"Probing the Energy Gap of High Temperature Superconductors with Tunneling Spectroscopy: Evidence for Exotic Pairing"
Prof. John Zasadzinski

Oct 21, 1999

講演題目：　Probing the Energy Gap of High Temperature Superconductors with Tunneling Spectroscopy: Evidence for Exotic Pairing
講　師　：　Prof. John Zasadzinski
　　　　　(Illinois Institute of Technology and Argonne National Laboratory)
日　時　：　平成11年10月21日 (木) 16:30-17:00
場　所　：　北海道大学理学部２号館大学院講義室（2-2-11）

要　旨　：
TTunneling spectroscopy is presented for a variety of high temperature　superconducting cuprates. The data for electron doped compounds such as　Nd-Ce-Cu-O which have a relatively low Tc = 22K are consistent with an　s-wave energy gap and ordinary phonon mediated pairing. However, the　tunneling data for hole-doped cuprates such as Bi-Sr-Ca-Cu-O (Bi2212) with　an optimal Tc = 95 K are more consistent with a d-wave energy gap. The　doping dependence of the energy gap parameter in Bi2212 is anomalous,　increasing to very large values in the underdoped regime even as Tc　decreases. This gap parameter follows the trend of the antiferromagnetic　exchange energy, J, which indicates that the pairing is of magnetic origin.　 The high bias tunneling spectra reveal a dip feature that is often quite　pronounced and may be related to the resonance peak in the magnetic　excitations probed by neutron scattering. Also, the high bias features　include a secondary peak which might be an indication of a competing　pseudogap of magnetic origin.

小田研　（北海道大学大学院理学研究科）
（moda@sci.hokudai.ac.jp）

"Dissipative Quantum Brain Dynamics"
Prof. Giuseppe Vitiello

Jun 02, 1999

講演題目：　Dissipative Quantum Brain Dynamics
講　師　：　Prof. Giuseppe Vitiello
　　　　　(Dipartimento di Fisica, Universita' di Salerno, Italy)
日　時　：　平成11年6月2日 (水) 16:30-17:30
場　所　：　北海道大学理学部２号館大学院講義室（2-2-11）

要　旨　：
The quantum model of brain has been extended to dissipative dynamics in order to solve the problem of memory capacity. This gave rise to an interesting framework to study consciousness related mechanisms. The brain is modeled as an open system interacting with the environment and the information storage is represented through the dynamical generation of collective modes under external stimuli. We find different life-times for modes of different momentum. This suggests interesting features related with memory persistence time. Also stimulated by the increasing interest in quantum features of neural net dynamics, either in connection with information processing in biological systems, or in relation with a computational strategy based on the system quantum evolution　(quantum computation), we study the classical limit of the dissipative quantum brain dynamics in order to exhibit a link with neural net dynamics.

世話人　 石川健三
（ishikawa@particle.sci.hokudai.ac.jp）
北海道大学大学院理学研究科

"NMR in Mesoscopic Magnetic　Molecular Rings and Clusters"
Prof　F. Borsa

Feb 12, 1999

講演題目：　NMR in Mesoscopic Magnetic　Molecular Rings and Clusters
講　師　：　Prof　F. Borsa
　　　　　(Pavia University (Italy) and Iowa State University)
日　時　：　平成11年2月12日 (金) 11:00-
場　所　：　北海道大学理学部２号館大学院講義室（2-2-11）

要　旨　：
Molecular magnets are mesoscopic magnetic systems which can be synthesized in bulk quantities by chemical techniques and can be prepared in crystalline form whereby each molecule is an independent magnetic entity with negligible intermolecular magnetic interactions. After reviewing briefly some recent NMR studies [1,2,3] of the spin dynamics in different types of magnetic rings and clusters I will focus the attention on low temperature proton NMR and mSR results in the antiferromagnetic (AFM) molecule [Fe10(OCH3)20(C2H2O2Cl)10] (in short Fe10) and the ferrimagnetic cluster [Mn12O12(CH3COO A)16(H2O)42CH3-COOH4H2O] (in short Mn12). The Fe10 is an antiferromagnetically coupled ring with nearest neighbor exchange coupling　constant J/kB =13.8 K and a total S=0 non magnetic ground state. The 1H nuclear relaxation data as a function of applied magnetic field (performed in part at the Grenoble high field facility) show　spectacular cross relaxation effects at the critical field for which the energy levels of the singlet ground state and the first few magnetic excited states become almost degenerate (level crossing) [5]. Mn12 is a molecular magnet with a high spin ground state and a large crystal field easy axis anisotropy. At low temperatures one can observe quantum tunneling effects in the relaxation of the magnetization of the molecule. I will present proton and muon relaxation data versus temperature and applied magnetic field. The data can be explained in terms of thermal fluctuations of the direction of the Mn12 magnetization in its S=10 ground state [4]. Spin-echo 1H NMR experiments in conditions off equilibrium demonstrate the possibility of monitoring the very slow relaxation of the　Mn12 magnetization at T << 3K from the time dependence of of the amplitude of echo signal [5].
1) A. Lascialfari, D. Gatteschi, F. Borsa, A. Cornia, Phys. Rev. B 55, 14341 (1997).
2) A. Lascialfari , Z.H.Jang , F.Borsa, D.Gatteschi and A.Cornia, Journal of Applied Physics 83, 6946 (1998).
3) A. Lascialfari, D. Gatteschi, F. Borsa, A. Shastri, Z. H. Jang, P. Carretta, Phys. Rev. B 57, 514 (1998).
4) A.Lascialfari, Z.H.Jang, F.Borsa, P.Carretta and D.Gatteschi, Phys. Rev. Letters 3773 (1998).
5) Z.H.Jang, A.Lascialfari, F. Borsa, A.Cornia, D.Gatteschi, M.H.Julien, (Unpublished).

世話人　 熊谷健一
（kumagai@phys.sci.hokudai.ac.jp）
北海道大学・大学院理学研究科・物理学専攻

"Phase Diagram and Hole Pairing of High Tc Superconductors Based on Extended　t-J Hamiltonian"
Prof. Sung-Ho Salk

Feb 10, 1999

講演題目：　Phase Diagram and Hole Pairing of High Tc Superconductors Based on Extended　t-J Hamiltonian
講　師　：　Prof. Sung-Ho Salk
　　　　　(Pohang University)
日　時　：　平成11年2月10日 (水) 15:30-17:00
場　所　：　北海道大学理学部２号館大学院講義室（2-2-11）

要　旨　：
A variety of experimentally observed phase diagrams of high Tc cuprates are briefly reviewed with emphasis on pseudogap phase. Based on the experimental results, we focus our attention to both the spin gap phase and the d-wave superconducting phase for hole doped high Tc cuprates. By introducing an extended t-J Hamiltonian with a hole-hole interaction term and its slave-boson representaion, we derive a computed phase diagram and discuss the pseudogap phase including the superconducting phase of holon pair condensation. Further both the U(1) and SU(2) slave boson theory of Lee and coworkers will be compared with our results of holon pair condensation. It is shown that the spin gap size remains nearly unchanged below the holon pair condensation temperature. We find that the s-wave holon pairing under the condition of d-wave singlet pairing is preferred, thus allowing the formation of d-wave hole pair.

世話人　 小田　研
（moda@sci.hokudai.ac.jp）
北海道大学・大学院理学研究科・物理学専攻

"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
　　　　　MIT
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
(toyagi@es.hokudai.ac.jp)
北海道大学・大学院理学研究科・物理学専攻