Crystal symmetry and high-magnetic-field specific heat of SrCu2(BO3)2

We report measurements of the specific heat of the quantum spin liquid system SrCu2(BO3)2 in continuous magnetic fields H of up to 33T. The specific heat data, when combined with a finite temperature Lanczos diagonalization of the Shastry-Sutherland Hamiltonian, indicate the presence of a nearest neighbor Dzyaloshinsky-Moriya (DM) interaction that violates the crystal symmetry. Moreover, the same DM interaction is required to explain the observed electron spin resonance lines for H‖c.