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arxiv: 2508.15566 · v1 · submitted 2025-08-21 · ❄️ cond-mat.stat-mech · cond-mat.mes-hall

From Near-Integrable to Far-from-Integrable: A Unified Picture of Thermalization and Heat Transport

Pith reviewed 2026-05-18 21:49 UTC · model grok-4.3

classification ❄️ cond-mat.stat-mech cond-mat.mes-hall
keywords thermalizationheat transportintegrability breakingphase diagramdiatomic hard-point gasBogoliubov hypothesishydrodynamic relaxationnonequilibrium dynamics
0
0 comments X p. Extension

The pith

The relaxation dynamics of a one-dimensional diatomic hard-point gas fall into three universal regimes organized by a phase diagram in system size and integrability-breaking strength, with the same regimes governing heat transport.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper studies how a one-dimensional diatomic hard-point gas approaches equilibrium when a tunable parameter δ controls the strength of integrability breaking. It maps the full range of δ and system size N to produce a phase diagram that identifies three distinct dynamical regimes for local energy relaxation. Near integrability, kinetic processes produce exponential decay of energy fluctuations whose time scale grows as the inverse square of δ. Far from integrability, hydrodynamic processes dominate and the relaxation time grows linearly with system size. An intermediate regime realizes the Bogoliubov crossover from kinetic to hydrodynamic behavior, and the identical scaling structure appears in the decay of equilibrium heat-current fluctuations.

Core claim

The central claim is that relaxation in the diatomic hard-point gas is organized by a phase diagram in the (N, δ) plane containing three regimes: near-integrable kinetic relaxation with exponential decay and thermalization time τ ∝ δ^{-2}, far-from-integrable hydrodynamic relaxation with power-law decay and τ scaling linearly with N, and an intermediate regime in which kinetic relaxation precedes hydrodynamic relaxation; heat transport obeys the same three-regime structure, so that the observed dynamics in the thermodynamic limit depend on the order in which the limits N → ∞ and δ → 0 are taken.

What carries the argument

The phase diagram in the plane of system size N and integrability-breaking parameter δ that separates kinetic, intermediate, and hydrodynamic relaxation processes.

If this is right

  • In the thermodynamic limit the relaxation behavior depends on whether system size or integrability-breaking strength is taken to its limiting value first.
  • Hydrodynamic relaxation can appear in small systems once the system is sufficiently far from integrability.
  • The decay of heat-current fluctuations in equilibrium follows the same three-regime scaling as nonequilibrium thermalization.
  • The Bogoliubov sequence of kinetic followed by hydrodynamic stages appears explicitly as the intermediate regime in the phase diagram.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Analogous phase diagrams may classify relaxation in other one-dimensional systems whose integrability can be tuned continuously.
  • Small-system simulations can already access hydrodynamic scaling by choosing sufficiently large integrability-breaking strength.
  • The unified picture suggests that thermalization and transport can be studied together in quantum many-body systems with comparable parameter tuning.

Load-bearing premise

The specific collision rules of the diatomic hard-point gas and the chosen measure of integrability breaking capture the generic crossover between kinetic and hydrodynamic relaxation without model-specific artifacts.

What would settle it

Numerical measurement of local energy relaxation time versus δ and N that fails to exhibit the predicted δ^{-2} scaling in the near-integrable limit or the linear-in-N scaling in the far-from-integrable limit would falsify the phase diagram.

Figures

Figures reproduced from arXiv: 2508.15566 by Hong Zhao, Weicheng Fu, Yisen Wang, Yong Zhang, Zhen Wang.

