We study the transverse susceptibility of the fully frustrated antiferromagnetic Ising Δ-chain, extending Minami's transfer-matrix method to the transverse susceptibility of general-type Ising linear-chains [J. Phys. Soc. Jpn. 67 (1998), 2255]. For the transverse fields Γ1 in tip spin sites and Γ2 in bottom spin sites, we calculate zero-field transverse susceptibilities χxtip = lim Γ1 →0Mxtip/Γ1 and χxbottom = lim Γ2 →0Mxbottom/Γ2 while keeping the ratio Γ1/Γ2 fixed, where Mxtip (bottom) denotes the magnetization for tip (bottom) spin sites. Both the transverse susceptibilities follow Curie's law at low temperatures. We also calculate χxbottom(Γ1 > 0), the transverse susceptibility of the bottom spin chain under a finite tip-spin transverse field, to understand the Curie-type behavior in zero-field susceptibility. Using the second-order perturbation theory, we also discuss the Γ1 dependence of χxbottom (Γ1) at zero temperature.
The transverse susceptibility of the one-dimensional antiferromagnetic XY model is calculated for a sequence of finite chains with periodic boundary conditions. The infinite-chain susceptibility is obtained by extrapolation, and good convergence is found for most temperatures. The results are compared with the c-axis susceptibility of Cs2CoCl4, which has been measured over the temperature interval 40 mK-4.2 K. The agreement is excellent, confirming that this compound can be well approximated by a set of weakly coupled antiferromagnetic XY chains.