Thermal conductivity of normal conducting and superconducting bulk metallic glasses at very low temperatures

Bulk metallic glasses (BMG) have been produced for more than one decade, but their low temperature properties are still mostly unexplored although BMGs represent a new and very interesting kind of amorphous material with a wide range of electric and magnetic properties. At low temperatures the physical properties of non-magnetic BMGs should be governed by atomic tunneling systems, phonons and conduction electrons as well as the interactions among these degrees of freedom. We recently started the investigation of the thermal properties of superconducting BMGs well below T_c, as in these samples the electronic degree of freedom can be switched on and off by an external magnetic field. We present the thermal conductivity of a Zr based BMG in the superconducting state down to 6 mK. Our results show that sufficiently far below T_c the thermal conductivity can be described by the thermal diffusion of phonons and their resonant scattering with tunneling systems. Furthermore we present the thermal conductivity of the normal conducting BMG Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 in the temperature range between 10 mK and 250 mK. Both measurements were performed with a SQUID-based contact free technique with extremely small parasitic heating.