We extend beyond the Euler scales the hydrodynamic theory for quantum and classical integrable models developed in recent years, accounting for diffusive dynamics and local entropy production. We review how the diffusive scale can be reached via a gradient expansion of the expectation values of the conserved fields and how the coefficients of the expansion can be computed via integrated steady-state two-point correlation functions, emphasising that PT-symmetry can fully fix the inherent ambiguity in the definition of conserved fields at the diffusive scale. We develop a form factor expansion to compute such correlation functions and we show that, while the dynamics at the Euler scale is completely determined by the density of single quasiparticle excitations on top of the local steady state, diffusion is due to scattering processes among quasiparticles, which are only present in truly interacting systems. We then show that only two-quasiparticle scattering processes contribute to the diffusive dynamics. Finally we employ the theory to compute the exact spin diffusion constant of a gapped XXZ spin-1/2 chain at finite temperature and half-filling, where we show that spin transport is purely diffusive.
Cited by 128
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- 1 Universiteit Gent / Ghent University
- 2 Laboratoire de Physique Théorique [LPTENS]
- 3 King's College London [KCL]
- Agence Nationale de la Recherche [ANR]
- Erwin Schrödinger International Institute for Mathematics and Physics [ESI]
- Gouvernement du Canada / Government of Canada
- Ministry of Research and Innovation (Canada, Ontario, Min Res&Innov) (through Organization: Ministry of Research, Innovation and Science - Ontario [MRIS])
- Scuola Internazionale Superiore di Studi Avanzati / International School for Advanced Studies [SISSA]
- Thomas Young Centre / Thomas Young Centre [TYC]