Influence of quantum fluctuations on zero-temperature phase transitions between collinear and noncollinear states in frustrated spin systems

Abstract

We study a square-lattice spin-$\frac{1}{2}$ Heisenberg model where frustration is introduced by competing nearest-neighbor bonds of different signs. We discuss the influence of quantum fluctuations on the nature of the zero-temperature phase transitions from phases with collinear magnetic order at small frustration to phases with noncollinear spiral order at large frustration. We use the coupled-cluster method for high orders of approximation (up to LSUB6) and an exact diagonalization of finite systems (up to 32 sites) to calculate ground-state properties. The role of quantum fluctuations is examined by comparing ferromagnetic-spiral and antiferromagnetic-spiral transitions within the same model. We find clear evidence that quantum fluctuations ``prefer'' collinear order, and that they may favor a first-order transition instead of a second-order transition when there are no quantum fluctuations.