Field-induced quantum phase transitions in the spin-1/2 triangular-lattice antiferromagnet Cs₂CuBr₄

In classical magnetic spin systems, geometric frustration leads to a large number of states of identical energy. We report here evidence from magnetocaloric and related measurements that in Cs₂CuBr₄ - a geometrically frustrated Heisenberg S= 1/2 triangular 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 fields directed along the c axis. Only the first of these quantum phase transitions has been theoretically predicted, suggesting that quantum effects continue to dominate at fields much higher than previously considered.