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Transport through interacting defects and lack of thermalisation
by Giuseppe Del Vecchio Del Vecchio, Andrea De Luca, Alvise Bastianello
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Submission summary
Authors (as registered SciPost users):  Alvise Bastianello 
Submission information  

Preprint Link:  https://arxiv.org/abs/2104.13887v3 (pdf) 
Date accepted:  20211223 
Date submitted:  20211119 17:02 
Submitted by:  Bastianello, Alvise 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We consider 1D integrable systems supporting ballistic propagation of excitations, perturbed by a localised defect that breaks most conservation laws and induces chaotic dynamics. Focusing on classical systems, we study an outofequilibrium protocol engineered activating the defect in an initially homogeneous and far from the equilibrium state. We find that large enough defects induce full thermalisation at their center, but nonetheless the outgoing flow of carriers emerging from the defect is nonthermal due to a generalization of the celebrated Boundary Thermal Resistance effect, occurring at the edges of the chaotic region. Our results are obtained combining abinitio numerical simulations for relatively smallsized defects, with the solution of the Boltzmann equation, which becomes exact in the scaling limit of large, but weak defects.
Author comments upon resubmission
Dear Editor,
we thank you for your time in considering our submitted manuscript "Transport through interacting defects and lack of thermalisation" for publication in Scipost Physics. In her/his report, the referee raised some points which helped us in further clarifying our exposition: we are thankful for the useful input. Hereafter we address in detail the questions of the referee and the consequent improvements on the manuscript. We hope the revised version of our work can satisfy the Referee's request and meet the publication criteria of Scipost Physics.
Detailed comments:
1) Referee: "This is a nice and original work on a classical 1d model written as if it were a paper on a quantum model, which makes reading it quite confusing. It considers a classical system of quadratic oscillators with a nonlinear potential confined in a region of size L."
1) Answer: We apologize for the misunderstanding, of course it was not our intention to confuse the reader. As stated already in the previous version of the manuscript, we focus on the classical setup to have access to largescale numerical simulations which are beyond the reach of quantum numerical methods. Nonetheless, the microscopic mechanism at the origin of the boundary generalized resistance is explained in terms of simple kinetic arguments which are quantitatively captured by the Boltzmann approach (which can be formulated within the quantum framework as well). Since we look at the classical model just as a convenient tool to check our general picture and the same mechanism is expected to hold in the quantum scenario, we did not sufficiently advertise the use of the classical setup. In the revised version of the manuscript, we repair this mistake through the following changes
 In the abstract, we replace "quasiparticles"> "carriers". While we think the concept of quasiparticle can be used in the classical scenario as well, we agree the reader could be induced to believe we are focusing mostly on quantum models. For the same reason, in the abstract we now explicitly added "Focusing on classical systems, we study..."
We explicitly state in the introduction that we focus on the classical scenario. Nevertheless, let us point us that classical physics has been an irreplaceable tool to assess the validity of nonequilibrium scenarios also in connection with integrability, as shown for example by Refs. [5457] of the revised manuscript.
We changed the title of Section 2 to make the use of a classical model explicit.

In Eq.(1), \psi^\dagger > \psi^. The use of \psi^\dagger instead of \psi^ was a typo, indeed it never appeared again in the manuscript.

Below Eq. 3, we removed the explicit mention to the Wigner distribution. Even though the local phasespace distribution of the classical model is defined similarly to the quantum Wigner distribution, we agree it could be confusing for the reader.
2) Referee: "Another point: while in the quantum transport setup drivendissipative boundaries (which lead to the Lindblad equation) are used to mimic the leads attached to a mesoscopic object (in the markovian limit etc.), in classical mechanics I am not sure this can be easily justified nor I have seen it used before (no reference is quoted here, so I assume this is an invention of the authors). The authors should justify this setup more convincingly."
2) Answer: To the best of our knowledge, we are not aware of the use of this strategy within the classical scenario and we could not find a proper reference. This is also the main motivation behind appendix A, which provides a detailed account of how the strategy works. In the revised version of the manuscript, we summarize in a few words the general idea before digging into the details: essentially, outgoing carriers flowing out of the defect are removed from the system thanks to the dissipation. On the other hand, the tunable drive allows injecting an excitation distribution of our choice. The effect of the boundaries is then captured by Eq. 16: numerically solving for the stationary solution \partial_t n=0 allows for a determination of the injected mode density as a function of the tunable drive s(k). This allows for an exact characterization of the injected mode density in terms of the boundary noise: for example, in Fig. 3 the injected mode density (k>0) of the microscopic simulations (symbols) is exactly tuned to the target value (solid curve) obtained by solving Eq. 16.
Published as SciPost Phys. 12, 060 (2022)