Speaker
Description
The Shastry-Sutherland lattice is one of the paradigmatic geometries used in frustrated magnetism, and the properties of the Heisenberg and Ising models on this lattice have been studied extensively over the last two decades. The Heisenberg model has several well identified properties, including a spin gap and a remarkable series of magnetization plateaus at $1/8$, $2/15$, $1/6$, $1/4$, $1/3$, $2/5$ and $1/2$, all observed in an almost perfect realization of that model, SrCu$_2$(BO$_3$)$_2$. In comparison, the Ising model on the same lattice is expected to exhibit simpler physics, with only a $1/3$ plateau, and for a long time no clean experimental realization was known. Recently, a family of rare-earth compounds with Shastry-Sutherland geometry has been synthetized, and one of its members, Er$_2$Be$_2$GeO$_7$, has a magnetization curve typical of an Ising model, with nearly vertical jumps between plateaus [1]. However, quite surprisingly, there are two plateaus at $1/4$ and $1/2$, and no plateau at $1/3$, in clear contradiction with the theoretical expectation. In this talk, I will argue that this very peculiar plateau sequence has its source in a small orthorhombic distortion visible in single-crystal neutron scattering experiment. This distortion induces two different values of the exchange couplings on the dimers of the Shastry-Sutherland lattice, and, thanks to extensive analytical and numerical investigations, we have shown that this is sufficient to replace the $1/3$ plateau by two plateaus at $1/4$ and $1/2$. A complete account of all properties, including specific heat and neutron scattering inside the plateaus, can be achieved if residual farther range dipolar interactions are included. However, without the orthorhombic distortion, the correct plateau structure could not be explained, as revealed by ground state simulations of the dipolar model (J. A. Koziol and K. P. Schmidt, private communication).
Acknowledgements
This work was supported by NSF award number DMR-2327555 (Duke University) and the Swiss National Science Foundation grant number 212082 (EPFL).
References
[1] L. Yadav et al., “Unprecedented Fractional Magnetization Plateaus in a New Shastry-Sutherland Ising Compound,” 2024, arXiv. https://doi.org/10.48550/ARXIV.2405.12405
