Many-body large polaron optical conductivity in ${\text{SrTi}}_{1-x}{\text{Nb}}_x{\text{O}}_3$

Recent experimental data on the optical conductivity of niobium-doped ${\text{SrTiO}}_3$ are interpreted in terms of a gas of large polarons with effective coupling constant ${\alpha }_{\text{eff}}\approx 2$. The theoretical approach takes into account many-body effects, the electron-phonon interaction with multiple LO-phonon branches, and the degeneracy and the anisotropy of the $\text{Ti}{t}_{2g}$ conduction band. Based on the Fröhlich interaction, the many-body large polaron theory provides an interpretation for the essential characteristics, except—interestingly—for the unexpectedly large intensity of a peak at $\sim 130\text{meV}$, of the observed optical conductivity spectra of ${\text{SrTi}}_{1-x}{\text{Nb}}_x{\text{O}}_3$ without any adjustment of material parameters.

Figure 1

(Color online) Optical conductivity of ${\text{SrTi}}_{1-x}{\text{Nb}}_x{\text{O}}_3$ for 0.1%, 0.2%, 0.9%, and 2% at 300 K [panel (a)] and 7 K [panel (b)]. For clarity, the mid-infrared conductivities of $x=0.1\mathrm{\%}$ and 0.2% are magnified by a factor 4 (from Ref. 15).

Figure 2

(Color online) The many large polaron optical conductivity compared with the experiment (Ref. 15) at $T=7\text{K}$. The doping level is (a) $x=0.1\mathrm{\%}$, (b) 0.2%, (c) 0.9%, and (d) 2%.

Figure 3

(Color online) The many large polaron optical conductivity compared with the experiment (Ref. 15) at $T=300\text{K}$. The doping level is (a) $x=0.1\mathrm{\%}$, (b) 0.2%, (c) 0.9%, and (d) 2%.