U(1)-Symmetry protected Dirac nodal loops of triplons in SrCu2(BO3)2

We demonstrate the appearance of symmetry protected triplon Dirac modal lines in the low energy excitation spectrum of a realistic microscopic model of the geometrically frustrated quantum magnet SrCu2(BO3)2 in its high symmetry phase. The symmetry-allowed Dzyaloshinskii-Moriya interactions induce dispersive trilpon bands within the bond-operator formalism that cross linearly over an extended closed path in the Brillouin zone. Our results establish that the nodal lines are protected by a $U(1)$-symmetry and robust against perturbations that preserve this symmetry. In the presence of a longitudinal field, the nodal loop shrinks and vanishes for a sufficiently strong field.