JPS Hokkaido Branch



"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

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"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

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"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

Category: 支部講演会 | Static URL: /seminar/19981120.htm | Edit

"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

Category: 支部講演会 | Static URL: /seminar/19981111.htm | Edit

"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

Category: 支部講演会 | Static URL: /seminar/19980316.htm | Edit
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