By high-temperature series expansion and exact complete diagonalization the magnetic susceptibility $\chi \mathrm{}\left(T\right)\mathrm{}$ and the specific heat $C\left(T\right)\mathrm{}$ of a two-leg $S=1/2\mathrm{}$ ladder with cyclic (four-spin) exchange are computed. Both methods yield convincing results for not too small temperatures. We find that a small amount of cyclic exchange influences the thermodynamic properties significantly. Our results can serve as reliable basis for an efficient analysis of experimental data.

• Received 30 October 2002

DOI:https://doi.org/10.1103/PhysRevB.67.134428