Semimetallic behavior in Heusler-type Ru2TaAl and thermoelectric performance improved by off-stoichiometry

Abstract

We report a study of the temperature-dependent electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and 27 Al nuclear magnetic resonance (NMR) in Heusler-type Ru 2 TaAl, to shed light on its semimetallic behavior. While the temperature dependence of the electrical resistivity exhibits semiconductorlike behavior, the analysis of low-temperature specific heat reveals a residual Fermi-level density of states (DOS). Both observations can be realized by means of a semimetallic scenario with the Fermi energy located in the pseudogap of the electronic DOS. The NMR Knight shift and spin-lattice relaxation rate show activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap in Ru 2 TaAl. From the first-principles band structure calculations, we further provide a clear picture that an indirect overlap between electron and hole pockets is responsible for the formation of a pseudogap in the vicinity of the Fermi level of Ru 2 TaAl. In addition, an effort for improving the thermoelectric performance of Ru 2 TaAl has been made by investigating the thermoelectric properties of Ru 1.95 Ta 1.05 Al. We found significant enhancements in the electrical conductivity and Seebeck coefficient and marked reduction in the thermal conductivity via the off-stoichiometric approach. This leads to an increase in the figure-of-merit ZT value from 6.1 × 10 −4 in Ru 2 TaAl to 3.4 × 10 −3 in Ru 1.95 Ta 1.05 Al at room temperature. In this respect, a further improvement of thermoelectric performance based on Ru 2 TaAl through other off-stoichiometric attempts is highly probable.

Magnetic and transport properties in metallic and disordered Ru2VAl and Ru2VGa

“…2 and 3. An extra line with a small intensity at ∼ 37 • is observed in the XRD patterns for both of these compounds, similar to that often reported in many Ru based Heusler alloys and assigned to a Ru-rich phase [24,25,26,31]. All the other diffraction lines could be indexed within the B2 structure for Ru 2 VAl whereas Ru 2 VGa crystallizes in the L2 1 structure.…”

“…As Ru and Fe have similar outermost electronic configurations, some Ru-based Heusler compounds have been recently added to this family. Ru 2 NbAl, Ru 2 NbGa and Ru 2 TaAl exhibit semimetallic ground state and a value of ZT ∼ 5.2 × 10 −3 at 300 K, relatively larger than that of Febased compounds, has been found in Ru 2 NbAl [24,25,26]. Similar to some of the Fe-based compounds having VEC = 24, presence of superparamagnetism as well as ferromagnetically correlated clusters have been reported in Ru 2 NbAl as well [24].…”

Electronic structures of 24-valence-electron full Heusler compounds investigated by density functional and GW calculations

“…However, the difference between the band structures of two functionals is dramatically diminished and the band gap becomes very consistent after the many-body GW correction, and more importantly the results consist with experimental observations [31]. Similar behavior for the Ru 2 TaAl [34] full Heusler compounds has also been observed recently. It is evident that GW calculations could remedy the inconsistency of band structures that obtained at DFT level.…”

Coexistence of spin semimetal and Weyl semimetal behavior in FeRhCrGe

“…Figure 3(f) shows the C P /T vs. T 2 along with the linear fit. Using the free electron gas model, the DoS at E F can be estimated from 29…”