Thermal properties of vitreous silica with He impurities

The specific heat and thermal conductivity of vitreous silica (Suprasil W) doped with He at 420°C and 200 bar has been measured at low temperatures (0.5 K to 8 K). While with respect to an undoped sample the specific heat shows an excess contribution which varies as T^1.7, no effect of doping on the thermal conductivity is observed. This shows that the coupling to phonons of the excitations introduced by He doping is weak compared to that of intrinsic low energy excitations common to glasses. The excess specific heat can be attributed to excitations of He atoms in cavities.