Subterahertz Momentum Drag and Violation of Matthiessen’s Rule in an Ultraclean Ferromagnetic ${\mathrm{SrRuO}}_3$ Metallic Thin Film

${\mathrm{SrRuO}}_3$, a ferromagnet with an approximately 160 K Curie temperature, exhibits a $T^2$-dependent dc resistivity below $\approx 30\mathrm{K}$. Nevertheless, previous optical studies in the infrared and terahertz range show non-Drude dynamics at low temperatures, which seem to contradict Fermi-liquid predictions. In this work, we measure the low-frequency THz range response of thin films with residual resistivity ratios, ${\rho }_{300K}/{\rho }_{4K}\approx 74$. At temperatures below 30 K, we find both a sharp zero frequency mode which has a width narrower than $k_{B}T/\hslash$ as well as a broader zero frequency Lorentzian that has at least an order of magnitude larger scattering. Both features have temperature dependences consistent with a Fermi liquid with the wider feature explicitly showing a $T^2$ scaling. Above 30 K, there is a crossover to a regime described by a single Drude peak that we believe arises from strong interband electron-electron scattering. Such two channel Drude transport sheds light on reports of the violation of Matthiessen’s rule and extreme sensitivity to disorder in metallic ruthenates.

Figure 1

(a) dc resistivity as a function of the temperature for the ${\mathrm{SrRuO}}_3$ film for the two orthogonal directions. Inset: resistivity minus residual resistivity as a function of $T^2$. Fits to the data in the temperature range 2–32 K are shown as black lines. (b) Real part of the THz conductivity ${\sigma }_1$ from 5 K to room temperature along with corresponding dc values. (c) One Drude vs two Drude fit of dc and THz data at 3 K. The dc conductivity and real and imaginary parts of the complex conductivity are fitted simultaneously.

Figure 2

(a) Real and imaginary THz conductivity with dc values at 5 K. The red and blue solid lines show fitting with two Drude terms. The green and blue shaded regions correspond to a narrow and wider Drude term, respectively. The green shade is offset to distinguish from the other. (b)–(f) Real and imaginary parts of the complex conductivity with dc values for 5 K [same data as in (a)], 15, 25, 35, and 80 K. The data are presented with one Drude (dashed lines) and two Drude modeling (solid lines). Note that for 35 and 80 K, two Drude terms from fitting have the same scattering rates, so the two Drude overlaps with one Drude fitting. In (b)–(f) the same line types are used.

Figure 3

(a) Scattering rate of the wide (red) and narrow (blue) Drude peaks as a function of $T$. The dashed line is a quadratic fit to the data below 30 K, and extended to higher $T$. (b) Spectral weight of the two Drude peaks (obtained from two Drude fitting) and their total spectral weight, from 5 to 30 K. Above 30 K, single Drude fit parameters are given. The spectral weight is proportional to ${\omega }_p^2$.