Temperature-driven evolution of phonon-coupled carrier dynamics in lanthanum hexaferrite probed by terahertz spectroscopy: Implications for THz photonics

Abstract

Carrier–phonon coupling plays a central role in charge transport, energy relaxation, and device stability in the terahertz (THz) regime, making its investigation crucial for condensed-matter physics and future high-frequency technologies. We report a systematic study of the temperature-dependent THz time-domain response of lanthanum hexaferrite (LaFe12O19, LaM) across 296–683 K. Increasing temperature produces a clear decrease in transmission amplitude and a corresponding increase in time delay, highlighting the strongly dispersive nature of LaM. The reduction in transmission and enhanced absorption at elevated temperatures are linked to intensified phonon-carrier interactions and disorder-induced scattering. A nonlinear change in the refractive index indicates enhanced ionic polarizability from thermally activated lattice vibrations. At room temperature, LaM shows a high dielectric constant (ɛ′ ≈ 13.9 at 1 THz) and low dielectric loss (tan δ ≈ 0.03 at 1 THz), dominated by ionic polarization. Temperature- and frequency-dependent optical conductivity trends provide direct evidence of carrier scattering and phonon coupling. Lorentz oscillator fitting of permittivity and conductivity confirms phonon-assisted resonances with temperature-driven shifts in resonance frequency, oscillator strength, damping, scattering time, and quality factor. These findings demonstrate the strong influence of carrier–phonon interactions on LaM's THz response and its promise for tunable THz devices.