Ground state properties of intermediate valence SmB₆

    Strongly correlated narrow-gap semiconductors, denoted also as Kondo insulators, have attracted much attention in the last few years. The formation of the small transport and spin gap close to the Fermi level E_F in these compounds is believed to
originate from the hybridization of the narrow and localized f-states with the broad s, p or d-type conduction band. The
intermediate valence compound SmB₆ was the first of these materials to be discovered and it is one of the most often studied of this class. Experiments have shown that the hybridization gap, which is responsible for the decrease of electrical conductivity below 70K, has a width of E_g» 10-20meV. The further decrease of conductivity below 15K can be associated with a direct activation energy E_d »3-5 meV, which corresponds to transitions of electrons between the bottom of the conduction band and the in-gap states at E_F. These states also seem to be responsible for the residual conductivity of this material below 3-5 K, and its properties appear to depend on the content of impurities and imperfections in the sample. Activation energy calculations have shown that the residual conductivity is of non-activated (metallic) nature. However, there are still open questions about the gap structure and the properties of in-gap states. The purpose of the present study was to investigate SmB6 by point-contact (PC) spectroscopy and specific heat measurements and thus to contribute to the understanding of its ground state properties.
The differential conductance dI/dU of SmB₆-SmB₆ junctions has been investigated between 0.1 and 4.2 K as function of lateral contact size. The zero-bias PC conductance varied between 0.01mS and 1mS. Two different regimes of charge transport have been distinguished. Large junctions were in the diffusive regime, in which the conductance is dominated by the bulk conductivity. Small junctions were in the tunnelling regime and electrons can tunnel through the contact either by single hopping event s liken the bulk material or as evanescent waves when the zero-bias conductance is smaller than about 10mS. The position of the spectral anomalies, which are related to the different activation energies and band gaps of mB₆ did not depend on the contact size. To estimate the average density of states of our tunnel junctions we normalized the spectra taken at 0.1K with respect to the voltage-dependent background, averaged the normalized spectra, and fitted the average spectrum by functional dependencies of the density of states, assuming a constant transmission probability. A good fit is obtained by assuming a constant background of » % 45 and two energy-dependent parts of the density of states as shown in Fig. 2. The one part, g₁(E), has a gap of » 21meV (full width at half maximum), while the other part, g₂(E), has a gap of »4.5meV. These gaps may be attributed to E_g and to E_d, respectively. These results agree with those of planar SmB6-Pb tunnel junctions, and the values of E_g and E_d fit well those derived from other experiments. The large background conductance originates in this case from states at lattice distortions and surface of the PC area.
 

Figure 2: Average dI/dU(U) tunnel spectrum (solid circles) and a fit (white line through the data points) calculated using the density of states (dotted line). g(E) is the sum of g₁(E) and g₂(E) plus a constant background.

    In order to obtain additional information about the properties of the background (in-gap) states at E_F, the heat capacity was measured. Fig. 3 shows the specific heat of samples a and b as C(T)/T in the 0.1-3K range. Since sample b with a higher content of impurities has a larger C than sample a, impurities clearly influence the density of states at E_F. Additional contributions to C of sample b, which is between 1 and 3K by a factor 3-4 larger than C of sample a, may come from contributions at higher T and from surface states as this sample was polished. Another remarkable feature is the increase of C(T)/T towards lower temperatures, which resembles the enhancement of the specific heat in heavy fermion systems. As this enhancement is more pronounced for the sample with less impurities (a) it can not be associated with the presence of impurity states.
 

Figure 3: Specific heat of SmB₆ in C/T-T² plot.

    It is unlikely to explain the enhancement of C/T by a magnetic contribution or by a formation of a magnetic state
since no corresponding signal for such processes was observed in the magnetic susceptibility down to 10mK. We can also exclude a very low temperature Schottky anomaly because of additional activation energy was detected below 2 K. Thus the electronic part, which is connected with the metallic (in-gap) states at E_F, is probably responsible for the observed behavior of the heat capacity of SmB₆. Moreover, the increase of C(T)/T below about 2K very likely demonstrates the formation of an intrinsic coherent state within these in-gap states.

K. Flachbart, S. Gabáni, V. Pavlík, M. Reiffers, P. Samuely
K. Gloos (Tech. Univ. Darmstadt), M. Orendáč (P.J.Š.Univ. Košice), E. Konovalova, Y. Paderno (IPM UAS Kiev).