Magnetoelastic interactions in SrCu₂(BO₃)₂ studied by first principles calculations and Raman experiments at high magnetic fields

SrCu­­₂(BO₃)₂ is both a Mott-Hubbard insulator and well-known physical realization of the Shastry-Sutherland model [1]. This compound is rich in magnetic phases and textures, due in large part to the strong geometric frustration of the magnetic Cu²⁺-Cu²⁺ dimers. A signature of this compound is the existence of magnetization plateaus at low temperatures, which have been probed with both dilatometry [2] and measurements of the magnetocaloric effect [2–3]. Here we present measurements of the dynamic and static crystal lattice properties of SrCu­­₂(BO₃)₂ using Raman scattering, magnetostriction, and thermal expansion measurements at magnetic fields of up to 45 T. The Raman-active “pantograph modes”—previously shown to be a major cause of the strong spin-lattice coupling in SrCu­­₂(BO₃)₂ [4]—show anomalous hardening with increasing magnetic field and temperature, and are likely responsible for stabilizing the high field magnetization plateaus. Dilatometry measurements are used to correlate the field-dependent Raman modes with the closing of the spin gap, as well as fractional-magnetization stripe states M = 1/4 M_s and M = 1/3 M_s, where M_s is the saturation magnetization.

[1] B. S. Shastry and B. Sutherland, Physica B+C 108, 1069 (1981).

[2] Jaime et al., Proc. Nat. Acad. Sci. 109, 12404 (2012).

[3] Imago et al., arXiv:2203.07607

[4] Radke et al., Proc. Nat. Acad. Sci. 112, 1971 (2015).