Thermal Conductivity of the Two-Dimensional Spin-Gap System SrCu₂(BO₃)₂ in Magnetic Fields

We have measured the thermal conductivity of the two-dimensional spin-gap system SrCu₂(BO₃)₂ single crystal in magnetic fields up to 14T. In zero field, both the in-plane and out-of-plane thermal conductivities exhibit anomalous enhancement at low temperatures below ∼ 11K. The enhancement is suppressed with increasing magnetic field. The enhancement in zero field is attributed to the increase of the thermal conductivity due to phonons, which is caused by the suppression of the phonon-magnon scattering owing to the spin-gap formation. The suppression of the enhancement by the application of magnetic field is explained by the reduction of the spin gap.