Anomalous thermal broadening in the Shastry-Sutherland model and SrCu$_2($BO$_3)_2$

Abstract

5 pages, 5 figuresInternational audienceIn the quantum magnet SrCu$_2$⁢(BO$_3$)$_2$, an anomalous thermal broadening of the triplon modes has been measured at relatively low temperatures compared to the triplon gap $\Delta$ using both inelastic neutron scattering and Raman spectroscopy. Given how accurately a broad variety of physical phenomena inSrCu$_2$⁢(BO$_3$)$_2$ are captured by the spin $S=1/2$ Shastry-Sutherland model, it remains an open question whether the anomalous thermal broadening is also an intrinsic feature of this minimal model. However, few techniques are available for computing the finite-temperature dynamics of strongly interacting many-body systems. To address this problem, we have developed a broadly applicable numerical simulation method based on matrix-product states to simulate dynamical spectral functions at nonzero temperatures accurately, detailed in a companion paper [Phys. Rev. B 113, 024406 (2026)]. Using this technique, we demonstrate that the experimentally observed broadening is captured by the Shastry-Sutherland model. Perturbative calculations identify the origin of this phenomenon as singlet bound two-triplon states being thermally excited at an energy scale small compared to the gap $\Delta$ to the single triplon excitations at the experimentally relevant model parameters