Transverse and Longitudinal Magnetothermopower Promoted by Ambipolar Effect in Half-Heusler Topological Materials

Abstract

Topologically trivial and non-trivial semimetals with a high degree of carrier compensation are well known for demonstrating large transverse magnetothermopower ($S_{yx}$). However, in such systems, the longitudinal magnetothermopower ($S_{xx}$) is typically suppressed due to nearly perfect electron-hole compensation. Here, we show that the half-Heusler topological semimetal DyPtBi exhibits simultaneously large $S_{xx}$ and $S_{yx}$ magnetothermopowers, defying this conventional trade-off. In $B=14$\,T, thermopower of DyPtBi reaches peak values of $S_{xx}=131\,μ\rm{V/K}$ at $T=149$\,K and $S_{yx}=-297\,μ\rm{V/K}$ at $T=200$\,K, and transverse component remains significantly large even at $290$\,K ($S_{yx}=-213\,μ\rm{V/K}$). Remarkably, at $T=290$\,K and in relatively weak magnetic field of $1$\,T, both relevant for practical applications, DyPtBi shows $S_{yx}=-18\,μ\rm{V/K}$, which is one of the largest values reported under such conditions. The large transverse thermopower originates from an ambipolar effect associated with thermal excitation occurring in zero-gap semiconductors. Due to the imperfect electron-hole compensation, an intrinsic asymmetry between hole- and electron-type carriers enables pronounced values of both $S_{xx}$ and $S_{yx}$, resulting in high effective thermopower ($S_{xx}+|S_{yx}|=379\,μ\rm{V/K}$) in DyPtBi at 200\,K. A comparative analysis with DyPdBi, another half-Heusler material that demonstrates large $S_{xx}=123\,μ\rm{V/K}$ but small $S_{yx}=-16\,μ\rm{V/K}$ (both values obtained at $T=293$\,K and $B=14$\,T), highlights the critical role of band structure and compensation tuning. These findings underscore the potential of chemical doping and band engineering in rare-earth-based half-Heusler materials for optimizing both transverse and longitudinal thermoelectric properties.

Figures (12)

• Figure 1: Schematics of the electronic structure for a semimetal (a) and a zero-gap semiconductor (b) at $T$=0 K. The corresponding Fermi-Dirac distribution is also shown in panel (b). (c) Schematic representation of electronic structure and Fermi-Dirac distribution in a zero-gap semiconductor at $T>0$ K, where thermal broadening enables the thermal excitation of carriers across the band touching point, leading to the multiband conduction involving both electons and holes. Panels (d) and (e) show the calculated electronic band structure of DyPtBi and DyPdBi, respectively, obtained for Dy $f$ states treated as core states. Band structures are plotted along high-symmetry directions of the Brillouin zone shown in the inset to panel (e). (f) Schematic illustration of the electronic band structure of DyPdBi in zero magnetic field and in finite magnetic field ($B>0$). The Zeeman effect causes band splitting, resulting in the emergence of multi-band transport.
• Figure 2: (a) Temperature dependence of the electrical resistivity of DyPtBi and DyPdBi, for both compounds measured with electrical current applied along [$\bar{1}10$] crystallographic direction. (b) Temperature dependence of the longitudinal thermopower of DyPtBi and DyPdBi, for both compounds measured with temperature gradient applied along [$\bar{1}10$] crystallographic direction.
• Figure 3: Magnetic field dependence of longitudinal thermopower ($S_{xx}$) of DyPtBi (a, b) and DyPdBi (c) at several different temperatures. $S_{xx}$ as a function of temperature at zero magnetic field and at several different values of applied magnetic field for DyPtBi (d) and DyPdBi (e). Temperature gradient was applied along [$\bar{1}10$] crystallographic direction, magnetic field was applied along [001] crystallographic direction.
• Figure 4: Nernst effect in DyPtBi and DyPdBi. The magnetic field dependence of the transverse thermopower ($S_{yx}$) of DyPtBi (a, b) and DyPdBi (c) at several different temperatures. $S_{yx}$ as a function of temperature at several different values of applied magnetic fields for DyPtBi (e) and DyPdBi (f). The temperature gradient was applied along [$\bar{1}10$] crystallographic direction, and the magnetic field was applied along [001] crystallographic directions.
• Figure 5: Magnetoresistance isotherms as a function of magnetic field for DyPtBi (a) and DyPdBi (b). Hall resistivity as a function of magnetic field for DyPtBi (c) and DyPdBi (d).