THz conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$

TL;DR

This study measures the terahertz optical conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$ thin films across a magnetic transition, revealing frequency-dependent metallic behavior and the impact of disorder and interband transitions.

Contribution

It provides detailed THz conductivity data for Sr$_{1-x}$Ca$_x$RuO$_3$ across the ferromagnetic to paramagnetic transition, including theoretical analysis of interband effects.

Findings

Ferromagnetic SrRuO$_3$ and paramagnetic CaRuO$_3$ show frequency-dependent metallic response below 20 K.

Intrinsic electronic contributions to optical scattering rate are accessible and exceed residual scattering at 1.4 THz.

Deviations from Drude response start around 0.7 THz, with increased disorder leading to featureless spectra in intermediate compositions.

Abstract

We investigate the optical conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$ across the ferromagnetic to paramagnetic transition that occurs at $x=0.8$. The thin films were grown by metalorganic aerosol deposition with $0 \leq x \leq 1$ onto NdGaO$_3$ substrates. We performed THz frequency domain spectroscopy in a frequency range from 3~cm$^{-1}$ to 40~cm$^{-1}$ (100~GHz to 1.4~THz) and at temperatures ranging from 5~K to 300~K, measuring transmittivity and phase shift through the films. From this we obtained real and imaginary parts of the optical conductivity. The end-members, ferromagnetic SrRuO$_3$ and paramagnetic CaRuO$_3$, show a strongly frequency-dependent metallic response at temperatures below 20~K. Due to the high quality of these samples we can access pronounced intrinsic electronic contributions to the optical scattering rate, which at 1.4~THz exceeds the residual scattering rate by more than a factor of three. Deviations from a Drude response start at about 0.7~THz for both end-members in a remarkably similar way. For the intermediate members a higher residual scattering originating in the compositional disorder leads to a featureless optical response, instead. The relevance of low-lying interband transitions is addressed by a calculation of the optical conductivity within density functional theory in the local density approximation (LDA).