"Towards Bulk 49K Superconductivity in the Intermetallic CaFe2As2"
Mr. Tyler Drye

Nov 18, 2013

講演題目: Towards Bulk 49 K Superconductivity in the Intermetallic CaFe2As2
講 師 : Mr. Tyler Drye
     Department of Physics, University of Maryland College Park
(Visiting scholar by ICMR International Research Fellowship)
日 時 : 平成25年11月18日 (月) 15:00-16:00
場 所 : 北海道大学理学部2号館409室(2-4-09)

要 旨 :
The highest achievable superconducting critical temperature (Tc) in intermetallic iron-pnictide superconductors (specifically, the 122 family) was heald to be 39K when K is substituted for Ba in BaFe2As2. Our group was among the first to report traces of superconductivity in the 40-50K range when rare earth elements are doped for Ca in CaFe2As2. Unfortunately, these materials display a drastically low shielding volume fraction and correspondingly low critical current densities. Recent studies aimed at increasing this volume fraction, including high pressure synthesis and unusual doping schemes, have met with varying levels of success. This talk will detail these efforts and the outlook for bulk superconductivity in this system.

世話人  柳澤達也

タイラー氏の講演の様子 タイラー氏の講演の様子

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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


Category: 支部講演会 | Static URL: /seminar/20131023.htm | Edit


Oct 01, 2013

1.北海道支部費として、学会事務局に予算請求した額が(年間約10万円)支給されます。 主な支出は支部講演会謝金、支部委員会開催時の旅費、通信文房具費、支部会事務謝金(1万円)です。


・講演の計画は、会員皆様への連絡の都合上、原則として一ヶ月前までに、講演者の略歴紹介および講演内容を添えて 支部三役まで連絡下さい。支部の行事として適切かどうか検討させて頂くことになっています。いうまでもなく、講演会は北大以外の他機関でも行うことが出来ます。




Category: 組織 | Static URL: /org/first_post.htm | Edit

"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
ジョセフィンさんの講演の様子 ジョセフィンさんの講演の様子

Category: 支部講演会 | Static URL: /seminar/20130919.htm | Edit

"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

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"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

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"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

Category: 支部講演会 | Static URL: /seminar/20130321.htm | Edit

"Electronic ferroelectricity in carbon-based systems: from reality of organic conductors to promises of polymers"
Prof. Dr. Natasha Kirova

Jan 21, 2013

日本物理学会北海道支部講演会 共催: 第185回エンレイソウの会共催
講演題目: Local and non-equilibrium processes in charge density waves
講 師 : Prof. Dr. Natasha Kirova
     CNRS, University Paris-Sud, France
日 時 : 平成25年1月21日(月) 13:30-16:30
場 所 : 北海道大学 工学部大会議室(A1-17)

要 旨 :
Ferroelectricity is a demanded effect in fundamental and applied solid state physics. Till now, the ferroelectrics were available mostly in electronically and optically inert, usually inorganic, materials. The electronic ferroelectricity was discovered in conducting organic stacks of (TMTTF)2X at their Mott insulator phase, and now it is studied in layered BEDT-compounds with the charge ordering, and in complexes with neutral-ionic transitions; this research is very active in Japan. In this talk we discuss the existing and the expected ferroelectricity in electronically and optically active carbon-based materials: organic crystals, conducting polymers and even graphene ribbons. We indicate the type of conducting polymers ? the substituted polyacetylene - where the ferroelectricity should be present. The theory predicted an existence of solitons with non-integer variable charges, both with and without spin, which are the walls separating domains with opposite electric polarisation. Their physics will serve to relate transient ferroelectric processes and the visible range optics. We shall interpret the experimental data for (TMTTF)2X allowing to separate the critical relaxation within ferroelectric domains and the repolarization via sweeping of domain walls. We determine the critical slowing-down near the transition temperature, and the low frequency absorption coming from the creep of domain walls.

世話人  市村 晃一

Category: 支部講演会 | Static URL: /seminar/20130121_2.htm | Edit

"Local and non-equilibrium processes in charge density waves"
Prof. Dr. Serguei Brazovskii

Jan 21, 2013

日本物理学会北海道支部講演会 共催: 第184回エンレイソウの会共催
講演題目: Local and non-equilibrium processes in charge density waves
講 師 : Prof. Dr. Serguei Brazovskii
     CNRS, University Paris-Sud, France
日 時 : 平成25年1月21日(月) 13:30-16:30
場 所 : 北海道大学 工学部大会議室(A1-17)

要 旨 :
This seminar will review resent experimental observations and their theoretical modeling in charge density waves (CDW). The results may be relevant to a broad class of low dimensional electronic systems with symmetry broken states ? the nearest extension is the ferroelectric charge ordering in organic conductors. The selected observations will be related to strong, topologically nontrivial perturbations of the order parameter under an external impact. The pattern may be static, induced by electric field or dynamic, under the optical pumping ? these are the two new trends in solid state physics [1]. Recent optical study of a far-from-equilibrium CDW recovered coherent aperiodic undulations of the order parameter, critical slowing down of the collective mode, and evolution of the particle-hole gap. Numerical modeling allowed interpreting the observations, particularly the spatio-temporal distortions arising from “earthquakes” - annihilation events of topological defects in depth of the sample. Strong perturbations were also found at the microscopic scale accessed by the STM; there are the amplitude solitons expected to play a role of neutral spin carriers - spinons. Experiments on nano-junctions in CDWs indicate on the intrinsic reconstruction by creating stationary and transient electronic vortices ? dislocations. The modeling shows that vortices are formed stepwise in the junction when the voltage across, or the current through, exceed a threshold. The vortex core concentrates the total voltage drop, working as a self-tuned microscopic tunnelling junction.
[1] http://lptms.u-psud.fr/impact2012/

世話人  丹田 聡

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