Anisotropic lattice thermal conductivity in topological semimetal ZrGeX (X = S, Se, Te): a first-principles study

Abstract

Topological semimetals have attracted significant attentions owing to their potential applications in numerous fields such as low-power electron devices and quantum computation, which are closely related to their thermal transport properties. In this work, the phonon transport properties of topological Dirac nodal-line semimetals ZrGeX (X = S, Se, Te) with the PbClF-type structures are systematically studied using the first-principles calculations combined with the Boltzmann transport theory. The obtained lattice thermal conductivities show an obvious anisotropy, which is caused by the layer structures of ZrGeX (X = S, Se, Te). The room-temperature lattice conductivity of ZrGeTe along c direction is found to be as low as 0.24 W m−1 K−1, indicating that it could be of great significance in the fields of thermal coating materials and solar cell absorber. In addition, we extract each phonon branch from group velocities, phonon scattering rates, Grüneisen parameters, and phase space volumes to investigate the mechanism underlying the low thermal conductivity. It is concluded that the difference of thermal conductivities of three materials may be caused by the number of scattering channels and the effect of anharmonic. Furthermore, the phonon mean free path along a direction is relatively longer. Nanostructures or polycrystalline structures may be effective to reduce the thermal conductivity and improve the thermoelectric properties.

Searching for a promising topological Dirac nodal-line semimetal by angle resolved photoemission spectroscopy

“…On the other hand, we observe these inner Dirac bands cross at nonsymmorphic-symmetry protected four-fold degeneracy X point at −432 meV (Dirac point 2, labeled as DP2) (figures 4(a) and (c)). The Dirac point of outer band locates partially above E F , which is invisible by standard ARPES measurement, similar to the results of other ZrHM (H = Si/Ge, M = S/Se/Te) materials [20,22,23,27,32,35], as predicted by theoretical calculations [21,30,31]. By extrapolating the outer linear bands, the Dirac point is estimated to be located at 519 meV.…”

“…In addition, we investigate the electronic band structures of ZrGeS around X point (figure 4). Again, in the large energy range from E F to −1.5 eV, the linear Dirac bands were observed, intersecting with the surface state (SS) along Γ-X direction, in consistence with theoretical prediction [21,30,31] and similar to the results of other ZrGeS's isostructural compounds [23,27,48]. By measuring the slope of these linear bands, we find the Fermi velocity of the inner (labeled with black dashed lines), outer bands (labeled with green dashed lines) along Γ-X and the bands along X-M directions to be ν F ∼ 4.5 eV Å, 4.9 eV Å and 5.9 eV Å, respectively, similar to its isostructural compound ZrGeSe [20].…”

“…The DP3 here is located closer to E F than that of along Γ-X direction (DP2, −432 meV), similar to other isostructural compounds [20, 23-25, 32, 35, 42]. According to theoretical calculations [21,30,31] and photon energy dependent ARPES measurements of ZrSiS , ZrGeTe [23] and ZrSnTe [27], the linear Dirac bands along Γ-X and X-M directions are originated from bulk and surface states, respectively, which cross at X point with different energies. Besides, the Fermi wave vector along X-M direction k F,XM = 0.038 (1/Å) (figure 4(b)) is smaller than the inner bands along Γ-X direction k F,ΓX,inner = 0.100 (1/Å) (figure 4(a)), in consistence with the elliptical FS property around X point (figure 2(a)).…”

“…Figures 4(b) and (d) also show the predicted Dirac-like surface states [21,30,31] along the M-X-M high-symmetry direction with the Fermi velocity ν F ∼ 5.9 eV Å and the Dirac point DP3 located at −342 meV. The DP3 here is located closer to E F than that of along Γ-X direction (DP2, −432 meV), similar to other isostructural compounds [20, 23-25, 32, 35, 42].…”

“…The interlayer coupling between the adjacent Zr-S layers is relatively weak, which provides a natural cleavage plane in between [19] and yields a (001) surface with S termination, similar to its isostructural compound ZrGeSe [20]. The experimental lattice constants are a = b = 3.63 Å and c = 8.02 Å [21]. After cleavage, the high quality of crystallinity is verified by the sharp and clean (00L) x-ray diffraction peaks, as shown in figure 1(b), informing a (001) surface obtained.…”