Date and Time: Friday, May 17, 2019, 13：00-14：30
Place: IMRAM West Building 1, 2nd Floor Main Seminar room, Katahira Campus,
Tohoku University

“NMR studies of 2D quantum magnets”
Prof. Raivo Stern, National Institute of Chemical Physics & Biophysics, Tallinn, Estonia

Abstract:
Han Purple (BaCuSi2O6) 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 inter-dimer 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.* Novel Sr0.1Ba0.9CuSi2O6 with suppressed structural phase transition and just a single set of dimers at low-T is promising a new exciting extension of this study.**

The spin-dimer antiferromagnet SrCu2(BO3)2 was investigated in great detail over the past two decades, as it represents the most prominent realization of the Shastry-Sutherland lattice model. In this material, electronic spins of Cu2+ ions within the Cu2(BO3)2 layers forma lattice of mutually orthogonal spin-singlet dimers with significant inter-dimer interaction, giving rise to pronounced magnetic frustration. We performed NMR measurements in pulsed magnetic fields up to 54T using 11B nuclei.*** We observed a transition from a high-temperature, paramagnetic state to a low-T, commensurate superstructure of field-induced spin-dimer triplets in the 1/3 magnetization plateau. Moreover, the technical approach to measure broadband NMR in pulsed magnetic fields, that was developed in the course of this work, opens the door not only to the exploration of the higher-field ground states of SrCu2(BO3)2, but also to studies of many other quantum magnets with complex interactions that stabilize new phases of matter in very strong magnetic fields.

If time permits I will also introduce our original cryoMAS powder NMR technique in Tallinn and demonstrate some examples on Han Purple, TiPO4, NaTiSi2O6, and other interesting compounds.

* 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).
*** J. Kohlrautz, J, Haase, E.L. Green, Z. T. Zhang, J. Wosnitza, T. Herrmannsdörfer, H. A. Dabkowska, B. D. Gaulin, R. Stern, and H. Kühne, J. Magn. Reson. 271, 52 (2016).

Organizer: Quantum Spin Physics
Co-organizers: Network Joint Research Center for Materials and Devices
Dynamic Alliance for Open Innovation Bridging Human, Environment and Material