Fig. 3

Opposing responses of three- and four-phonon scattering strengths to pressure. a–d Total three-phonon and process-wise four-phonon scattering rates of the acoustic phonons in BAs as a function of the phonon frequency (ν) at 300 K and at different pressures. As pressure increases, total three-phonon scattering rates and AAAA four-phonon scattering rates strengthen, but AAAO and AAOO four-phonon scattering rates weaken. The competing responses to increasing pressure from three-phonon processes and the dominant AAOO four-phonon processes below 20 GPa cause a thermal conductivity peak at ~17.5 GPa (shown by the dashed box). Beyond 50 GPa, three-phonon scattering completely dominates over four-phonon processes; thus the strengthening AAAA four-phonon scattering rates have a very weak effect on the pressure dependence of κ⁽³⁺⁴⁾. The oval regions in a and b show the frequency range that contributes the most to the κ of BAs. e–h Spectral contributions to κ⁽³⁾ and κ⁽³⁺⁴⁾ as a function of ν at different pressures for acoustic phonons. The frequency range of 4–8 THz produces the largest contribution to both κ⁽³⁾ and κ⁽³⁺⁴⁾