ESR studies of the quasi-2D frustrated Cs2CuBr4*

We report low-temperature electron spin resonance (ESR) studies of single-crystalline samples of Cs$_2$CuBr$_4$, a spin-1/2 antiferromagnet with a triangular spin-lattice structure. A remarkable angular dependence of the resonance field, including the splitting of the ESR line for some orientations of the magnetic field, and the presence of a gap in the ESR excitation spectrum at temperatures above $T_N\sim$ 1.3~K have been revealed. Our observations suggest that uniform Dzyaloshinskii-Moriya interaction affects the low-energy excitation spectrum in this frustrated compound. The results are compared with that obtained recently for the isostructural material Cs$_2$CuCl$_4$ [Povarov et al., Phys. Rev. Lett. 107, 037204 (2011)].