Energy transport between two integrable spin chains

We study the energy transport in a system of two half-infinite XXZ chains initially kept separated at different temperatures, and later connected and let free to evolve unitarily. By changing independently the parameters of the two halves, we highlight, through bosonization and time-dependent matrix-product-state simulations, the different contributions of low-lying bosonic modes and of fermionic quasiparticles to the energy transport. In the simulations we also observe that the energy current reaches a finite value which only slowly decays to zero. The general picture that emerges is the following. Since integrability is only locally broken in this model, a preequilibration behavior may appear. In particular, when the sound velocities of the bosonic modes of the two halves match, the low-temperature energy current is almost stationary and described by a formula with a nonuniversal prefactor interpreted as a transmission coefficient. Thermalization, characterized by the absence of any energy flow, occurs only on longer time scales which are not accessible with our numerics.