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2008年度 物理科学談話会



Exsolution Lamellae in Minerals from Ultrahigh-Pressure Metamorphic Rocks

講演者 : Dr. Yousif Osman Mohammad (University of Slaimani, Iraq)
日時 : 2008年 1月 12日(木) 16:15〜17:45
場所 : 理学部会議室(A2棟)
要旨 : Abstract : Exsolution microstructure has been recently been identified in minerals of UHP ultramafic rocks and eclogite from several orogenic belts, Detailed characterization and experimental investigations regarding exsolution microstructures of these and other minerals in the rocks will improve our understanding of mantle processes and continental subduction.

問い合わせ : 物理科学専攻 前川 寛和
( 内線 2735 / TEL 072-254-9734 (ダイヤルイン) / email maekawa@p.s.osakafu-u.ac.jp )




Bound States in Continuum (BIC) and Quasi BIC (QBIC)
in Low Dimensional Nano-device

講演者 : Tomio Petrosky 博士 (テキサス大学オースチン校)
日時 : 2009年 2月 12日(木) 16:15〜17:45
場所 : 理学部会議室(A2棟)
要旨 : We report the prediction of quasibound states (resonant states with very long lifetimes) in a low dimensional nano-device that occur in the eigenvalue continuum of propagating states for certain systems (quasibound states in continue : QBIC ) in which the continuum is formed by two overlapping energy bands. We illustrate this effect using a quantum wire system with two channels and an attached adatom. When the energy bands of the two channels overlap, a would-be bound state that lays just below the upper energy band is slightly destabilized by the lower energy band and thereby becomes a resonant state with a very long lifetime (a second such state lays above the lower energy band). Unlike the bound states in continuum (BIC) predicted by von Neumann and Wigner, these states occur for a wide region of parameter space, so that observation of QBIC might be much easier that that of BIC.

問い合わせ : 物理科学専攻 田中 智
( 内線 2823 / TEL 072-254-9710 (ダイヤルイン) / email stanaka@p.s.osakafu-u.ac.jp )




ナノ光加工、光操作の新展開

講演者 : 伊藤 正 教授 (大阪大学基礎工学部)
日時 : 2008年 12月 19日(金) 16:00〜17:30
場所 : 理学部会議室(A2棟)
問い合わせ : 物理科学専攻 溝口 幸司
( 内線 3708 / TEL 072-254-9712 (ダイヤルイン) / email k.mizoguchi@p.s.osakafu-u.ac.jp )




分子性伝導体・磁性体の基底状態と相転移

講演者 : 中澤 康浩 教授 (大阪大学理学部)
日時 : 2008年 12月 16日(火) 16:00〜17:30
場所 : 理学部会議室(A2棟)
要旨 :  有機分子のもつπ電子や金属錯体中のd電子が伝導性や磁性に関与する分子性伝導体や磁性体は、低次元構造の結晶格子の中で、強い電子相関と電子格子相互作用が協奏的に働き、多様な電子物性を引き起こすことが知られています。これらの分子性化合物は、構成単位が分子という空間的な広がりをもつユニットであるため、結晶中に欠陥などが入り難く、また溶液中から低温で時間をかけてつくるため良質な結晶ができ、スピンや電荷が関係した電子物性の研究をする良い舞台となります。
代表的な有機ドナー分子であるBEDT-TTFとカウンターイオンからなる2:1の電荷移動塩は、2次元的な分子配列をもつ強相関電子系のモデル物質として注目されています。BEDT-TTF分子からなる有機超伝導体の中には、比較的高い10 K-14 K程度の転移温度を示す物質が多数存在します。こうした超伝導物質の類縁体のひとつであるk-(BEDT-TTF)2Cu2(CN)3という塩は、非常に良い三角格子構造をもった分子性Mott絶縁体化合物としてその低温のスピン状態に関心がもたれています。 我々は、この三角格子系化合物に対して、板上の微小サイズ(100-200mg程度)の単結晶試料を用いた低温熱容量測定を、75mKまでの極低温まで行ったところ、約15±5 mJK-2mol-1程度のg項(温度に比例する項)が存在することを明らかにしました。磁場印加の影響が殆どないことから、基底状態からの励起がギャップレスになっており、その励起構造を反映した熱容量が観測されたと考えられます。 また、この物質には約6 K付近に他の塩では見られないようなCpT -1のブロードな熱異常が存在することもわかり、この温度付近を境にハイゼンベルグ的な状態から量子的なスピン液体の状態にクロスオーバーがおこっている可能性が示唆されます。同様に三角格子構造をもつMott絶縁体塩であるEtMe3[Pd(dmit)2]2と対比させながら三角格子系の熱力学的な性質を議論します。
 同様の単結晶熱測定を、多核金属錯体で単分子磁石としての性質を示すMn4クラスターをネットワーク型に連結した化合物でも行いました。このような連結系では、単分子磁石の大きなスピンが長距離的に秩序化する現象や、磁場によって抑制される効果が見いだされました。さらに、磁場をわずかにかけることによって生じる磁場に誘起されたガラス化現象を観測することができました。  上記の物質を中心に、分子性化合物の単結晶を用いた熱容量測定の最近の話題を紹介します。

