Thermal Conductivity Of Ipa-Cucl3: Evidence For Ballistic Magnon Transport And The Limited Applicability Of The Bose-Einstein Condensation Model

The heat transport of the spin-gapped material (CH3)(2)CHNH3CuCl3 (IPA-CuCl3), a candidate quantum magnet with Bose-Einstein condensation (BEC), is studied at ultralow temperatures and in high magnetic fields. Due to the presence of the spin gap, the zero-field thermal conductivity (kappa) is purely phononic and shows a ballistic behavior at T < 1 K. When the gap is closed by magnetic field at H = H-c1, where a long-range antiferromanetic (AF) order of Cu2+ moments is developed, the magnons contribute significantly to heat transport and exhibit a ballistic T-3 behavior at T < 600 mK. In addition, the low-T kappa(H) isotherms show sharp peaks at H-c1, which indicates a gap reopening in the AF state (H > H-c1) and demonstrates limited applicability of the BEC model to IPA-CuCl3.