Anomalous Physical Properties of Strongly Correlated Electron System with Frustration and Spin Chirality

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Studies focused mainly on strongly correlated electrons with geometrical frustration have been carried out. (i)Unusual behaviors of the anomalous Hall resistivity(AHR) of ferromagnetic systems with non-trivial magnetic structure have been studied to clarify if the behavior is related to the ordering of spin chirality χ locally defined for three spins S_1, S_2 and S_3 as χ≡S_1(S_2×S_3). Ground states of frustrated spin systems have been studied experimentally for various pyrochiore type oxides. (ii) Superconductivity of Na_xCoO_2・yH_2O has been studied mainly by means of NMR. (iii)We have also studied the physics of high-T_c Cu oxides mainly by studying the characteristics of their magnetic excitation spectra.To see the origin of unusual behaviors of AHR of Nd_2Mo_2O_7, the magnetic structure has been analyzed as a function of the applied magnetic field Hand the H-dependence of the chirality (or AHR) has been directly calculated from the obtained results. Mo→Ti substitution effects on the behaviors of the AHR has also been studied. The results of these studies clearly indicate that the chiral order mechanism proposed theoretically does not, at least, have a primary role in determining the behavior of the AHR of Nd_2Mo_2O_7. In the study on AHR of a reentrant spin glass system Fe_<1-x>Al_x, it has been shown that the uniform chirality induced by the coupling of the frozen chirality with the uniform magnetization contributes to AHR. In the study on so-called spin ice system, Ho_2Ti_2O_7, we hav.e shown what kind of spin correlation this strongly frustrated spin system has at T→0.To clarify if the superconductivity of Na_xCoO_2・yH_2O with triangular lattice of CoO_2, The NQR 1/T_1T and the NMR Knight shift K have been measured, where a very small (or no) coherence peak of 1/T_1T and the decrease of K have been observed. Extensive studies are now being carried out.