09:00 to 09:45 
Yuan Li (Peking University, China) 
Topological magnon Dirac points in a threedimensional antiferromagnet The recent discovery of topological semimetals, which possess distinct electronband crossing with nontrivial topological characteristics, has stimulated intense research interest. By extending the notion of symmetryprotected band crossing into one of the simplest magnetic groups, namely by including the symmetry of timereversal followed by spaceinversion, we predict the existence of topological magnonband crossing in threedimensional (3D) antiferromagnets. The crossing takes the forms of Dirac points and nodal lines, in the presence and absence, respectively, of the conservation of the total spin along the ordered moments. In a concrete example of a Heisenberg spin model for a “spinweb” compound, Cu_{3}TeO_{6}, we demonstrate the presence of Dirac magnons over a wide parameter range using linear spinwave approximation [1]. Inelastic neutron scattering experiments have been carried out to detect the bulk magnonband crossing in a singlecrystal sample. The highly interconnected nature of the spin lattice suppresses quantum fluctuations and facilitates our experimental observation, leading to remarkably clean experimental data and very good agreement with the linear spinwave calculations. The predicted topological Dirac points are confirmed [2].
[1] K. Li et al., PRL 119, 247202 (2017).
[2] W. Yao et al., Nature Physics 14, 1011 (2018).



09:45 to 10:30 
Kazuhiro Nawa (Tohoku University, Japan) 
Topological triplon band and edge states in the spin dimer antiferromagnet Ba2CuSi2O6Cl2 The search for topological insulators has been actively promoted in the field of condensed matter physics. Recently, the concept of topologically insulating state and associated edge states have been extended to bosonic quasiparticles, such as magnons in solids [1, 2]. In the talk, we demonstrate that also triplons can construct topological bands in the spin1/2 twodimensional dimerized antiferromagnet Ba_{2}CuSi_{2}O_{6}Cl_{2} [3].
Inelastic neutron scattering experiments were performed by using the coldneutron disk chopper spectrometer AMATERAS [4] installed in MLF, JPARC. Twenty pieces of single crystals were coaligned so that an a* or b*direction for every crystal coincided with each other. A color contour map of the scattering intensities is shown in Fig. a. The two dispersive branches correspond to the band which is dispersive along both H and K directions. In addition, the decrease in intensity was observed at 2.6 meV, irrespective of scattering wave vectors. This can be explained by alternation in interdimer interactions along the adirection: alternation induces an energy gap between the low and high energy bands, and small structure factor of the latter band at high energies results in “splitting” of the triplon branch, as illustrated in Fig. b. The topology of the bands can be understood by regarding Ba_{2}CuSi_{2}O_{6}Cl_{2} as a pseudoonedimensional variant of SuSchriefferHeeger (SSH) model [5]. The correspondence indicates the presence of thermally excited topologically protected edge states induced by a bipartite nature of the lattice [6].
(a) Excitation spectrum at 0.3 K.
(b) Schematic dispersion relations of the triplon bands.
[1] R. Shindou, R. Matsumoto, S. Murakami, J. Ohe, Phys. Rev. B 87 144427 (2013).
[2] L. Zhang, J. Ren, J. S. Wang, and B. Li, Phys. Rev. B 87, 144101 (2013).
[3] M. Okada et al., Phys. Rev. B 94, 094421 (2016).
[4] K. Nakajima et al., J. Phys. Soc. Jpn. 80, SB028 (2011).
[5] W. P. Su, J. R. Schrieffer and A. J. Heeger, Phys. Rev. Lett. 42, 1698 (1979).
[6] K. Nawa et al., condmat arXiv:1810.08931 (2018).



10:30 to 11:00 
 
Tea Break 


11:00 to 11:45 
Jianda Wu (TsungDao Lee Institute, China) 
String excitations in the antiferromagnetic HeisenbergIsing chain (Remote Talk) Although the low energy fractional excitations of onedimensional integrable models are often wellunderstood, exploring quantum dynamics in these systems remains challenging in the gapless regime, especially at intermediate and high energies. Based on the algebraic Bethe ansatz formalism, we study spin dynamics in the antiferromagnetic spin1/2 XXZ chain with the Ising anisotropy via the formfactor formulae. Various excitations at different energy scales are identified crucial to the dynamic spin structure factors under the guidance of sum rules. At small magnetic polarization, gapless excitations of psinons and antipsinons dominate the low energy spin dynamics. In contrast, spin dynamics at intermediate and high energies is characterized by the two and threestring states. The dynamic spectra of the identified dominant excitations evolve with clear energy separations when tuning the magnetic field, conveying a simple and straightforward way to clearly identify the novel string excitations in proper condensed matter systems. Our predictions have been experimentally confirmed on the quasionedimensional material SrCo2V2O8, where the details of the experimental observations will also be discussed.



