Neutron scattering from the static and dynamic lattice of SrCu2(BO3)2 in its Shastry-Sutherland singlet ground state

Elastic and inelastic neutron scattering results show that SrCu2(BO3)2 enters its low-temperature singlet ground state below 10 K without an obvious accompanying structural phase transition, despite suggestions emanating from earlier heat capacity measurements. However, evidence for significant spin-phonon coupling is found in the energy widths, and the corresponding lifetimes, of transverse acoustic phonons propagating in the (H00) direction of the Shastry-Sutherland, tetragonal basal plane. Transverse acoustic phonons with energies comparable to and higher than the onset of the two-triplet continuum show substantially increased lifetimes on entering the singlet ground state below ∼10 K. This is qualitatively consistent with the removal of a decay channel for the phonons due to the gapping of the spin excitation spectrum in SrCu2(BO3)2 at low temperatures.