Metal-insulator transition in transition-metal oxides has drawn much attention because such a phenomenon reflects interplay among spin, charge, lattice, and orbital degrees of freedom. Our group has been studying their behavior by neutron scattering technique with use of pulse and steady-source neutron beams, and the triple axis neutron spectrometer owned by the ISSP is shown in the left bottom figure. The hole-doping dependences of the incommensurability and the transition temperature of the stripe ordering for hole concentration x up to x = 0.5 have been observed in the highly-doped La_{2-x}Sr_{x}NiO_{4} which is one of the isomorphic compounds of the High T_{c} cuprate superconductors. The ordering temperatures of the stripe charge as well as spin orderings have the maxima at x = 1/3, and they decrease beyond x = 1/3. Beyond x = 1/2, the Ni system gradually becomes metallic and show the insulator to metal transition at x ~ 0.9. Our recent pulse neutron studies on the Ni system revealed that the characteristics of the spin dynamics changes at x = 0.5, and further detail studies of such unusual spin dynamics and their relation to the transport properties are now ongoing.

Triple-axis spectrometer installed at the JRR-3 in the Japan Atomic Energy Agency, Tokai, Ibaraki.

Doping dependence of the stripe ordering in highly-doped La_{2-x}Sr_{x}NiO_{4}, which is an isomorphic compounds of one of High T_{c} cuprate superconductors La_{2-x}Sr_{x}CuO_{4}. Unlike the expectation from the previous studies, the transition temperatures of the stripe charge and spin orderings exhibit the maximums at x = 1/3, and decreases above x = 1/3. The periodicity of the stripe order is approximately linear in x, but levels off beyond x = 1/2. The system gradually becomes metallic and shows the insulator to metal transition near x ~ 0.9.

Research Subjects

Neutron scattering study of metal-insulator transition, magnetic ordering, charge/orbital orderings in transition-metal oxides

Neutron scattering study of spin sturucture and dynamics in noncentrosymmetric magnetic superconductors

^{†}Crystalline Electric Field Level Scheme of the Non‐ Centrosymmetric CePtSi_{3}: D. Ueta, T. Kobuke, M. Yoshida, H. Yoshizawa, Y. Ikeda, S. Itoh and T. Yokoo, Physica B536 (2018) 21-23.

2.

^{†}Magnetic and Thermodynamic Studies on the Charge and Spin Ordering in the highly-doped La_{2-x}Sr_{x}CoO_{4}: M. Yoshida, D. Ueta, Y. Ikeda, T. Yokoo, S. Itoh and H. Yoshizawa, Physica B536 (2018) 338-341.

3.

^{†}Anisotropic pressure effects on superconductivity in Fe_{1+y}Te_{1-x}S_{x}: K. Yamamoto, T. Yamazaki, T. Yamanaka, D. Ueta, H. Yoshizawa and H. Yaguchi, J. Phys. Soc. Jpn.87 (2018) 054705.

Inelastic Neutron Scattering Study of Stripe and Overdoped Checkerboard Ordering in Layered Nickel Oxide Nd_{2−}_{x}Sr_{x}NiO_{4}: Y. Ikeda, S. Suzuki, T. Nakabayashi, H. Yoshizawa, T. Yokoo and S. Itoh, J. Phys. Soc. Jpn.85 (2016) 023701.

^{†*}Superconductivity and Non-Fermi-Liquid Behavior in the Heavy-Fermion Compound CeCo_{1−}_{x}Ni_{x}In_{5}: R. Otaka, M. Yokoyama, H. Mashiko, T. Hasegawa, Y. Shimizu, Y. Ikeda, K. Tenya, S. Nakamura, D. Ueta, H. Yoshizawa and T. Sakakibara, J. Phys. Soc. Jpn.85 (2016) 094713.