Classical and quantum two-dimensional anisotropic Heisenberg antiferromagnets.

Holtschneider,M.,~ Wessel,S.,~ Selke, W.~
Institut für Theoretische Physik, Technische Hochschule Aachen,
D-52056 Aachen, Germany; Institut für Theoretische Physik, Universität Stuuttgart, Stuttgart, Germany


The classical and the quantum, spin~$S=\frac{1}{2}$, versions of the uniaxially anisotropic Heisenberg antiferromagnet on a square lattice in a field parallel to the easy axis are studied using Monte Carlo techniques. For the classical version, the role of biconical fluctuations at low temperatures in the transition region between the antiferromag netic and spin-flop phases is elucidated. For the quantum version, the previously proposed scenario of a first-order transition between the antiferromagnetic and spin-flop phases with a critical endpoint and a tricritical point is scrutinized