Giant suppression of phononic heat transport in a quantum magnet BiCu2PO6

Thermal transport of quantum magnets has elucidated the nature of low energy elementary excitations and complex interplay between those excited states via strong scattering of thermal carriers. BiCu 2 PO 6 is a unique frustrated spin-ladder compound exhibiting highly anisotropic spin excitations that contain both itinerant and localized dispersion characters along the b - and a -axes respectively. Here, we investigate thermal conductivity κ of BiCu 2 PO 6 under high magnetic fields ( H ) of up to 30 tesla. A dip-feature in κ , located at ~15 K at zero- H along all crystallographic directions, moves gradually toward lower temperature ( T ) with increasing H , thus resulting in giant suppression by a factor of ~30 near the critical magnetic field of H c ≅ 23.5 tesla. The giant H- and T- dependent suppression of κ can be explained by the combined result of resonant scattering of phononic heat carriers with magnetic energy levels and increased phonon scattering due to enhanced spin fluctuation at H c , unequivocally revealing the existence of strong spin-phonon coupling. Moreover, we find an experimental indication that the remaining magnetic heat transport along the b -axis becomes almost gapless at the magnetic quantum critical point realized at H c .