Specific heat of SmB6 at very low temperatures

Ground state properties of the intermediate valence narrow-gap semiconductor SmB6 have been experimentally studied by means of specific heat C(T) measurements down to 0.04K and in magnetic field up to 6 T. The most remarkable feature of the observed results are the high values of C(T) below 2K with a small increase below about 0.1K for both investigated samples. This behaviour can be attributed to the formation of a coherent state within the metallic-like in-gap states of SmB6 and suggests growth of the electronic part of specific heat gamma like in heavy fermion systems. The in-gap states seem to be influenced by impurities which, together with nuclei of boron and samarium isotopes, contribute to the magnetic part of C(T). Moreover, impurities play an important role in the hyperfine interaction in SmB6.