11:45 to 12:30 
Shintaro Ishiwata (University of Tokya, Japan) 
Topological and helical spin structures in centrosymmetric cubic perovskites Topological spin texture consisting of multipleq spin spiral is of great interest for novel quantum transport phenomena and spintronic functions. A recent interesting example is a magnetic skyrmion, which is a topologically stable, vortexlike spin object discovered in noncentrosymmetric systems allowing the DzyaloshinskiiMoriya (DM) interaction [1,2]. The title compound SrFeO_{3} is a promising candidate of the centrosymmetric compound hosting a novel skyrmion lattice in the absence of the DM interaction. SrFeO_{3} has been known as a rare oxide showing both helimagnetism and metallic conduction while preserving the centrosymmetric cubic lattice. While the magnetic ground state has been believed to be a simple properscrewtype spin order for long time, we have found that the magnetic phase diagram of SrFeO_{3} hosts a rich variety of helimagnetic phases, two of which show novel topological helimagnetic orders [3].
In this presentation, I will show the topologically nontrivial helimagnetic phases in the simple cubic perovskite SrFeO_{3}, which were discovered by the polarized and unpolarized small angle neutron scattering (SANS) measurements on the single crystalline samples. We found that SrFeO_{3} shows two kinds of multipleq helimagnetic structures:_{ }an anisotropic doubleq spin spiral and an isotropic quadrupleq spiral hosting a threedimensional lattice of topological singularities [4]. As a related topic, our recent discovery of a novel helimagnetic phase in the isostructural cubic perovskite Sr_{1x}Ba_{x}CoO_{3} by Sakai et al. will be also presented [5]. These perovskitetype oxides not only diversify the family of SkX host materials, but furthermore provides an experimental missing link between centrosymmetric lattices and topological helimagnetic order.
This work was done in collaboration with T. Nakajima, J. H. Kim, D. S. Inosov, Y. Tokunaga, S. Seki, N. Kanazawa, Y. W. Long, Y. Kaneko, R. George, K. Seemann, J. S. White, J. L. Gavilano, Y. Taguchi, T. Arima, B. Keimer, and Y. Tokura. This work is supported by JST PRESTO Hypernanospace design toward Innovative Functionality (Grant No. JPMJPR1412) and the JSPS GrantinAid for Scientific Research (A) Grant No. 17H01195.
[1] S. Mühlbauer et al., Science 323, 915 (2009).
[2] S. Seki, X. Z. Yu, S. Ishiwata, and Y. Tokura, Science 336, 198 (2012).
[3] S. Ishiwata et al., Phys. Rev. B 84, 054427 (2011).
[4] S. Ishiwata et al., arXiv:1806.02309.
[5] H. Sakai et al., Phys. Rev. Mater. 2, 104412 (2018).



12:30 to 14:30 
 
Lunch Break 


14:30 to 15:15 
Zi Yang Meng (Institute of Physics Chinese Academy of Sciences, China) 
Dynamical signatures in quantum matter beyond LandauGinzburgWilson paradigm 


15:15 to 16:00 
Kedar Damle (TIFR, Mumbai, India) 
Frustrated transverse field ising models 


16:00 to 16:30 
 
Tea Break 


16:30 to 17:15 
Srimanta Middey (IISc, India) 
Interface and Lattice Engineering of Complex Oxides Interface engineering of complex oxides has become a popular approach to realize fascinating collective phenomena, which are very often “hidden” or unattainable in the constituent bulk materials. While the strong interplay among spin, charge, orbital, lattice degrees of freedom facilitate interesting manybody quantum phenomena in correlated oxides, the additional broken symmetries and frustrated couplings across the interface of artificial heterostructures may give rise to new electronic, magnetic states. Geometrical lattice engineering (GLE) has been presented as another potential way in recent times to realize novel topological and quantum manybody states. The key idea behind the GLE is to design fully epitaxial fully epitaxial ultrathin heterostructures with an artificial lattice geometry (e.g. buckled honeycomb lattice, Kagome lattice etc.) generated by stacking of a very specific number of atomic planes along a particular orientation.
As a prototype example of such interface and lattice engineering, I will talk about our ongoing work on rare earth nickelate heterostructures. The results of synchrotron diffraction, xray absorption spectroscopy, resonant xray scattering to elucidate the response of underlying lattice, spin, orbital and charge degrees of freedom combined with strain and quantum confinement will be presented.

S. Middey et al., Annual Review of Materials Research 46, 305334 (2016).

S. Middey et al., Phys. Rev. Lett. 116, 056801 (2016).

S. Middey et al., Phys. Rev. Lett. 120, 156801 (2018); Phys. Rev. B 98, 045115 (2018); Appl. Phys. Lett. 113, 081602 (2018).



17:15 to 18:00 
Vikram Tripathi (TIFR, Mumbai, India) 
Strong ferromagnetic Kitaev correlations in the honeycomb iridate Na2IrO3: evidence from highfield magnetometry The magnetic field response of the Mottinsulating honeycomb iridate Na_{2}IrO_{3} is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peakdip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering (≈ 15K) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that at high magnetic fields, long range spin correlation functions decay rapidly, signaling a transition to a longsoughtafter fieldinduced quantum spin liquid beyond the peakdip structure. Kitaev systems are thus revealed to be excellent candidates for fieldinduced quantum spin liquids, similar physics having been suggested in another Kitaev material α−RuCl3.


