Thermodynamics of anisotropic triangular magnets with ferro- and antiferromagnetic exchange

Abstract

We investigate thermodynamic properties like specific heat c _V and susceptibility χ in anisotropic J _1 – J _2 triangular quantum spin systems ( $S=1/2$ ). As a universal tool we apply the finite temperature Lanczos method (FTLM) based on exact diagonalization of finite clusters with periodic boundary conditions. We use clusters up to N = 28 sites where the thermodynamic limit behavior is already stably reproduced. As a reference we also present the full diagonalization of a small eight-site cluster. After introducing model and method we discuss our main results on c _V ( T ) and $\chi (T)$ . We show the variation of peak position and peak height of these quantities as function of control parameter ${J}_{2}/{J}_{1}$ . We demonstrate that maximum peak positions and heights in Néel phase and spiral phases are strongly asymmetric, much more than in the square lattice J _1 – J _2 model. Our results also suggest a tendency to a second side maximum or shoulder formation at lower temperature for certain ranges of the control parameter. We finally explicitly determine the exchange model of the prominent triangular magnets Cs _2 CuCl _4 and Cs _2 CuBr _4 from our FTLM results.

Keywords

• quantum spin models
• magnetic susceptibility
• heat capacity
• exact diagonalization