Charge transport and thermopower in the electron-doped narrow gap semiconductor Ca1−xLaxPd3O4

We have investigated the charge transport property, thermoelectric effect, and electronic state for the electron-doped narrow gap semiconductor ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Pd}}_{3}{\mathrm{O}}_{4}$ by means of the transport measurement, optical/photoemission spectroscopy, and ab initio calculation. The high-quality polycrystalline samples of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Pd}}_{3}{\mathrm{O}}_{4}$ were synthesized by using the high-pressure synthesis technique. In the undoped system $x=0$, the optical conductivity spectra show a charge gap of about 0.1 eV, which is qualitatively consistent with the results of ab initio calculation. The electron doping causes the rapid reduction of resistivity ${\ensuremath{\rho}}_{xx}$, resulting in the metallic state at the doping level of as small as $x=0.01$. On the other hand, the magnitude of Seebeck coefficient $|S|$ moderately decreases with increasing $x$. Consequently, the power factor ${S}^{2}/{\ensuremath{\rho}}_{xx}$ reaches about $7\phantom{\rule{4pt}{0ex}}\mathrm{\textmu{}}\mathrm{W}/{\mathrm{K}}^{2}\mathrm{cm}$ at 350 K at $x=0.03$, which is much higher than that of hole-doped analog of ${\mathrm{CaPd}}_{3}{\mathrm{O}}_{4}$. Combined with the results of photoemission spectra and ab initio calculation, the large power factor in the present material likely originates from the relatively low resistivity, probably high electron mobility ($\ensuremath{\sim}30\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}$ at 350 K), due to the dispersive Pd $4{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ conduction band.