Magnetic Susceptibility of Ideal Spin 1 $/$2 Heisenberg Antiferromagnetic Chain Systems, ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ and ${\mathrm{SrCuO}}_2$

Magnetic susceptibility is measured on the one-dimensional spin 1/2 antiferromagnets ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ and $\mathrm{Sr}{\mathrm{CuO}}_2$ using single crystals. Large, nearly contamination-free single crystals enable us to measure the intrinsic spin susceptibility over a wide temperature range between 5 and 800 K. The results are in excellent agreement with the recent calculation by Eggert, Affleck, and Takahashi, and the exchange interaction energy $J$ is estimated to be $2200\pm 200$ and $2100\pm 200\mathrm{K}$, respectively. In ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$, an isotropic susceptibility drop is observed below about 20 K, which is also consistent with the result of this rigorous calculation.