"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