Multi-Triplet Magnons in SrCu2(BO3)2 Studied by Thermal Conductivity Measurements in Magnetic Fields

terest, because the thermal conductivity due to magnons, κmagnons, has been found to be large in some low-dimensional copper-oxides.1) Theoretically, the existence of a ballistic κmagnon has been predicted in integrable systems,2) and actually a large thermal conductivity associated with the ballistic κmagnon has been observed in one-dimensional Heisenberg chain systems SrCuO2 and Sr2CuO3. 3) As for non-integrable systems such as spin-gap systems, on the other hand, κmagnon has theoretically been predicted to be diffusive and small. Experimentally, however, a large κmagnon has been observed in a spin-gap state of the two-leg spin-ladder system Sr14Cu24O41, 4–8) owing to the marked decrease of the magnon-magnon scattering. Therefore, the mechanism of large κmagnon has not yet been understood so clearly. The compound SrCu2(BO3)2 is a layered material composed of the two-dimensional CuBO3 plane, as shown in the inset of Fig. 2(a). The ground state is a spin-singlet state with the spin gap of 34 K9–11) and can be expressed in terms of the Shastry-Sutherland model.9,12, 13) In the spin gap state of SrCu2(BO3)2, it has been reported that the thermal conductivity due to phonons, κphonon, is enhanced, owing to the decrease of the phononmagnon scattering, and that magnons do not contribute to the thermal conductivity.14–16)