"STM vortex core spectroscopy and non conventional pairing in high temperature superconductors"
Christophe Renner

Nov 11, 1998

講演題目: STM vortex core spectroscopy and non conventional pairing in high temperature superconductors
講 師 : Christophe Renner
     University of Geneva
日 時 : 平成10年11月20日 (金) 15:30-17:00
場 所 : 北海道大学理学部2号館4階409講義室

要 旨 :
The quasiparticle density of states is a key ingredient in the quest for the mechanism of high temperature superconductivity. One of the common features of high- and low-temperature superconductors is a gap in the quasiparticle excitation spectrum at the Fermi level related to the carrier pairing below the superconducting transition temperature ( T c). However, in high temperature superconductors (HTS), its characteristics are very unusual and depart highly from the BCS predictions. One of the striking spectroscopic signatures of HTS is the persistence of a gap in the normal state above T c. Tunneling spectroscopy shows that this normal state gap is intimately related to the superconducting gap. Recently, it was put into a new perspective as it was also found at low temperature inside the vortex cores of Bi2Sr2CaCu2O8 by scanning tunneling spectroscopy. These latest results are consistent with the existence of incoherent pairing states in the normal state above T c and inside the vortex cores below T c. They suggest that these materials are in an intermediate regime between BCS and Bose-Einstein condensation.

世話人  小田 研

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"Ultrafast Laser-generated Coherent Waves in Advanced Materials: From the Academic Lab to the Real World and Back"
Prof. Keith A. Nelson

Nov 11, 1998

講演題目: Ultrafast Laser-generated Coherent Waves in Advanced Materials: From the Academic Lab to the Real World and Back
講 師 : Prof. Keith A. Nelson
日 時 : 平成10年11月11日 (水) 14:00-
場 所 : 電子科学研究所講堂(事務棟 2F)

要 旨 :
Coherent waves are generated and monitored optically in bulk and thin film materials to provide fundamental insight into their structures and dynamics; to exert optical control over their behavior; and to provide information ofimmediate practical value. On picosecond and nanosecond time scales, acoustic waves are characterized to learn about structural phase transitions and relaxation dynamics in crystals, polymers, and viscous liquids. In thin films, the measured acoustic responses yield the film elastic moduli, thermal diffusivities, and thicknesses, as well as a simple check for film-substrate delamination. This has led to the successful commercialization of the measurement method which is now used in the microelectronics industry. In ferroelectric crystalline solids, high-frequency vibrational waves which move through the lattice at a significant fraction of the speed of light are generated with femtosecond pulses and pulse sequences. Their responses teach us about ferroelectric phase transitions, anharmonic potential energy surfaces and nonlinear lattice dynamics, and the prospects for optical control over collective material behavior. The results illustrate the interplay between fundamental spectroscopic research on complex materials and the practical applications that may emerge.

世話人  八木 駿郎

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"High-Pressure Phase Transitions of Low-Coordination Oxides"
Prof. Karl Syassen

Mar 16, 1998

講演題目: High-Pressure Phase Transitions of Low-Coordination Oxides
講 師 : Prof. Karl Syassen
     Max-Planck-Institut fuer Festkoerperforschung, Stuttgart
日 時 : 平成10年3月16日 (月) 16:00-
場 所 : 北海道大学理学部2号館211室(2-2-11)

要 旨 :
The combination of high-resolution synchrotron x-ray diffraction, optical spectroscopies, and large hydrostatic pressures produced by diamond anvil cells offers a unique approach to studying structural, vibrational, and band structure related properties of solids. Since the introduction of the DAC, high pressure investigations of oxide materials have remained at the forefront of experimental high pressure research. This talk will be concerned with recent pressure studies of phase transitions in oxides [1-4] which have in common a low coordination of their ambient pressure phases. Topics to be discussed are the rich PT phase diagram of the spin-Peierls compound CuGeO$_3$ and new insights into pressure-induced amorphization of berlinite-type compounds, as obtained from combined x-ray diffraction, optical spectroscopy and Moessbauer studies.
[1] A. R. Goni et al., Phys. Rev. Lett. 77, 1079 (1996).
[2] S. Braeuninger et al., Phys. Rev. B 56, R11357 (1997).
[3] M. P. Pasternak et al., Phys. Rev. Lett. 79, 4409 (1997).
[4] T. Zhou et al., Phys. Rev. B 57, Jan. 1, 1998.

世話人  中原 純一郎

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"Intermittently Flowing Rivers of Quantized Magnetic Flux: Vortex Motion, Noisy Fractal Networks, and Flux Avalanches Superconductors"
Prof. Franco Nori

Mar 16, 1998

講演題目: Intermittently Flowing Rivers of Quantized Magnetic Flux: Vortex Motion, Noisy Fractal Networks, and Flux Avalanches Superconductors
講 師 : Prof. Franco Nori
日 時 : 平成10年6月29日 (月) 10:30-11:30
場 所 : 北海道大学工学部 A108教室(1階)

要 旨 :
This general talk will be mostly about vortex dynamics and a bit about granular media. After a pedagogical introduction, the main ideas and several results will be presented, sometimes using videos. A more technical description follows below. Vortices in superconductors exhibit a variety of equilibrium phases, including liquid, lattice, and glassy states. In addition, vortices might be driven, producing several dynamical phases with steady states of plastic and elastic motion. We characterize the dynamical instabilities (i.e., flux avalanches or cascades producing voltage bursts), as well as the evolution of the topological order and vortex flow paths ("vortex streets" surrounded by regions of pinned flux). Our analysis of the microscopic spatio-temporal dynamics of individual flux-lines in superconductors leads insight to commonly measured bulk macroscopic quantities, such as magnetization and critical currents. We have studied [1] flux-gradient-driven flux lines (i.e., there is no artificial uniform external force on the vortices) as an external field H(t) is quasi-statically ramped up and down. We explore a wide variety of relevant parameters which are difficult to continuously tune experimentally, such as the density, strength, radius, and location of the pinning sites. Our predictions (e.g., magnetization hysteresis loops) can be directly compared with commonly-measured experimental quantities. We analyze both global (e.g., M(H), J_c(H)) and local (e.g.,B(x,y,H(t)), M(x,y,H(t)), J_c(x,y,B)) measurable quantities. Our results elucidate the topological order dynamics of a driven plastic lattice interacting with a rigid disordered substrate, a problem that has recently attracted considerable attention [2].
[1] C. Reichhardt et al, Phys. Rev. B 52, 10441 (1995); 53, R8898 (1996); 54, 16108 (1996); Phys. Rev. Lett. 78, 2648 (1997); J. Groth et al, Phys. Rev. Lett. 77, 3625 (1996); Olson et al, Phys. Rev. B 56, 6175 (1997); Phys. Rev. Lett. 80, 2197 (1998); and preprints.
[2] F. Nori, Science 271, 1373 (1996).

世話人  田村 信一朗


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