Properties of the in-gap states in SmB6

We have studied the properties of the in-gap states of the intermediate val ence narrow-gap semiconductor SmB6 by means of electrical conductivity and specific heat measurements at temperatures between 0.1 and 15 K. The electr ical conductivity results imply that the residual conductivity of SmB6 whic h is observed below 3 K is non-activated and the corresponding state, which is formed within the impurity dependent in-gap states, has a metallic-like nature. The heat capacity measurements confirm the metallic-like propertie s of the in-gap states and reveal, moreover, an enhancement of the specific heat of SmB6 below about 2 K, The observed behaviour can be attributed to the formation of a coherent state within the metallic-like state of this co mpound. (C) 2001 Elsevier Science Ltd. All rights reserved.