209Bi NMR and NQR investigation of the small-gap semiconductor Ce3Bi4Pt3

Thermoelectricity in correlated narrow-gap semiconductors

We review many-body effects, their microscopic origin, as well as their impact on thermoelectricity in correlated narrow-gap semiconductors. Members of this class-such as FeSi and FeSb₂-display an unusual temperature dependence in various observables: insulating with large thermopowers at low temperatures, they turn bad metals at temperatures much smaller than the size of their gaps. This insulator-to-metal crossover is accompanied by spectral weight-transfers over large energies in the optical conductivity and by a gradual transition from activated to Curie-Weiss-like behaviour in the magnetic susceptibility. We show a retrospective of the understanding of these phenomena, discuss the relation to heavy-fermion Kondo insulators-such as Ce₃Bi₄Pt₃ for which we present new results-and propose a general classification of paramagnetic insulators. From the latter, FeSi emerges as an orbital-selective Kondo insulator. Focussing on intermetallics such as silicides, antimonides, skutterudites, and Heusler compounds we showcase successes and challenges for the realistic simulation of transport properties in the presence of electronic correlations. Further, we explore new avenues in which electronic correlations may contribute to the improvement of thermoelectric performance.

Comment on the reference compound for chemical shift and Knight shift determination of (209)Bi nuclei

Several groups exploring the (209)Bi NMR in solids, including usual insulators, metallic and magnetic materials and recently diamagnetic topological materials, use different standards (usually old and invalid) for chemical shift (Knight shift) determination, ignoring IUPAC recommendations. As a consequence the published shift values exhibit considerable differences (up to 17,500 ppm).

Closing the spin gap in the Kondo insulator Ce3Bi4Pt3 at high magnetic fields

Kondo insulator materials--such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3 and SmB6--are 3d, 4f and 5f intermetallic compounds that have attracted considerable interest in recent years. At high temperatures, they behave like metals. But as temperature is reduced, an energy gap opens in the conduction band at the Fermi energy and the materials become insulating. This contrasts with other f-electron compounds, which are metallic at all temperatures. The formation of the gap in Kondo insulators has been proposed to be a consequence of hybridization between the conduction band and the f-electron levels, giving a 'spin' gap. If this is indeed the case, metallic behaviour should be recovered when the gap is closed by changing external parameters, such as magnetic field or pressure. Some experimental evidence suggests that the gap can be closed in SmB6 (refs 5, 8) and YbB12 (ref. 9). Here we present specific-heat measurements of Ce3Bi4Pt3 in d.c. and pulsed magnetic fields up to 60 tesla. Numerical results and the analysis of our data using the Coqblin-Schrieffer model demonstrate unambiguously a field-induced insulator-to-metal transition.