Figure 1
Figure 1. Figure 1: FIG. 1. Diagram of characteristic timescales versus [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Dependence of [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Evolution of energy [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: (b) reveals a collapse for small δ when t is rescaled by δ 2 . The curve approximates exponential de￾cay (see the inset in semi-logarithmic scale) toward its minimum value 6/N, indicating that Teq ∝ τk ∝ δ −2 ≫ τh as δ → 0. This suggests that the system remains in region (I) (see [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: (a) shows the evolution of hΦ(t)i for different system sizes N at the fixed δ = 0.05, exploring the in￾fluence of finite-size effects on thermalization behavior. The time is rescaled by √ N, as it follows from Eq. (20) that changing the number of particles at a fixed tem￾perature or changing the temperature at a fixed particle number is equivalent in the initial stage. Therefore, the change in system size … view at source ↗
Figure 6
Figure 6. Figure 6: (a) shows the normalized HCAF for various values of δ at the fixed small N = 50. For small δ, the kinetic predictions and numerical results agree well. However, as δ increases, deviations emerge. The kinetic region rapidly shrinks, and hydrodynamic effects begin to dominate, as evidenced by the oscillations arising from the collision of acoustic modes at the boundary [65]. These results suggest that even s… view at source ↗
Figure 7
Figure 7. Figure 7: (a) shows the HCAF for different values of δ at a large system size, N = 104 . For all values of δ, over short timescales (t < τf), the HCAF remains constant, corresponding to a linear increase in heat con￾ductivity (κ), as shown in [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
read the original abstract

Whether and how a system approaches equilibrium is central in nonequilibrium statistical physics, crucial to understanding thermalization and transport. Bogoliubov's three-stage (initial, kinetic, and hydrodynamic) evolution hypothesis offers a qualitative framework, but quantitative progress has focused on near-integrable systems like dilute gases. In this work, we investigate the relaxation dynamics of a one-dimensional diatomic hard-point (DHP) gas, presenting a phase diagram that characterizes relaxation behavior across the full parameter space, from near-integrable to far-from-integrable regimes. We analyze thermalization (local energy relaxation in nonequilibrium states) and identify three universal dynamical regimes: (i) In the near-integrable regime, kinetic processes dominate, local energy relaxation decays exponentially, and the thermalization time $\tau$ scales as $\tau \propto \delta^{-2}$. (ii) In the far-from-integrable regime, hydrodynamic effects dominate, energy relaxation decays power-law, and thermalization time scales linearly with system size $N$. (iii) In the intermediate regime, the Bogoliubov phase emerges, characterized by the transition from kinetic to hydrodynamic relaxation. The phase diagram also shows that hydrodynamic behavior can emerge in small systems when sufficiently far from the integrable regime, challenging the view that such effects occur only in large systems. In the thermodynamic limit, the system's relaxation depends on the order in which the limits ($N \to \infty$ or $\delta \to 0$) are taken. We then analyze heat transport (decay of heat-current fluctuations in equilibrium), demonstrating its consistency with thermalization, leading to a unified theoretical description of thermalization and transport. Our approach provides a pathway for studying relaxation dynamics in many-body systems, including quantum systems.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript studies relaxation in a one-dimensional diatomic hard-point gas, constructing a phase diagram in the (δ, N) plane that identifies three dynamical regimes for local energy thermalization: (i) near-integrable (exponential decay with τ ∝ δ^{-2}), (ii) far-from-integrable (power-law decay with τ ∝ N), and (iii) an intermediate Bogoliubov crossover. It further shows that heat-current fluctuations in equilibrium are consistent with these regimes and discusses the non-commuting limits N → ∞ and δ → 0, claiming a unified picture of thermalization and transport.

Significance. If the reported scalings and regime boundaries are robust, the work supplies a concrete, tunable realization of the kinetic-to-hydrodynamic crossover in a many-body system and demonstrates that hydrodynamic relaxation can appear at modest N when δ is sufficiently large. The explicit link between nonequilibrium thermalization times and equilibrium current fluctuations is a positive feature. The model dependence on the specific hard-point collision rules remains a limitation that affects how far the phase diagram can be viewed as generic.

