# Ballistic transport and boundary resistances in inhomogeneous quantum spin chains

## TL;DR

This paper investigates the complex transport behavior in inhomogeneous quantum spin chains, revealing coexistence of thermalization and ballistic transport regions, and discusses implications for boundary resistance and integrability effects.

## Contribution

It introduces a matrix-product-state approach to analyze relaxation dynamics in joined ballistic quantum spin chains, highlighting the coexistence of different transport regimes.

## Key findings

- Inner region tends towards thermalization
- Outer region supports ballistic transport
- Possible non-zero boundary resistance at infinite time

## Abstract

Transport phenomena are central to physics, and transport in the many-body and fully-quantum regime is attracting an increasing amount of attention. It has been recently revealed that some quantum spin chains support ballistic transport of excitations at all energies. However, when joining two semi-infinite ballistic parts, such as the XX and XXZ spin-1/2 models, our understanding suddenly becomes less established. Employing a matrix-product-state ansatz of the wavefunction, we study the relaxation dynamics in this latter case. Here we show that it takes place inside a light cone, within which two qualitatively different regions coexist: an inner one with a strong tendency towards thermalization, and an outer one supporting ballistic transport. We comment on the possibility that even at infinite time the system supports stationary currents and displays a non-zero Kapitza boundary resistance. Our study paves the way to the analysis of the interplay between transport, integrability, and local defects.