Thermodynamics of anisotropic triangular magnets with ferro- and antiferromagnetic exchange

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We investigate thermodynamic properties like specific heat cᵥ and susceptibility χ in anisotropic J₁-J₂ 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 cᵥ(T) and χ(T). We show the variation of peak position and peak height of these quantities as function of control parameter J₂/J₁. We demonstrate that maximum peak positions and heights in Néel phase and spiral phases are strongly asymmetric, much more than in the square lattice J₁-J₂ 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 Cs₂CuCl₄ and Cs₂CuBr₄ from our FTLM results.