Ballistic magnon transport in the antiferromagnet Nd2CuO4

The in-plane thermal conductivity of the antiferromagnetic insulator Nd$_{2}$CuO$_{4}$ was measured down to 50 mK. In zero magnetic field, the system is in a noncollinear state with gaps in all magnon branches so that only phonons carry heat. In an in-plane field of 10 T, a spin-flop transition causes the system to be in a collinear state, in which significant magnon transport has been observed above 2 K. We show that below 0.5 K these magnons travel ballistically, with a thermal conductivity that varies as $T^{3}$ which confirms the existence of acoustic magnons and allows us to extract their velocity. At higher temperature, we can extract the temperature dependence of: 1) magnon scattering of acoustic phonons; 2) phonon scattering of acoustic magnons.