Spin-specific heat determination of the ratio of competing first- and second-neighbor exchange interactions in frustrated spin-$frac{1}{2}$ chains

The magnetic susceptibility $chi(T)$ of spin-1/2 chains is widely used to quantify exchange interactions, even though $chi(T)$ is similar for different combinations of ferromagnetic $J_1$ between first neighbors and antiferromagnetic $J_2$ between second neighbors. We point out that the spin specific heat $C(T)$ directly determines the ratio $alpha = J_2/|J_1|$ of competing interactions. The $J_1-J_2$ model is used to fit the isothermal magnetization $M(T,H)$ and $C(T,H)$ of spin-1/2 Cu(II) chains in LiCuSbO$_4$. By fixing $alpha$, $C(T)$ resolves the offsetting $J_1$, $alpha$ combinations obtained from $M(T,H)$ in cuprates with frustrated spin chains.