Self-Doping and the Mott-Kondo Scenario for Infinite-Layer Nickelate Superconductors

We give a brief review of the Mott-Kondo scenario and its consequence in the recently-discovered infinite-layer nickelate superconductors. We argue that the parent state is a self-doped Mott insulator and propose an effective t- J-K model to account for its low-energy properties. At small doping, the model describes a low carrier density Kondo system with incoherent Kondo scattering at finite temperatures, in good agreement with experimental observation of the logarithmic temperature dependence of electric resistivity. Upon increasing Sr doping, the model predicts a breakdown of the Kondo effect, which provides a potential explanation of the non-Fermi liquid behavior of the electric resistivity with a power law scaling over a wide range of the temperature. Unconventional superconductivity is shown to undergo a transition from nodeless (d+is)-wave to nodal d-wave near the critical doping due to competition of the Kondo and Heisenberg superexchange interactions. The presence of different pairing symmetry may be supported by recent tunneling measurements.

Seebeck and Figure of Merit Enhancement by Rare Earth Doping in Yb14-xRExZnSb11 (x = 0.5)

Yb14ZnSb11 has been of interest for its intermediate valency and possible Kondo designation. It is one of the few transition metal compounds of the Ca14AlSb11 structure type that show metallic behavior. While the solid solution of Yb14Mn1-xZnxSb11 shows an improvement in the high temperature figure of merit of about 10% over Yb14MnSb11, there has been no investigation of optimization of the Zn containing phase. In an effort to expand the possible high temperature p-type thermoelectric materials with this structure type, the rare earth (RE) containing solid solution Yb14-xRExZnSb11 (RE = Y, La) was investigated. The substitution of a small amount of 3+ rare earth (RE) for Yb2+ was employed as a means of optimizing Yb14MnSb11 for use as a thermoelectric material. Yb14ZnSb11 is considered an intermediate valence Kondo system where some percentage of the Yb is formally 3+ and undergoes a reduction to 2+ at ~85 K. The substitution of a 3+ RE element could either replace the Yb3+ or add to the total amount of 3+ RE and provides changes to the electronic states. RE = Y, La were chosen as they represent the two extremes in size as substitutions for Yb: a similar and much larger size RE, respectively, compared with Yb3+. The composition x = 0.5 was chosen as that is the typical amount of RE element that can be substituted into Yb14MnSb11. These two new RE containing compositions show a significant improvement in Seebeck while decreasing thermal conductivity. The addition of RE increases the melting point of Yb14ZnSb11 so that the transport data from 300 K to 1275 K can be collected. The figure of merit is increased five times over that of Yb14ZnSb11 and provides a zT ~0.7 at 1275 K.