Spin dynamics in the triangular-lattice antiferromagnet Cs$_2$CuBr$_4$*

We present results of our recent electron spin resonance (ESR) studies of Cs$_2$CuBr$_4$, a spin-1/2 Heisenberg antiferromagnet with a distorted triangular-lattice structure, in magnetic fields up to 50 T. Studying the magnetic excitation spectrum in the magnetically saturated phase ($H_{sat} \sim 30$ T) allowed us to accurately determine exchange parameters of Cs$_2$CuBr$_4$. In addition, we report on the observation of a substantial zero-field gap, $\sim 10$ K, whose origin will be discussed. The peculiarities of the ESR spectrum above and below $H_{sat}$ are described taking into account the effect of the Dzyaloshinskii-Moriya interaction.