Thermal conductivity of isotopically modified single crystal diamond

We present new experimental results on the thermal conductivity of isotopically enriched $^{12}\mathrm{C}$ diamond crystals at low temperatures. To our knowledge, the measured value for a 99.9% $^{12}\mathrm{C}$ crystal at 104 K, 410 W/cm K, is the highest measured thermal conducitivity for a solid above liquid nitrogen temperature. Our measured temperature dependent conductivities for the isotopically enriched diamond and natural abundance diamond specimens are well described by Callaway's theoretical model. We predict that the thermal conductivity of a 99.999% $^{12}\mathrm{C}$ diamond crystal should exceed 2000 W/cm K at \ensuremath{\sim}80 K.