10/21より陽子ビーム受け入れ開始、10/27に施設検査。
早稲田大学にて開催（リモートも含めて100名程度参加）
KDS: https://kds.kek.jp/category/2873/
◆ 研究成果
Dynamic exchange striction in the Kondo insulator YbB12
Kiril Nemkovski1, Kazuhiko Ikeuchi2,3,4, Tetsuya R. Yokoo2,3,4,5, Fumitoshi Iga6,7, Toshiro Takabatake8
1ISIS Neutron and Muon source, STFC Rutherford Appleton Laboratory, UKRI, Didcot OX11 0QX, UK
2Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan
3Materials & Life Science Division, J-PARC Center, Tokai, Ibaraki 319-1106, Japan
4Graduate Institute for Advanced Studies, SOKENDAI, Tsukuba, Ibaraki 305-0801, Japan
5University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
6Comprehensive Research Organization Society (CROSS), Tokai, Ibaraki 319-1106, Japan
7Ibaraki University, Mito, Ibaraki 310-8512, Japan
8Hiroshima University, Higashi-Hiroshima 739-8530, Japan
YbB12 is an archetypal system exhibiting so-called Kondo insulator behaviour at low temperature. Despite the intensive investigations, the nature of the ground state, as well as the transition to the Kondo-insulating regime remains a subject of debate. Though theoretical approaches to this problem usually deal with the electron subsystem, one cannot disregard a possible interplay with lattice vibrations. Previous study [1] demonstrated an anomalous additional intensity in the phonon spectrum of YbB12 around 15 meV, which correlates with low-energy magnetic excitations in temperature, energy and intensity distribution within the Brillouin zone. The purpose of this study was to clarify the nature of this effect by studying its Q-dependence on the single-crystalline sample.
The experiment has been performed at POLANO instrument (BL23). The sample of Yb11B12 (99.5% 11B enrichment) was the same as in [1]. It was oriented with <1-10> axis vertically, so that the horizontal scattering plane includes all three main symmetry directions of the FCC structure. We have performed measurements at 5K and 130K in a wide range of momentum transfer. The main incident energy was 121 meV, with resolution of 6.7 meV (FWHM) at elastic line. Fig.1 shows the inelastic intensity at 15 meV as a function of Q at 5K and 130 K. Although at small Q some magnetic scattering occurs, too, the overall intensity is dominated by phonon contribution. In agreement with previous results [1], after correction for Bose factor the intensity at 5K clearly exceeds the one at 130 K. The differential signal increases with Q in the same manner as original intensities before subtraction, demonstrating typical phonon behaviour. Thus, we can conclude that the observed effect is not due to any kind of hybrid phonon-magnetic nature and certainly not because of the residual magnetic intensity expanding to higher Q. Most likely interpretation is a dynamic exchange striction of the crystal lattice, which occurs in the temperature range where YbB12 demonstrates an antiferromagnetic excitation [2] and disappears upon increase of the temperature and transition to a paramagnetic regime.
[1] P. A. Alekseev, J.-M. Mignot, K. S. Nemkovski et al., JPCM 24, 205601 (2012).
[2] K. S. Nemkovski, J.-M. Mignot, P. A. Alekseev et al., PRL 99, 137204 (2007).
Fig.1. Q-dependence of inelastic intensity at 15 meV (spectra were integrated over energy in the range from 13 to 17 meV) at Brillouin zone boundaries along <111>.
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