講演会のお知らせ （11/10：Prof. Raivo Stern (National Institute of Chemical Physics & Biophysics, Estonia))

講演題目：High field induced Bose-Einstein condensation in strongly correlated BaCiSi₂O₆
講師：Prof. Raivo Stern (National Institute of Chemical Physics & Biophysics, Estonia)
日時：２０１６年１１月１０日（木）１６：００－１７：３０
場所：コラボレーション棟３階　コラボレーション室
要旨：
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 Sr_0.1Ba_0.9CuSi₂O₆ 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).


連絡先：　理学部物理学科　鄭　国慶　（内線７８１３）