Transport and electrical properties of cryogenic thermoelectric FeSb2: the effect of isoelectronic and hole doping.

Thermoelectric materials operating at cryogenic temperatures are in high demand for efficient cooling and power generation in applications ranging from superconductors to quantum computing. The narrow band-gap semiconductor FeSb2, known for its colossal Seebeck coefficient, holds promise for such applications, provided its thermal conductivity value can be reduced. This study investigates the impact of isoelectronic substitution (Bi) and hole doping (Pb) at the Sb site on the transport properties of FeSb2, with a particular focus on thermal conductivity (κ). Polycrystalline FeSb2powder, along with Bi- and Pb-doped samples, were synthesized using a simple co-precipitation approach, followed by thermal treatment in an H2atmosphere. XRD and SEM analysis confirms the formation of the desired phase pre- and post-consolidation using spark plasma sintering. The consolidation process resulted in a high compaction density and the formation of submicrometer-sized grains, as substantiated by electron backscattered diffraction analysis. Substituting 1% of Bi and Pb at the Sb site successfully suppressed the thermal conductivity (κ) from ∼15 W (m·K)-1in pure FeSb2to ∼10 and ∼8.7 W (m·K)-1, respectively. Importantly, resistivity measurements revealed a metal-to-insulator transition at around 6.5 K in undoped FeSb2and isoelectronically Bi-substituted FeSb2, suggesting the existence of metallic surface states and provides valuable evidence for the perplexing topological behavior exhibited by FeSb2.

Spin and current transport in the robust half-metallic magnetc-CoFeGe.

Spintronics is an emerging form of electronics based on the electrons' spin degree of freedom for which materials with robust half-metallic ferromagnet character are very attractive. Here we determine the structural stability, electronic, magnetic, and mechanical properties of the half-Heusler (hH) compound CoFeGe, in particular also in its cubic form. The first-principles calculations suggest that the electronic structure is robust with 100% spin polarization at the Fermi level under hydrostatic pressure and uni-axial strain. Both the longitudinal and Hall current polarization are calculated and the longitudinal current polarization (PL) is found to be>99%and extremely robust under uniform pressure and uni-axial strain. The anomalous Hall conductivity and spin Hall conductivity of hH cubic CoFeGe (c-CoFeGe) are found to be∼-100S cm-1and∼39 ℏ/eS cm-1, respectively. Moreover, the Curie temperature of the alloy is calculated to be ∼524 K with a 3µBmagnetic moment. Lastly, the calculated mechanical properties indicate thatc-CoFeGe is ductile and mechanically stable with a bulk modulus of ≈154 GPa. Overall, this analysis reveals that cubic CoFeGe is a robust half-metallic ferromagnet and an interesting material for spintronic applications.