Theoretical analysis of the experiments on the double-spin-chain compound

We have analyzed the experimental susceptibility data of (Formula presented) and found that the data are well explained by the double-spin-chain models with strong antiferromagnetic dimerization. Large quantum Monte Carlo calculations were performed in the spin systems with frustration. This was made possible by removing the negative-sign problem with the use of the dimer basis that has spin-reversal symmetry. The numerical data agree with the experimental data within 1% relative errors in the whole temperature region. We also present a theoretical estimate for the dispersion relation and compare it with recent neutron-scattering experiments. Finally, the magnitude of each interaction bond is predicted.