JPS Hokkaido Branch



"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)

Category: 支部講演会 | Static URL: /seminar/19991021.htm | Edit

"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

Category: 支部講演会 | Static URL: /seminar/19990602.htm | Edit

"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

Category: 支部講演会 | Static URL: /seminar/19990212.htm | Edit

"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

Category: 支部講演会 | Static URL: /seminar/19990210.htm | Edit

"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
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