报告题目：Magnetism of spin-chain systems and the role of spin-orbital coupling
Low-dimensional and geometrically frustrated spin systems exhibit some of the most interesting physical phenomena seen in condensed matter physics. Due to the low site connectivity and competing interactions, classical order is often suppressed by quantum and thermal fluctuations, giving rise to novel ground states and quasiparticle excitations. Among the low-dimensional systems, the spin-chain systems with general formula A3MM’O6 (A = alkaline-earth metal, M/M’ = transition metals) have attracted much attention in recent years, due to their reduced dimensionality in the presence of geometrical frustration [1-4]. The crystal structure consists of one-dimensional (1D) chains that are oriented along the c-axis and arranged in a triangular lattice in the ab plane. Beside the strongly one-dimensional crystal structure these compounds show strong spin anisotropy and possibility to investigate the role of spin-orbital coupling on the magnetism. We have been investigating the magnetism of Sr3NiIrO6, Sr3ZnIrO6 and Sr3NiPtO6 using neutron scattering, both elastic and inelastic, as well as muon spin rotation/relaxation (µSR) measurements. Despite the presence of two magnetic phase transitions (TN1 = 75 K and TN2 = 20 K) in the magnetization of Sr3NiIrO6, neutron diffraction study reveals only one long range magnetic ordering below 70 K with antiferromagnetic coupling between Ni2+ and Ir4+ moment along the c-axis. On the other hand the non-collinear magnetic structure of Sr3ZnIrO6 reveals that the moments of Ir4+ are tilted away from the c-axis, indicating the role of Dzyaloshinskii-Moriya (DM) interaction. Our inelastic neutron scattering study reveals a quasi-1D magnon excitations with a giant spin gap of 30 meV in Sr3NiIrO6. The analysis of the spin wave spectrum reveals strongly coupled Ising-like chains (Jz >> Jxy) along the c-axis that are weakly coupled into a frustrated triangular lattice in the ab-plane. The magnetic excitations survive up to 200 K well above the magnetic ordering temperature of TN ∼ 75 K, also indicating a quasi-1D nature of the magnetic interactions in Sr3NiIrO6. Our microscopic model is in agreement with ab initio electronic structure calculations  and explains the giant spin-flip field observed in bulk magnetization measurements. We will compare the magnetic excitations observed in Sr3NiIrO6 with that of Sr3ZnIrO6 and Sr3NiPtO6 and discuss the nature of spin-liquid ground state in Sr3NiPtO6 .
 A. D. Hillier, D. T. Adroja, W. Kockelmann, L. C. Chapon, S. Rayaprol, P. Manuel, H. Michor, and E. V. Sampathkumaran, Phys. Rev. B83, 024414 (2011).
 E. Lefrancois, L. C. Chapon,V. Simonet, P. Lejay, D. Khalyavin, S. Rayaprol, E.V. Sampathkumaran, R. Ballou, andD. T.Adroja, Phys. Rev. B90, 014408 (2014).
 S. Toth, W. Wu, D. T. Adroja, S. Rayaprol, and E. V. Sampathkumaran, Phys. Rev. B93, 174422 (2016)
 P. McClarty, A.D. Hillier, D.T. Adroja, S. Rayaprol and E.V. Sampathkumaran, Phys. Rev., submitted (2017): arXiv:1610.00038v1
 X. Ou and H. Wu, Sci. Rep. 4, 4609 (2014); E. E. Gordon, H. Xiang, J. Köhler, and M.-H. Whangbo, J. Chem. Phys. 144, 114706 (2016).
. D.T. Adroja et al, unpublished (2017).
Dr Adroja is a senior scientist at the ISIS facility and is responsible for the MERLIN spectrometer and he is also a visiting professor at Johannesburg University, South Africa. His research programme is focused on strongly correlated electron systems with more than 24 years of experience in neutron scattering and muon spin rotation technique. Dr Adroja obtained his Ph.D. from IIT Bombay (in 1991), India, and then worked as a research fellow at Southampton, Hiroshima (JSPS fellowship 1997), St Andrews universities before joining ISIS in 1999. Since 2001, he work at ISIS as an instrument scientist in Excitations group, then from 2007 to present, he become a Senior Scientist at ISIS. He was the pioneer in discovering the Kondo insulator CeRhS, and the related materials Ce(NiCo)Sn and CePdSb. Recently he has made significant contributions to the field of spin and charge gap formation in strongly correlated electron systems and published two review papers on this topic. He has published over 290 papers in peer-review journals.