平成２８年度（２０１６年度）第9回 物理教室談話会

題目	High field induced Bose-Einstein condensation in strongly correlated BaCiSi2O6
講師	Prof. Raivo Stern (National Institute of Chemical Physics & Biophysics, Estonia)
日時	平成28年11月10日(木)　16：00～
場所	コラボレーション棟３階　コラボレーション室
Abstract	Han Purple (BaCiSi2O6) is not only a remarkable ancient pigment, but has also served physicists as a valuable model material for studying Bose-Einstein condensation (BEC) of magnons in high magnetic fields. We have characterized the BEC phase by copper and silicon NMR at 50 mK and around 23-27 T. In parallel, we have succeeded to establish low-T structural model of BaCuSi2O6 via scattering techniques. Using these precise low-T structural data and extensive density-functional calculations, we elucidate magnetic couplings in this compound. The resulting magnetic model comprises two types of nonequivalent spin dimers, in excellent agreement with the 63,65Cu NMR data. We further argue that leading interdimer couplings connect the upper site of one dimer to the bottom site of the contiguous dimer, and not the upper-to-upper and bottom-to-bottom sites, as assumed previously. This finding is verified by inelastic neutron scattering data and implies the lack of magnetic frustration in BaCuSi2O6, thus challenging existing theories of the magnon BEC in this compound.1) Novel Sr0.1Ba0.9CuSi2O6 with suppressed structural phase transition and apparently just single set of dimers at low-T is promising new exciting extention of this study.2) * V.V.Mazurenko, M.V.Valentyuk, R.Stern, A.A.Tsirlin, PRL 112, 107202 (2014). ** P. Puphal, D. Sheptyakov, N. van Well, L. Postulka, I. Heinmaa, F. Ritter, W. Assmus, B.Wolf, M.Lang, H. O. Jeschke, R. Valenti, R. Stern, C. Rüegg, C. Krellner, PRB 93, 174121 (2016).
世話人	鄭　国慶（内線：7813）