Browsing by Author "Okamoto, Y"

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    Magnetic order in frustrated Kagome-Triangular lattice antiferromagnet NaBa2Mn3F11
    (American Physical Society, 2017-03) Hayashida, S; Ishikawa, H; Okamoto, Y; Okubo, T; Hiroi, Z; Avdeev, M; Manuel, P; Hagihala, M; Soda, M; Masuda, T
    We performed powder neutron diffraction experiments on NaBa2Mn3F11 [1], a model compound of \textit{Kagome-Triangular} lattice where three of six next-nearest neighbor interactions are non-negligible. More than 10 magnetic Bragg peaks are clearly observed below T= 2 K, meaning that the ground state is a magnetically ordered state. From indexing the magnetic Bragg peaks, magnetic propagation vector of \textbf{\textit{q}}0= (0, 0, 0) and two incommensurate vectors which are close to (1/3, 1/3, 0) are identified. Combination of representation analysis and Rietveld refinement reveals that the propagation vector of \textbf{\textit{q}}0 exhibits the 120º structure in the \textit{ab}-plane. Our calculation of the ground state suggests that the non-negligible magnetic dipolar interaction is responsible for the determined 120º structure in NaBa2Mn3F11. © 2021 American Physical Society
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    Magnetic state selected by magnetic dipole interaction in the kagome antiferromagnet NaBa2Mn3F11
    (American Physical Society, 2018-02-12) Hayashida, S; Ishikawa, H; Okamoto, Y; Okubo, T; Hiroi, Z; Avdeev, M; Manuel, P; Hagihala, M; Soda, M; Masuda, T
    We haved studied the ground state of the classical kagome antiferromagnet NaBa2Mn3F11. Strong magnetic Bragg peaks observed for d spacings shorter than 6.0 Å were indexed by the propagation vector of k0=(0,0,0). Additional peaks with weak intensities in the d-spacing range above 8.0 Å were indexed by the incommensurate vector of k1=[0.3209(2),0.3209(2),0] and k2=[0.3338(4),0.3338(4),0]. Magnetic structure analysis unveils a 120∘ structure with the tail-chase geometry having k0 modulated by the incommensurate vector. A classical calculation of the Heisenberg kagome antiferromagnet with antiferromagnetic second-neighbor interaction, for which the ground state a k0120∘ degenerated structure, reveals that the magnetic dipole-dipole (MDD) interaction including up to the fourth neighbor terms selects the tail-chase structure. The observed modulation of the tail-chase structure is attributed to a small perturbation such as the long-range MDD interaction or the interlayer interaction. ©2018 American Physical Society
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    Novel K rattling: a new route to thermoelectric materials?
    (AIP Publishing, 2014-01-15) Shoko, E; Okamoto, Y; Kearley, GJ; Peterson, VK; Thorogood, GJ
    We have performed ab initio molecular dynamics simulations to study the alkali-metal dynamics in the Al-doped (KAl0.33W1.67O6 and RbAl0.33W1.67O6) and undoped (KW2O6 and RbW2O6) defect pyrochlore tungstates. The K atoms exhibit novel rattling dynamics in both the doped and undoped tungstates while the Rb atoms do not. The KAl0.33W1.67O6 experimental thermal conductivity curve shows an unusual depression between ∼50 K and ∼250 K, coinciding with two crossovers in the K dynamics: the first at ∼50 K, from oscillatory to diffusive, and the second at ∼250 K, from diffusive back to oscillatory. We found that the low-temperature crossover is a result of the system transitioning below the activation energy of the diffusive dynamics, whereas the high-temperature crossover is driven by a complex reconstruction of the local potential around the K atoms due to the cage dynamics. This leads to a hardening of the K potential with increasing temperature. This unusual reconstruction of the potential may have important implications for the interpretation of finite-temperature dynamics based on zero-temperature potentials in similar materials. The key result is that the novel K rattling, involving local diffusion, leads to a significant reduction in the thermal conductivity. We suggest that this may open a new route in the phonon engineering of cage compounds for thermoelectric materials, where the rattlers are specifically selected to reduce the lattice thermal conductivity by the mechanism of local diffusion. © 2014 AIP Publishing LLC.