题目：High Magnetic Field Sciences at AHMF in Osaka University

报告人：Masayuki Hagiwara教授

       日本大阪大学先端强磁场科学研究中心主任

时间：2018年12月4日上午9:30

地点：国家脉冲强磁场科学中心B206

报告摘要：

I will talk about two topics studied in high magnetic fields at the Center for Advanced High Magnetic Field Science (AHMF) after brief introduction of our high magnetic field facilities and experimental apparatus.

The first topic is “the spin multipolar excitations observed in åkermanite compounds Sr₂CoB₂O₇(B=Ge and Si)”. In these compounds, Co²⁺ions have magnetic moments (S=3/2). These are magneto-electric multiferroic materials in which spontaneous electric polarization appears due to thed-phybridization mechanism. The Sr₂CoGe₂O₇, abbreviated as SCGO, has small magnetic anisotropy, while the Sr₂CoSi₂O₇, abbreviated as SCSO, has large magnetic anisotropy. In SCGO, we observed a two-magnon spin excitation appearing above the saturation magnetic field (H_sat) in the electron spin resonance spectra. Owing to small anisotropy, this excitation has purely spin-quadrupolar nature, and hence its observation is possible by electric component of the light (electro-magnon) due to the spin nematic interaction in SCGO. On the other hand, we observed not only two-magnon but also three-magnon spin excitations aboveH_satin SCSO. The three-magnon (spin-octupolar) excitation was observed due to large magnetic anisotropy which mixes magnetic dipole and quadrupole terms, resulting in making this excitation possible by magnetic component of the light. We discuss the similarity and difference in spin excitations between SCGO and SCSO.

The second topic is “the high-field magnetization of theS=1/2 honeycomb-lattice antiferromagnet Cu₂(pymca)₃(ClO₄) where pymca stands for pyrimidine-2-carboxylate”. Recently, a honeycomb-lattice magnet with bond-dependent exchange interactions has been extensively studied as the Kitaev model [3], but in this talk, I will present the studies on a simple Heisenberg honeycomb-lattice antiferromagnet. The magnetic susceptibility shows a broad peak near 25 K, and no long range order is observed down to 0.6 K in the specific heat measurements. The magnetization curve up to 70 T at 1.4 K shows three step-like increases. Assuming three different exchange bonds based on the structure, the calculated magnetization curve reproduces the observed one except for the magnetization near 70 T, where the observed magnetization indicates another step while the calculated magnetization becomes saturated. The relationship between the magnetization plateaus and exchange bonds is discussed based on the numerical calculations.

报告人简介：

Masayuki Hagiwara教授1992年在大阪大学获得理学博士学位,先后在日本东芝公司，日本理化所(RIKEN)任职。2004年任日本大阪大学先端强磁场科学中心教授并任中心主任。研究领域为强磁场下低维量子自旋体系的物性研究。尤其专长于一维体系，如Luttinger液体、Haldane自旋链以及Kagome磁阻挫体系在极低温和强磁场下的实验研究。在PRL、PRB等凝聚态物理领域期刊发表学术论文200余篇。