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

TL;DR

This study reveals a sequence of magnetic-field-induced quantum phase transitions in a frustrated triangular-lattice antiferromagnet, with experimental evidence for multiple stable spin states at fractional magnetizations, highlighting the role of quantum fluctuations.

Contribution

It provides experimental evidence for a cascade of quantum phase transitions in Cs₂CuBr₄, including states at fractions of saturation magnetization, previously only theoretically predicted.

Findings

Quantum fluctuations stabilize multiple spin states.

Transitions into these states are first-order.

Higher fractional states may be explained by specific spin arrangements.

Abstract

We report magnetocaloric and magnetic-torque evidence that in Cs$_{2}$CuBr$_{4}$ -- a geometrically frustrated Heisenberg $S=1/2$ triangular-lattice antiferromagnet -- quantum fluctuations stabilize a series of spin states at simple increasing fractions of the saturation magnetization $M_{s}$. Only the first of these states -- at $M={1/3}M_{s}$ -- 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 into those states is due to strong lowering of the energies by quantum fluctuations, with implications for the general character of quantum phase transitions in geometrically frustrated systems.