major comments (2)
  1. [§2] §2 (model definition): the integrability-breaking parameter δ is introduced via the mass ratio in the elastic two-mass collision rules. Because equal-mass hard-point dynamics maps to free streaming, the specific form of δ may generate additional near-conserved quantities whose relaxation competes with the intended kinetic and hydrodynamic channels. The central claim that the three regimes are universal therefore requires an explicit check that the exponential-versus-power-law distinction and the δ^{-2} scaling survive under a qualitatively different perturbation (e.g., a weak quartic potential).
  2. [§4] §4 (thermodynamic-limit discussion): the statement that relaxation depends on the order of limits (N → ∞ versus δ → 0) is load-bearing for the phase-diagram interpretation. The manuscript should supply either analytic bounds or numerical data showing how the crossover lines shift when the two limits are taken in opposite order, rather than inferring the dependence solely from finite-N, finite-δ trajectories.
minor comments (2)
  1. [Figure 2] Figure 2 (phase diagram): the boundaries separating the three regimes are drawn as sharp lines; adding uncertainty bands or indicating the fitting windows used to extract τ would improve clarity.
  2. [Abstract] Abstract, line 3: the phrase 'three universal dynamical regimes' should be qualified as 'three regimes observed in this model' to avoid implying model-independent universality before the generality check is performed.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive suggestions. We provide point-by-point responses to the major comments below.

read point-by-point responses
  1. Referee: [§2] §2 (model definition): the integrability-breaking parameter δ is introduced via the mass ratio in the elastic two-mass collision rules. Because equal-mass hard-point dynamics maps to free streaming, the specific form of δ may generate additional near-conserved quantities whose relaxation competes with the intended kinetic and hydrodynamic channels. The central claim that the three regimes are universal therefore requires an explicit check that the exponential-versus-power-law distinction and the δ^{-2} scaling survive under a qualitatively different perturbation (e.g., a weak quartic potential).

    Authors: The referee raises a valid point regarding the potential model-specific features of the mass-ratio perturbation. While we maintain that the diatomic hard-point gas provides a clean realization of the kinetic-to-hydrodynamic crossover, we acknowledge that an explicit verification with an alternative perturbation, such as a weak quartic potential, would help establish broader universality. In the revised manuscript, we will expand the discussion in §2 to address possible near-conserved quantities and argue based on existing literature that the δ^{-2} scaling is characteristic of weak integrability breaking in one-dimensional systems. However, performing new simulations with a different model is beyond the current scope, and we will note this as a direction for future work. revision: partial

  2. Referee: [§4] §4 (thermodynamic-limit discussion): the statement that relaxation depends on the order of limits (N → ∞ versus δ → 0) is load-bearing for the phase-diagram interpretation. The manuscript should supply either analytic bounds or numerical data showing how the crossover lines shift when the two limits are taken in opposite order, rather than inferring the dependence solely from finite-N, finite-δ trajectories.

    Authors: We agree that the non-commutativity of the limits is crucial and that our original presentation relied on inference from finite-parameter data. To address this directly, we have conducted additional numerical simulations in which the thermodynamic limit is approached by first sending N to infinity at fixed δ and subsequently taking δ to zero. The results, which will be included in the revised §4, show that the crossover lines in the phase diagram indeed shift depending on the order of limits, with the hydrodynamic regime becoming accessible at smaller δ when N is taken to infinity first. This provides concrete support for the phase-diagram interpretation. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation is model-driven and self-contained

full rationale

The paper examines relaxation in an explicit diatomic hard-point gas model by varying the integrability-breaking parameter δ and system size N, then reports observed dynamical regimes and scalings (exponential decay with δ^{-2}, power-law with N) from direct analysis of that model's collision rules and nonequilibrium evolution. No step reduces a claimed prediction to a fitted input by construction, nor does any central claim rest on a self-citation chain or imported uniqueness theorem; the phase diagram and unified thermalization-transport picture follow from the model's explicit dynamics rather than tautological redefinition. This is the normal non-circular outcome for a numerical study of a concrete microscopic system.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the representativeness of the DHP gas for generic many-body relaxation and on the validity of Bogoliubov's staged-evolution picture as a qualitative guide.

free parameters (1)
  • δ
    Integrability-breaking parameter (likely mass ratio or similar) whose value controls the distance from the integrable point and enters the reported scalings.
axioms (1)
  • domain assumption Bogoliubov's three-stage (initial, kinetic, hydrodynamic) evolution hypothesis
    Invoked as the qualitative framework that the quantitative regimes are compared against.

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Reference graph

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