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論文

Investigation of the phonon dispersion associated with superlattice reflections in the BiS $_{2}$ -based superconductor LaBiS $_{2}$ O $_{0.5}$ F $_{0.5}$

玉造博夢; 長谷川巧*; 佐賀山基*; 水牧仁一朗*; 村上洋一*; 梶谷丈*; 東中隆二*; 松田達磨*; 青木勇二*; 筒井智嗣*

Physical Review B, 107(2), p.024303_1 - 024303_8, 2023/01

https://doi.org/10.1103/PhysRevB.107.024303

被引用回数：0 パーセンタイル：0(Materials Science, Multidisciplinary)

The phonon dispersion of a BiS $_{2}$ -based superconductor LaBiS $_{2}$ O $_{0.5}$ F $_{0.5}$ is investigated by first-principles calculations and inelastic X-ray scattering experiments. The origin of superlattice (SL) reflections arising from transverse-type lattice modulation, which were recently reported in [J. Kajitani ., J. Phys. Soc. Jpn. , 103601 (2021)], is discussed in terms of lattice dynamics. Our first-principles calculations of phonon dispersion and the Fermi surfaces (FSs) demonstrate that the phonon mode corresponding to the transverse-type lattice modulation is unstable, and the propagation vector corresponding to the SL reflections is close to the FS nesting vector, which suggests that the phonon softening originates from the FS nesting. Against these calculated expectations, measured phonon dispersion in LaBiS $_{2}$ O $_{0.5}$ F $_{0.5}$ along the Z-A direction, where the SL point is located, shows no remarkable temperature dependence, and there are no steeply declining branches accompanied with a softening around the SL point. Based on these results, we discuss the two possibilities for the transverse lattice modulation in LaBiS $_{2}$ O $_{0.5}$ F $_{0.5}$ : the order-disorder type structural transition and the displacive structural transition with an overdamped mode, for both of which the local structure distortion or the short range correlation within the BiS $_{2}$ plane would be essential.

論文

Partial breakdown of translation symmetry at a structural quantum critical point associated with a ferroelectric soft mode

石井悠衣*; 山本有梨沙*; 佐藤直大*; 南部雄亮*; 河村聖子; 村井直樹; 尾原幸治*; 河口彰吾*; 森孝雄*; 森茂生*

Physical Review B, 106(13), p.134111_1 - 134111_7, 2022/10

https://doi.org/10.1103/PhysRevB.106.134111

被引用回数：0 パーセンタイル：0(Materials Science, Multidisciplinary)

We report that complete suppression of a phonon-driven structural phase transition causes partial breakdown of a three-dimensional translation symmetry in a well-defined sublattice. This state is revealed for a dielectric compound, Ba $_{1-x}$ SrAlO, that comprises an AlO network incorporated into a hexagonal Ba(Sr) sublattice. Pair distribution function analyses and inelastic neutron scattering experiments provide clear-cut evidence of the AlO network forming a continuum of Al-O short-range correlations similar to glasses, whereas the Ba(Sr) sublattice preserves the original translational symmetry. This glassy network significantly dampens the phonon spectrum and transforms it into the broad one resembling those typically observed in glass materials.

論文

Structural phase transitions in CsPbBr $_{3}$

弘津俊輔*; 原田仁平*; 飯泉仁; 下司和男

Journal of the Physical Society of Japan, 37(5), p.1393 - 1398, 1974/05

https://doi.org/10.1143/JPSJ.37.1393

被引用回数：234

ペロブスカイト型構造をもつCsPbBr $_{3}$ の構造相転移を中性子回折法により調べた。相転移は88 $^{circ}$ Cと130 $^{circ}$ Cにあって、それぞれ2次、1次である。130 $^{circ}$ Cの相転移はブリルアン帯のM点におけるM $_{3}$ フオノンの凍結によって起り、88 $^{circ}$ Cの相転移は正方晶ブリルアン帯のZ点におけるR $_{2}$ 5的モードの凍結によって起ることがわかった。これらの結果と群論的考察とにより、空間群は上の相から順次O $_{h}$ $^{1}$ -Pm3m、D $_{4}$ $_{h}$ $^{5}$ -P4/mbm、D $_{2}$ $_{h}$ $^{1}$ $^{6}$ Pmbnであることがわかった。又ソフト・モードの固有ベクトルから、各相での原子変位が求められた。