Theoretical analysis of the double spin chain compound KCuCl_3

We investigate thermal and magnetic properties of the double spin chain compound KCuCl_3 via an exactly solved ladder model with strong rung interaction. Results from the analysis of the thermodynamic Bethe Ansatz equations suggests the critical field values H_{c1}=22.74T and H_{c2}=51.34T, in good agreement with the experimental observations. The temperature dependent magnetic properties are directly evaluated from the exact free energy. Good overall agreement is seen between the theoretical and experimental susceptibility curves. Our results suggest that this compound lies in the strong dimerized phase with an energy gap $\Delta \approx 35$K at zero temperature.