arXiv subjects

Øven A. Grimenes

Publications and source records attributed to Øven A. Grimenes.

3 recordsLinked to original sources

arXiv2026

Exceptional thermoelectric properties in Na$_2$TlSb enabled by quasi-1D band structure

Materials with reduced dimensionality offer beneficial density-of-states (DOS) profiles for thermoelectric energy conversion, but can be impractical in realistic devices. Encouragingly, bulk high-symmetry materials can also exhibit similar quasi-low-dimensional band structures. A striking example is the full-Heusler compound Na$_2$TlSb, whose valence-band energy isosurfaces can form intersecting two-dimensional pockets, i.e., a box-like structure. The individual energy isosurface sheets resemble those of 1D quantum wires. The combination of high electron velocities (perpendicular to the pockets) and a rapidly increasing DOS with energy in the transport regime (due to the low dimensionality) makes Na$_2$TlSb a representative case where the band structure gives rise to attractive electronic transport properties. However, these beneficial features could be counteracted by high electronic scattering rates due to the large scattering space. In this first principles study of Na$_2$TlSb we find that the electronic scattering rates remain modest. This result is linked to the reduced matrix elements of large-momentum ($\mathbf{q}$) scattering across the delocalized energy isosurfaces. The enhanced free-carrier screening due to the large DOS also contributes to reducing scattering. In combination, the low-dimensional features and modest scattering result in excellent electronic transport properties. Combined with an ultra-low lattice thermal conductivity of $κ_\ell < 1$ W/mK reported in the literature, we predict a thermoelectric figure of merit ranging from 2.4 at 300 K to a 4.4 at 600 K. The $n$-type properties are also favorable, with $zT$ values from 1.5 at 300 K to 3.0 at 600 K.

cond-mat.mtrl-sci↗

arXiv2025

Common errors in BoltzTraP-based calculations

Boltzmann transport calculations based on band structures computed from first principles play an important role in modern thermoelectric materials research. Among available codes, the \textsc{BoltzTraP} code is the most widely adopted, but many recent studies contain systematic mistakes. We identify three error modes: (1) inserting the electronic thermal conductivity at zero electric field, $κ_0$, in place of the electronic thermal conductivity at zero electric current, $κ_e$, (2) computing the figure of merit $zT$ by combining a constant relaxation time of unity while keeping the lattice thermal conductivity $κ_\ell$ in standard units, and (3) doing both errors at once. We have found many examples of the third error, but since the first two are simpler, we suspect they are also present in the literature. For the single parabolic band model, we derive exact analytical limits in the non-degenerate and near-degenerate regimes, and we show how mistakes appear for the realistic case study of ZrNiSn. Our results illustrate how faulty calculations can appear reasonable at certain temperatures and Fermi levels, and we provide practical guidance for identifying faulty results and avoiding such pitfalls in thermoelectric transport studies.

cond-mat.mtrl-sci↗

arXiv2025

Thermoelectric transport of strained CsK$_2$Sb: The role of electron velocities and scattering within extended Fermi surfaces

In this first-principles study, we investigated the thermoelectric properties of the full-Heusler compound CsK$_2$Sb at different compressive strains. This material exhibits a valence band structure with significant effective mass anisotropy, forming tube-like energy isosurfaces below the band edge, akin to that of two-dimensional (2D) systems. Such systems can have a large number of high-mobility charge carriers and a beneficial density of states profile. In the calculations, we predicted a maximum p-type figure of merit ($zT$) of 2.6 at 800 K, in line with previous predictions of high $zT$. This high $zT$ arises from the low lattice thermal conductivity of 0.35 Wm$^{-1}$K$^{-1}$ and the beneficial electronic band structure. The high density of states significantly increased the electron-scattering space, but this effect was largely compensated by reduced scattering rates of electrons with large momentum ${\mathbf{q}}$. We further explored the effect of enhancing the low-dimensionality through compressive strain. This increased the p-type power factor by up to 66 %; partly due to more strongly pronounced 2D features of the valence band, but primarily due to increased Fermi velocities. However, compressive strain also increased phonon velocities and hence the lattice thermal conductivity. The maximum p-type $zT$ thus only increased slightly, to 2.7 at 1 % compressive strain. In the conduction band, strain aligned the $Γ$- and X-centered valleys, resulting in the optimal n-type $zT$ increasing from 0.9 to 2.3 at 2 % compressive strain. Thus, highly strained CsK$_2$Sb has the potential for both good p- and n-type thermoelectricity.

cond-mat.mtrl-sci↗