-like T-dependent superconducting gap, and a pseudogap (PG)-like feature at higher temperatures. In all optimally doped cuprates and iron-based pnictides, we observe a saturation of the superconducting relaxation component. By taking into account the optical constants such as penetration depth and reflectivity we can accurately calculate energy needed for destruction of the superconducting state. If we compare the magnitudes of the destruction and cond ensation energies we notice a significant discrepancy in cuprates and iron-based pnictides, which can be explained with a phonon-mediated QP bottleneck mechanism. In contrast, in the charge-density wave systems, the destruction is faster and electronic; therefore it can not be explained in the frame of the proposed QP bottleneck mechanism. The second moment of the Eliashberg function, obtained from the relaxation rate in the metallic state at higher temperatures, has similar values in different iron-based pnictides, which indicates a moderate electron phonon coupling. More recently we have observed a switching between an equilibrium state to a metastable in 1T-TaS2 charge-density wave system by means of ultrafast laser quench through a symmetry breaking transition. The switching is repeatable and accompanied with a change in reflectivity and DC resistance, opening a new possibility for ultrafast non-volatile memory devices.

世話人　 戸田 泰則
(toda@eng.hokudai.ac.jp)
北海道大学大学院工学研究院応用物理学部門

"Sliding charge-density-wave in two-dimensional rare-earth tellurides"
Prof. Pierre Monceau

Mar 21, 2013

応用物理学部門学術講演会、杉野目記念会海外学識者講演会、第188回エ ンレイソウの会共催
講演題目：　Sliding charge-density-wave in two-dimensional rare-earth tellurides
講　師　：　Prof. Pierre Monceau
　　　　　Institut Neel, CNRS and University Joseph Fourier, France
日　時　：　平成25年3月21日（木） 15:00-16:30
場　所　：　北海道大学 工学部大会議室(A1-17)

要　旨　：
Amplitude charge density wave (CDW) excitations in rare earth tritellurides (RTe3) were probed by Raman scattering and femtosecond pump-probe spectroscopy. However, collective charge phase excitations could not be observed in far-infrared measurements due to screening by the residual metallic component at the Fermi surface. But the phase collective mode is accessible through nonlinear transport properties. We report such nonlinear transport properties in the layered DyTe3 at temperatures below the CDW phase transition, TP = 302K. Conductivity is increasing sharply above a threshold field. Under application of a rf field Shapiro steps are clearly observed. These features demonstrate for the first time CDW sliding in two-dimensional compounds. In collaboration with A. A. Sinchenko (Kotel’nikov Institut, Moscow) and P. Lejay (Institut Neel, Grenoble).

世話人　 丹田　聡
(tanda@eng.hokudai.ac.jp)
北海道大学大学院工学研究院応用物理学部門