Nanostructured YbAgCu4 for potentially cryogenic thermoelectric cooling.

We have studied the thermoelectric properties of nanostructured YbAgCu4 materials. A high power factor of ∼131 µW cm(-1) K(-2) has been obtained at 22 K for nanostructured samples prepared by ball milling the arc melted ingot into nanopowder and hot pressing the nanopowder. The implementation of nanostructuring method decreased the thermal conductivity at 42 K by 30-50% through boundary scattering comparing with the previously reported value of polycrystalline YbAgCu4. A peak dimensionless thermoelectric figure-of-merit, ZT, of 0.11 has been achieved at 42 K, which may find potential applications for cryogenic cooling below 77 K. The nanostructuring approach can be extended to other heavy Fermion materials to achieve high power factor and low thermal conductivity and ultimately higher ZT.

Carrier localization and electronic phase separation in a doped spin-orbit-driven Mott phase in Sr3(Ir(1-x)Ru(x))2O7.

Interest in many strongly spin-orbit-coupled 5d-transition metal oxide insulators stems from mapping their electronic structures to a J(eff)=1/2 Mott phase. One of the hopes is to establish their Mott parent states and explore these systems' potential of realizing novel electronic states upon carrier doping. However, once doped, little is understood regarding the role of their reduced Coulomb interaction U relative to their strongly correlated 3d-electron cousins. Here we show that, upon hole-doping a candidate J(eff)=1/2 Mott insulator, carriers remain localized within a nanoscale phase-separated ground state. A percolative metal-insulator transition occurs with interplay between localized and itinerant regions, stabilizing an antiferromagnetic metallic phase beyond the critical region. Our results demonstrate a surprising parallel between doped 5d- and 3d-electron Mott systems and suggest either through the near-degeneracy of nearby electronic phases or direct carrier localization that U is essential to the carrier response of this doped spin-orbit Mott insulator.

Figure-of-merit enhancement in nanostructured FeSb(2-x)Ag(x) with Ag(1-y)Sb(y) nanoinclusions.

We present the figure-of-merit (ZT) improvement in nanostructured FeSb(2-x)Ag(x) with Ag(1-y)Sb(y) nanoinclusions through a metal/semiconductor interface engineering approach. Owing to the interfaces between FeSb(2-x)Ag(x) and Ag(1-y)Sb(y) phases, as well as the identical work functions, both thermal conductivity and electrical resistivity of the nanocomposites were significantly reduced in the lower temperature regime compared with pure FeSb(2). Overall, an improvement of 70% in ZT was achieved for the optimized nanocomposite FeSb(1.975)Ag(0.025)/Ag(0.77)Sb(0.23) sample, in which Ag(0.77)Sb(0.23) is about 10% by molar ratio. The results of this approach clearly demonstrated the metal/semiconductor interface concept and confirmed the potential of strongly correlated material systems as promising thermoelectric materials.