Cascade of magnetic-field-induced quantum phase transitions in a spin $\bm{1/2}$ triangular-lattice antiferromagnet

We report magnetocaloric evidence that in Cs$_{2}$CuBr$_{4}$ -- a geometrically frustrated Heisenberg $S=1/2$ triangular-lattice antiferromagnet -- quantum fluctuations stabilize a series of gapped collinear spin states bounded by first-order transitions at simple increasing fractions of the saturation magnetization for magnetic fields directed along the c axis. Only the first of these states has been theoretically predicted. We discuss how the higher fraction quantum states might arise and propose model spin arrangements. We argue that the first-order nature of the transitions is due to strong lowering of the energies of the collinear states by quantum fluctuations, with implications for the general character of quantum transitions in geometrically frustrated systems.