Topological triplon modes and bound states in a Shastry–Sutherland magnet

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Abstract

The twin discoveries of the quantum Hall effect1, in the 1980s, and of topological band insulators2, in the 2000s, were landmarks in physics that enriched our view of the electronic properties of solids. In a nutshell, these discoveries have taught us that quantum mechanical wavefunctions in crystalline solids may carry nontrivial topological invariants which have ramifications for the observable physics. One of the side effects of the recent topological insulator revolution has been that such physics is much more widespread than was appreciated ten years ago. For example, while topological insulators were originally studied in the context of electron wavefunctions, recent work has initiated a hunt for topological insulators in bosonic systems: in photonic crystals3,4,5,6, in the vibrational modes of crystals7, and in the excitations of ordered magnets8. Using inelastic neutron scattering along with theoretical calculations, we demonstrate that, in a weak magnetic field, the dimerized quantum magnet SrCu2(BO3)2 is a bosonic topological insulator with topologically protected chiral edge modes of triplon excitations.

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Figure 1: Symmetries and exchange on the magnetic lattice of SrCu2(BO3)2.
Figure 2: Evolution of the low-energy magnetic excitations of SrCu2(BO3)2 in a field along the [001] direction with cut taken in the [−1 + H, 1 + H] direction.
Figure 3: Chern numbers of triplon bands and lowest singlet bound state.
Figure 4: Edge states.

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Acknowledgements

We thank R. Bewley, R. Ewings, J. Thompson and D. Voneshen for useful discussions. The STFC Rutherford Appleton Laboratory is thanked for access to neutron beam facilities. Computing resources were provided by the STFC e-Science facility. P.A.M. acknowledges financial support from a Keeley-Rutherford fellowship. Support is also acknowledged from EPSRC Grants EP/K028960/1 (D.P.) and EP/M020517/1 (D.P. and R.C.).

Author information

D.P. grew the crystal. T.G., P.A.M., S.F.P. and A.W.P. devised and carried out the experiment. P.A.M. and F.K. worked out the theoretical interpretation. F.K. and P.A.M. wrote the manuscript. All authors discussed the data and contributed to the analysis.

Correspondence to P. A. McClarty or F. Krüger.

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McClarty, P., Krüger, F., Guidi, T. et al. Topological triplon modes and bound states in a Shastry–Sutherland magnet. Nature Phys 13, 736–741 (2017) doi:10.1038/nphys4117

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