Probing of the pseudogap via thermoelectric properties in the Au-Al-Gd quasicrystal approximant

Abstract

The pseudogap of the recently discovered Au-Al-Gd quasicrystal approximant crystal (AC) is investigated over a wide electron-per-atom $(e/a)$ ratio of $\ensuremath{\sim}0.5$ using thermoelectric properties as an experimental probe. This Au-Al-Gd AC provides an ideal platform for fine probing of the pseudogap among a number of known ACs because the Au-Al-Gd AC possesses an extraordinarily wide single-phase region with respect to the variation in the electron concentration [A. Ishikawa, T. Hiroto, K. Tokiwa, T. Fujii, and R. Tamura, Phys. Rev. B 93, 024416 (2016)], in striking contrast to, for instance, binary stoichiometric $\mathrm{C}{\mathrm{d}}_{6}\mathrm{R}$ ACs. As a result, a salient peak structure is observed in the Seebeck coefficient, $S$, with the composition as well as that of the power factor ${S}^{2}\ensuremath{\sigma}$, in addition to a gradual variation in the conductivity, $\ensuremath{\sigma}$, and $S$. These two features are directly associated with rapid and slow variations, respectively, of spectral conductivity $\ensuremath{\sigma}(E)$, and hence the fine structure inside the pseudogap, in the vicinity of the Fermi level ${E}_{\mathrm{F}}$. Based on the observed continuous variation of the Fermi wave vector reported in the previous experimental work, fine tuning of ${E}_{\mathrm{F}}$ toward an optimal position was attempted, which led to the successful observation of a sharp peak in ${S}^{2}\ensuremath{\sigma}$ with a value of $\ensuremath{\sim}270\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{W}/\mathrm{m}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{K}}^{2}$ at 873 K. This is the highest value ever reported among both Tsai-type and Bergman-type compounds. The dimensionless figure of merit was determined as 0.026 at 873 K, which is also the highest reported among both Tsai-type and Bergman-type compounds.