問い合わせ : 物理科学専攻 細越 裕子
( 内線 2711 / TEL 072-254-9699 (ダイヤルイン) / email yhoso@p.s.osakafu-u.ac.jp )




Coherent phonons in opaque materials: solved and unsolved problems

講演者 : Oleg Misochko教授 (ロシア科学アカデミー)
日時 : 2008年 10月 28日(火) 15:30〜17:30
場所 : 大阪府立大学 学術交流会館 小ホール
要旨 : There have been extensive experimental studies of the ultrafast dynamics of crystal lattice by making use of coherence of phonons. The common feature in all these experiments is the generation of superpositions of lattice states that evolve in time coherently. The necessary requirement for coherent phonon excitation is the availability of Raman phonons with the frequency smaller than the inverse of pulse duration. At low excitation strength, when the oscillation amplitude is small, the vibrational properties such as the spectrum and lifetime of coherent phonons are considered as intrinsic properties of the crystal. However, such simple and well understood picture changes radically as one enters the non-linear regime, where the intense laser pulses p! roduce the high density of phonons and free carriers thereby modifying the phonon-phonon and electron-phonon coupling. At this high excitation level, the interatomic forces that bind solids and determine many of their properties are substantially altered due to large-amplitude lattice excitation resulting in a variety of novel effects. The semimetals Bi and Sb, as well as the narrow band gap semiconductor Te, are the prototype materials in which coherent optical phonons are generated upon femtosecond laser excitation. In all these crystals, the electron-lattice interaction is strong and the lattice configuration is sensitive to the population distribution of electrons within the conduction bands. Femtosecond excitation in these materials can drive both the fully symmetric and doubly degenerate coherent phonons. The vibrational excitation in the case of fully symmetric phonons is generally believed to be displacive: the population redistribution of! valence electrons alters the potential energy surface of the lattice and gives rise to a restoring force that drives coherent atomic motion. The dynamics of this mode are determined by the curvature and minima location of the altered lattice potential. In contrast, the excitation of doubly degenerate phonons is thought to be impulsive: owing to the large spectral bandwidth available in a short laser pulse, it can generate nonstationary lattice states in a crystal with the two, different in frequency, electromagnetic fields acting as a driving force. Note that the fully symmetric mo! de is the Peierls distortion mode for all the crystals under study, an d in the unfolded Brillouin zone of the cubic structure, it corresponds to a longitudinal acoustic mode. For excitation strength below the melting threshold the reflected pump-probe signal in all crystals exhibits a pronounced oscillatory behavior superimposed on a background signal. The initial coherent atomic displacements approach one tenth of the nearest neighbor distance (the so-called Lindemann stability limit) and it may thus be speculated whether this can lead to a novel lattice state. We measure how these large amplitude phonons in Bi, Sb and Te depend on pumping pulse duration, pulse chirp, pulse energy and temperature. To this end, we performed a variety of experiments including coherent control. Splitting the 50 f! s transform-limited pulse into two identical pulses with a different interpulse separation we observe that if the total electronic contribution in is always the sum of the signals resulting from each pulse independently, the oscillatory component exhibits interferences resulting in a variation of the total amplitude as a function of the interpulse separation g. Depending on g, the fully symmetrical oscillatory component can be almost completely cancelled but not enhanced as compared to single pulse excitation. In addition, we disperse the probe beam to observe the large amplitude vibrations at different wavelengths. In this case apart from using transform-limited pulse as a probe we also utilized probing by a supercontinuum. Besides the dependence on the excitation wavelength, the coherent amplitude in the crystals under study is observed to depend on the probing wavelength. Based on the obtained results, we discuss the mechanisms responsible for the creation of lattice coherence in the pump-probe experiments and outline the unsolved problems.
問い合わせ : 物理科学専攻 溝口 幸司
( TEL 072-254-9712 (ダイヤルイン) / email k.mizoguchi@p.s.osakafu-u.ac.jp )




Complex Spectral Representation of the Liouville operator and H-theorem
in Non-equilibrium Statistical Mechanics

講演者 : Tomio Petrosky 博士 (テキサス大学オースチン校)
日時 : 2008年 9月 29日(月) 16:30〜17:30
場所 : 理学部会議室(A2棟)
要旨 : Broken time-symmetry of unstable dynamical systems is analyzed in terms of complex spectral representation of the Liouville operator, as well as the Hamiltonian operator. This analysis reveals that irreversibility is described as a rigorous dynamical property of phase-space functions (in classical systems) or density matrices (in quantum systems) that belong to an extended function space including distributions for non-integrable system with infinitely many degrees of freedom. In the extended function space, generating operators of motion may have complex eigenvalue in spite of the fact that they are symmetric operators. The imaginary part of the eigenvalue appears as a result of resonance singularity that is a characteristic singularity in non-integrable systems. The imaginary part breaks time-symmetry. As an example of applications of the complex representation, we construct a H-function for a spontaneously decaying process in a two-revel atom on a rigorous dynamical basis without any approximation. The H-function constructed through the complex spectral representation withstands Loschmidt’s famous criticism on Boltzmann’s H-function. Our H-function gives a dynamical model of entropy flow, as well as entropy production.

問い合わせ : 物理科学専攻 田中 智
( 内線 2823 / TEL 072-254-9710 (ダイヤルイン) / email stanaka@p.s.osakafu-u.ac.jp )