2006 Fiscal Year Final Research Report Summary
Theoretical Study of Frustration Effects in Strongly Correlated Electron Systems
| Project/Area Number |
16540313
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Condensed matter physics II
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| Research Institution | The University of Tokyo (2006)
Kyoto University (2004-2005) |
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Principal Investigator |
TSUNETSUGU Hirokazu The University of Tokyo, Institute for Solid State Physics, Professor, 物性研究所, 教授 (80197748)
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| Project Period (FY) |
2004 – 2006
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| Keywords | strongly correlated electron systems / frustration / spin liquid / Kagome lattice / pyrochlore lattice / metal-insulator transition / spin-orbital model / magnon crystal |
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| Research Abstract |
Mechanism of structure and magnetic transitions in vanadium spinel oxides, AV_2O_4 (A=Zn, Mg, Cd) was studied. We derived an effective low-energy model for d-electron orbital and spin degrees of freedom, and determined its phase diagram by a mean field analysis and Monte Carlo simulations. We showed that the observed structural transition is ascribed to orbital ordering. We also showed that this ordering leads to spatially anisotropy of spin-spin interactions and that this derives the observed magnetic ordering at a lower temperature.
Strongly correlated electrons on the Kagome lattice were studied at half filling by using a cluster extension of dynamical mean field theory. We showed the presence of Mott metal-insulator transition but that the metallic phase is much more stable than the case of unfrustrated lattices. We also found very unusual one-dimensional spin correlations near the Mott transition.
Thermodynamics of frustrated quantum spin systems was studied near saturation field. We proposed an effective lattice gas model to describe magnons in fully polarized state that are confined in a small finite region, and solved this model both analytically and numerically. We showed the presence of magnon liquid-solid transition, and identified its universality class for various frustrated systems.
The origin of spin-liquid like behavior observed in the triangular spin-1 material NiGa_2S_4 was also studied. We proposed a scenario of spin nematic phase where spin quadruples show long range order. We calculated the effects of quantum fluctuations of quadruples on various low-temperature properties and showed that the results are consistent with many of the observed experimental data.
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