Dirac polaron dynamics in the correlated semimetal of perovskite ${\mathrm{CaIrO}}_3$

We have investigated the charge dynamics of the correlated Dirac semimetallic perovskite ${\mathrm{CaIrO}}_3$ by infrared spectroscopy. The optical conductivity spectra show the $\omega$-linear-type interband transition and a tiny Drude response at low temperatures, suggesting that the Dirac node is pinned near the Fermi energy due to the Mott criticality. Moreover, a large polaron absorption appears at around 0.04 eV, when thermally excited carriers diminish and the plasma frequency decreases below the optical phonon energy. The electron correlation and resultant reduced plasma frequency are likely to enhance the electron-phonon interaction via the insufficient electronic screening, realizing the Dirac polaron.

Figure 1

(a) The crystal structure of ${\mathrm{CaIrO}}_3$. Illustration of (b) Dirac band dispersion and (c) location of line node (green line) in the momentum space. The line node is on the $k_b$=$\pi$ plane. (d) The band structure derived by the theoretical calculation employing the dynamical mean field theory (Ref. [16]). The color code (dashed line) represents the spectral intensity ($E_{\mathrm{F}}$). (e) Resistivity for single crystalline ${\mathrm{CaIrO}}_3$ and polycrystalline ${\mathrm{SrIrO}}_3$. The inset shows the inverse of the Hall coefficient. (f) $\sigma \left(\omega \right)$ spectra at 10 K. Inset shows the magnified view of the low energy region. The triangle indicates peak A. The circle and dashed line denote the dc conductivity and the expected spectra from the Hagen-Rubens extrapolation, respectively.

Figure 2

The optical conductivity spectra (a) above 150 K and (b) below 120 K. The triangle indicates peak A. The dashed line denotes the fitting results for interband transition at 10 K (see also the text). The fitting results (c) at 50 K and (d) at 90 K, respectively (magenta lines). The Drude response, interband transition, and peak A (polaron band) are shown by blue, green, and red dotted lines.

Figure 3

(a) The spectral weight of peak A (polaron band) and Drude weight. The dashed line is the fitting result by the model (see text). (b) The temperature dependence of renormalized plasma frequency (${\omega }_{\mathrm{p}}^{*}$) and energy of peak A (polaron band) (${\omega }_0$). The hatched region denotes the energy range of the three LO phonons [triangles in Fig. 3]. (c) The mass enhancement factor $1+\lambda$. (d) The loss function spectra ($\mathrm{Im}[-1/\epsilon (\omega \left)\right]$) of ${\mathrm{CaIrO}}_3$. The spectra are offset for clarity. The dotted lines (open triangles) indicate the plasmon resonance (the LO phonons). The hatched region denotes the energy range of peak A (polaron band). (e) $\mathrm{Im}[-1/\epsilon (\omega \left)\right]$ spectrum of ${\mathrm{SrIrO}}_3$ at 10 K.

Figure 4

(a) Temperature dependence of resistivity in ${\mathrm{CaIr}}_{1-x}{\mathrm{Rh}}_x{\mathrm{O}}_3$. The inset shows the electron mobility at 10 K. The $\sigma \left(\omega \right)$ spectra at 50 K for (b) $x=0$, (c) 0.025, and (d) 0.05, respectively. The magenta line denotes the fitting curve with the Drude response (blue dotted line) and polaron band (red dotted line). The closed circle at $\omega =0$ denotes the dc conductivity. (e) The Drude weight (blue), spectral weight of polaron band (red), and the mass enhancement factor $1+\lambda$ (green), all derived at 50 K.