The heat transport of the spin-gapped material (CH3)2CHNH3CuCl3 (IPA-CuCl3), a candidate quantum magnet with Bose-Einstein condensation (BEC), is studied at ultra-low temperatures and in high magnetic fields. Due to the presence of the spin gap, the zero-field thermal conductivity (κ) is purely phononic and shows a ballistic behavior at T < 1 K. When the gap is closed by magnetic field at H = Hc1, where a long-range antiferromanetic (AF) order of Cu 2+ 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 κ(H) isotherms show sharp peaks at Hc1, which indicates a gap re-opening in the AF state (H > Hc1) and demonstrates limited applicability of the BEC model to IPA-CuCl3.

Magnetism of the S = 1 Heisenberg antiferromagnets on the spatially anisotropic square lattice has been scarcely explored. Here we report a study of the magnetism, specific heat, and thermal conductivity on Ni[SC(NH 2) 2 ] 6 Br 2 (DHN) single crystals. Ni 2+ ions feature an S = 1 rectangular lattice in the bc plane, which can be viewed as an unfrustrated spatially anisotropic square lattice. A long-range antiferromagnetic order is developed at T N = 2.23 K. Below T N , an upturn is observed in the b-axis magnetic susceptibility and the resultant minimum might be an indication for the XY anisotropy in the ordered state. A gapped spin-wave dispersion is confirmed from the temperature dependence of the magnetic specific heat. Anisotropic temperature-field phase diagrams are mapped out and possible magnetic structures are proposed.