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Sustaining a temperature difference
by Matteo Polettini, Alberto Garilli
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Submission summary
As Contributors:  Alberto Garilli 
Arxiv Link:  https://arxiv.org/abs/2005.06289v1 (pdf) 
Date submitted:  20200514 02:00 
Submitted by:  Garilli, Alberto 
Submitted to:  SciPost Physics 
Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Computational 
Abstract
We derive an expression for the minimal rate of entropy that sustains two reservoirs at different temperatures $T_0$ and $T_\ell$. The law displays an intuitive $\ell^{1}$ dependency on the relative distance and a characterisic $\log^2 (T_\ell/T_0)$ dependency on the boundary temperatures. First we give a backofenvelope argument based on the Fourier Law (FL) of conduction, showing that the leastdissipation profile is exponential. Then we revisit a model of a chain of oscillators, each coupled to a heat reservoir. In the limit of large damping we reobtain the exponential and squaredlog behaviors, providing a selfconsistent derivation of the FL. For small damping "equipartition frustration" leads to a wellknown balistic behaviour, whose incompatibility with the FL posed a longtime challenge.
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Anonymous Report 2 on 202077 Invited Report
 Cite as: Anonymous, Report on arXiv:2005.06289v1, delivered 20200707, doi: 10.21468/SciPost.Report.1805
Report
The authors study analytically and numerically the entropy production rate in a system connected to two reservoirs with different temperatures. The microscopic model in consideration is a set of classical oscillators coupled to dissipative environments. For this model a formula for the minimal rate of entropy is given. In the overdamped limit of the model the the minimal rate of entropy follows the one which can be derived from Fourier law. On the other hand, in the lownoise limit the formula predicts the ”equipartition frustration” which cannot be derived from Fourier law. The paper’s novelty is in the derivation of the minimal entropy production rate which is consistent with the one derived from Fourier law in the overdamped limit. This can be ultimately interpreted as selfconsistent derivation of the Fourier law. The paper is clearly written with respect to the main points of the paper which are well explained. I can recommend for publication. However, before publication the authors should consider following minor comments:
•Above Eq. (3): The authors refer to ”the equation” without quoting the equation.
• Above Eqs. (5) in the discussions of the condition of nonoscillating ends  the condition $p_0$ = 0 is missing.
• The authors could add some reference for OrnsteinUhlenbeck process and Kramers diffusion equation which might be helpful for nonspecialized readers.
• Missing parenthesis in eq. (16).
• Since eq. (19) is one of the main result of the paper, more steps of the derivation of eq. (19) from eq. (18) should be provided.
• The discussion of discretized equivalents after eq. (24) should be improved. For example the connection of eqs. (23) and (A.2) is not clear to me.
• In appendix B: the reference to eq. (14) supposes to be a reference to eq.(18). The authors should doublecheck the crossreferencing of the equations.
Anonymous Report 1 on 202072 Invited Report
 Cite as: Anonymous, Report on arXiv:2005.06289v1, delivered 20200702, doi: 10.21468/SciPost.Report.1797
Report
This paper is clear and enjoyable to read, and I fully recommend publication once some minor points are addressed.
1. In the introduction the authors state "As a main new technical result we obtain explicit secondorder expressions for the stationary distribution and the EPR". The new result for the EPR is clear. I was unsure which new result for the stationary distribution the authors were referring to, since various such results appear in Refs. 1417.
2. The parameter \alpha in Eq. 4 seems a little redundant, as it is referred to only once.
3. The authors' selfconsistent derivation of the Fourier law is convincing. I wonder if they could comment on its relation with the other approaches referred to e.g. in Ref. 16, and those in Ref. 17 and PRB 11 2164 that are mentioned in the review of Bonetto et al.
4. The squaredlog curve in Fig. 4 is a difficult to see on the plot, even in colour.
There were a small few typos:
 balistic > ballistic (abstract and elsewhere)
 there seems to be a sign error in the defn of $F_{kh}$ below Eq. 22
 underamped > underdamped (Fig 4 caption)
We thank the reviewer for the nice report and the interesting considerations. We would like to answer every point one by one:
 Together with the expression for the EPR we also refer as a main technical result for the stationary distribution to the secondorder Lyapunov equation Eq. 15 (Eq. 17 in the new version).
 The parameter $\alpha$ in Eq. 4 (Eq. 6 in the new version) is necessary since the constant $\kappa$ is dimensional (heat conductivity) we are considering an extended system in 3 dimensions (assuming the temperature varying only in one dimension), then $\alpha$ represents the area of a section orthogonal to the dimension in which we let the temperature vary. This is slightly different from what we obtain in the stochastic approach on the 1dimensional chain of oscillators where such section $\alpha$ does not appear. If you refer to the fact that we also made use of the same symbol $\alpha$ as an index in the basis of the eigenstates of $A$ then it is sufficient to change symbol, even if we think it is not strictly necessary since there is no ambiguity.
 We added a comment (page 8 in the new version, after Eq. 27) in which we compare our model with the one in PRB 11 2164. The main difference is that the authors of that paper consider the equipartition to be satisfied by each oscillator, finding a linear temperature profile between the boundaries $T_0$ and $T_n$. In our work we considered instead the profile which minimizes the EPR, giving an exponential profile and reproducing the temperature frustration in the low damping limit. In this way it is possible to say that the best way to minimize EPR is to reduce the heat flow to the colder reservoirs.
 In the plot Fig. 4 the aim is to point out that the EPR scales with the number of oscillators as $1/n$. However, observing that the squaredlog curves are not easy to see is definitely a good point and we we modified the figure in order to make all the remarkable informations visible.
Thank you for noticing the typos, the sign on the definition of the thermodynamic force had to be inverted.
(in reply to Report 2 on 20200707)
We thank the reviewer for the nice report and the interesting considerations. We would like to answer your points one by one: