Magnetic Field Enhancement of Heat Transport in the 2D Heisenberg Antiferromagnet $K_2{V}_3{O}_8$

The thermal conductivity and heat capacity of single crystals of the spin $1/2$ quasi-2D Heisenberg antiferromagnet ${\mathrm{K}}_2{\mathrm{V}}_3{\mathrm{O}}_8$ have been measured from 1.9 to 300 K in magnetic fields from 0 to 8 T. The zero field thermal conductivity data are consistent with resonant scattering of phonons by magnons near the zone boundary. Application of a magnetic field greater than 1 T, however, produces a new magnetic ground state with substantial heat transport by long wavelength magnons.