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Typicality approach to the optical conductivity in thermal and many-body localized phases

We study the frequency dependence of the optical conductivity $\text{Re} \, σ(ω)$ of the Heisenberg spin-$1/2$ chain in the thermal and near the transition to the many-body localized phase induced by the strength of a random $z$-directed magnetic field. Using the method of dynamical quantum typicality, we calculate the real-time dynamics of the spin-current autocorrelation function and obtain the Fourier transform $\text{Re} \, σ(ω)$ for system sizes much larger than accessible to standard exact-diagonalization approaches. We find that the low-frequency behavior of $\text{Re} \, σ(ω)$ is well described by $\text{Re} \, σ(ω) \approx σ_\text{dc} + a \, |ω|^α$, with $α\approx 1$ in a wide range within the thermal phase and close to the transition. We particularly detail the decrease of $σ_\text{dc}$ in the thermal phase as a function of increasing disorder for strong exchange anisotropies. We further find that the temperature dependence of $σ_\text{dc}$ is consistent with the existence of a mobility edge.