Anisotropic thermal conductivity of CuO

We report measurements of the low-temperature ($5\mathrm{K}\ensuremath{\le}T\ensuremath{\le}300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) thermal conductivity ($\ensuremath{\kappa}$) of single-crystal CuO along its main symmetry directions. The influence of the antiferromagnetic phase transitions (${T}_{N1}\ensuremath{\simeq}213\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, ${T}_{N2}\ensuremath{\simeq}230\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) on $\ensuremath{\kappa}$ is modest. The application of an electric field ($\ensuremath{\lesssim}6\phantom{\rule{0.16em}{0ex}}\mathrm{k}\mathrm{V}/\mathrm{cm}$) in the multiferroic regime (${T}_{N1}\ensuremath{\le}T\ensuremath{\le}{T}_{N2}$) was explored but found to produce no measurable effect on $\ensuremath{\kappa}$ attributable to a manipulation of magnetic domains. In the collinear ground state, there is considerable anisotropy of $\ensuremath{\kappa}$, with substantially larger values along ($[10\overline{1}]$) and transverse to ($[101]$) the CuO chains for which superexchange interactions are strongest. Calculations of the lattice thermal conductivity employing the measured anisotropic phonon dispersions satisfactorily reproduce the conductivities in all but the $[10\overline{1}]$ and $[101]$ directions, suggesting a substantial magnon contribution (${\ensuremath{\kappa}}_{m}$) along the latter. A model employing the measured spin-wave dispersions and anisotropic spin gap produces reasonable agreement with the inferred ${\ensuremath{\kappa}}_{m}(T)$.