Universal low-temperature behavior of frustrated quantum antiferromagnets in the vicinity of the saturation field

We study the low-temperature thermodynamic properties of a number of frustrated quantum antiferromagnets which support localized magnon states in the vicinity of the saturation field. For this purpose we use 1) a mapping of the low-energy degrees of freedom of spin systems onto the hard-core object lattice gases and 2) an exact diagonalization of finite spin systems of up to N = 30 sites. The considered spin systems exhibit universal behavior which is determined by a specific hard-core object lattice gas representing the independent localized magnon states. We test the lattice gas description by comparing its predictions with the numerical results for low-lying energy states of finite spin systems. For all frustrated spin systems considered we find a strong variation of the low-temperature specific heat passing the saturation field and a maximum in the isothermal entropy at saturation field resulting in an enhanced magnetocaloric effect. PACS codes: 75.10.Jm Quantized spin models 75.45.+j Macroscopic quantum phenomena in magnetic systems 75.30.Sg Magnetocaloric effect, magnetic cooling 1 Introductory remarks Quantum spin antiferromagnets on geometrically frustrated lattices have attracted much attention during last years [1–3]. A new and rapidly developing direction in this field of quantum magnetism is a study of the lowtemperature properties of such systems in the presence of an external magnetic field. The Zeeman interaction of spins with a magnetic field competes with the frustrating antiferromagnetic interspin interactions that may lead to new phenomena. Recently it has been found that a wide class of geometrically frustrated quantum spin antiferromagnets (including the kagomé and pyrochlore antiferromagnets) has quite simple ground states in the vicinity of the saturation magnetic field [4–6] (for a review, see Refs. [7, 8]). These ground states consist of independent (i.e. isolated) localized magnons in a ferromagnetic environment. The localized magnon states were used to predict a ground-state magnetization jump at the saturation field [4–6], a magnetic field induced spin-Peierls instability [9,10], and a residual ground-state entropy at the saturation field [3,8,11–13]. Moreover, in Refs. [8,12,13] the concept of localized magnons was used for a detailed analysis of the low-temperature magnetothermodynamics in the vicinity of the saturation field for two representative systems, the sawtooth chain (or ∆-chain) and the kagomé lattice. In particular, the authors of these papers mapped the low-energy degrees of freedom of the sawtooth chain (the kagomé lattice) to the hard-dimer gas on a one-dimensional lattice (the hard-hexagon gas on a triangular lattice) and used the results for the classical lattice gases to discuss the properties of the spin systems. They also