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REVIEW 4 major objections 4 minor 90 references

Sub-diffusion in the Anderson model on random regular graph

T0 review · 4 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read An initially localized wave packet on a random regular graph spreads subdiffusively: both the wave front and the mean distance grow as $t^{\beta}$ with $\beta(W) \approx 1 - W/W_{\mathrm{AT}}$, evidence of a subdiffusive phase rather than…

desk verdict Plausible and well-organized numerical evidence for subdiffusive spreading on the RRG, but the phase claim leans on an inherited exponent that the collapses do not independently test. read the letter →

arxiv 1908.11388 v2 pith:IKLER7J5 submitted 2019-08-29 cond-mat.dis-nn cond-mat.stat-mechquant-ph

classification cond-mat.dis-nncond-mat.stat-mechquant-ph
keywords Andersonlocalizationrandomregulargraphsubdiffusionwave-packetdynamicsreturnprobabilitystretchedexponentialmany-bodynon-ergodicphase
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper studies how an initially localized wave packet spreads on a random regular graph, a tractable stand-in for the Fock space of a disordered interacting system. It claims that for a range of disorder strengths below the Anderson transition the dynamics is genuinely subdiffusive: both the wave front and the mean distance grow as $t^{\beta}$ with $\beta < 1$ over the entire range, not merely at a critical point. The exponent is the same $\beta(W) \approx 1 - W/W_{\mathrm{AT}}$ that controls the stretched-exponential decay of the return probability, connecting spatial propagation to the relaxation of the initial site. If correct, this establishes a full subdiffusive phase on the random regular graph and, through the many-body-localization proxy, suggests a mechanism for slow relaxation of local observables in disordered interacting systems.

What carries the argument

The central object is the distance-resolved probability distribution $\Pi(x,t)$, the probability that the particle is at graph distance $x$ from its initial site at time $t$, restricted to a microcanonical energy shell around mid-spectrum. The wave front $X_{\mathrm{front}}(t)$, defined as the distance at which $\Pi(x,t)$ has its maximum, is the moving boundary that separates frozen, front, and relaxed regions in spacetime. The argument is carried by the relation between spatial and temporal relaxation: the stretched-exponential return probability $\Pi(0,t) \sim \exp(-\Gamma t^{\beta(W)})$ fixes the exponent $\beta(W) = 1 - W/W_{\mathrm{AT}}$, and the same exponent collapses both the wave-front motion and the growth of the first moment $X(t)$. The space-time factorization behind the front, $\Pi(x,t)-\Pi(x,\infty) = g(x)[\Pi(0,t)-\Pi(0,\infty)]$, is what ties local relaxation to the front motion.

What would settle it

A direct test would be to evolve wave packets on random regular graphs larger than $L = 2^{20}$ and longer than the current maximum times, extracting $\beta(W)$ separately from the wave-front position, the first moment $X(t)$, and the return probability. If the three fitted exponents disagree, or if $X(t)$ visibly crosses over to linear growth at any fixed $W$ in $8 \le W \le 14$, the subdiffusive-phase claim as stated would be falsified.

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Extended reading notes

Core claim

On the Anderson model on a random regular graph, the full radial probability distribution $\Pi(x,t)$ of an initially localized wave packet is shown to organize into four spacetime regimes determined by a moving wave front. Before the front arrives, the distribution is frozen; near the front, $\Pi$ minus its infinite-time value collapses as a function of $X_{\mathrm{front}}(t) - x$; behind the front, it factorizes as $g(x)[\Pi(0,t)-\Pi(0,\infty)]$ and relaxes with the return probability; at very long times it saturates to the uniform-over-sites distribution. The central quantitative claim is that for disorder strengths $0.4W_{\mathrm{AT}} \lesssim W \lesssim 0.7W_{\mathrm{AT}}$, both the front $X_{\mathrm{front}}(t) \approx \Gamma(W) t^{\beta(W)}$ and the mean distance $X(t) \sim t^{\beta(W)}$ grow subdiffusively with $\beta(W) \approx 1 - W/W_{\mathrm{AT}}$. Because the same exponent also controls the stretched-exponential return probability from earlier work, the paper presents the spatial spreading and the temporal relaxation as two faces of one subdiffusive phase, in contrast to $\mathbb{Z}^d$ lattices with $d>2$ where subdiffusion appears only at the critical point.

Load-bearing premise

The paper's spatial exponents are not fitted independently: it assumes that the stretched-exponential return probability with $\beta(W) = 1 - W/W_{\mathrm{AT}}$, taken from its earlier study, is accurate and not a finite-time artifact, and pins the wave-front and mean-distance growth to this same $\beta$.

Editorial extensions

If this is right

  • If the claim holds, the random regular graph hosts a genuine subdiffusive phase for a finite interval of disorder, not just a critical point, with the exponent decreasing linearly to zero at the Anderson transition.
  • The Thouless time, when the front reaches the graph diameter, scales as $(\ln L / \ln K)^{1/\beta(W)}$, which diverges as a power of the logarithm near the transition; this is the same scaling found in subdiffusive many-body-localized systems.
  • Slow spreading on the random regular graph implies slow relaxation of local observables in systems that map to it, offering a route to subdiffusion in many-body systems that does not invoke Griffiths effects.
  • The observed slow dynamics rules out full ergodicity in the random-matrix sense for $W \ge 8$ even in parameter regions where eigenfunction-based measures such as the inverse participation ratio may look ergodic.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the stretched-exponential exponent survives at larger sizes, one could test whether the same $\beta$ controls entanglement growth in the corresponding many-body system, since the graph distance maps to the Hamming distance in Fock space.
  • The four-regime structure is likely generic to locally tree-like graphs with exponential growth of the number of sites with distance; a possible extension is to derive $\beta(W)$ from statistics of rare resonances along the boundary rather than importing it from the return probability.
  • A finite-time to diffusive crossover at larger graph sizes cannot be excluded from these data; the paper's own claim should be read as establishing the subdiffusive phase for the accessible time and size window, with the thermodynamic extrapolation as the open question.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 4 minor

Summary. This manuscript studies the finite-time dynamics of an initially localized wave packet in the Anderson model on a random regular graph (RRG), using the full probability distribution Π(x,t) as a function of distance from the initial site. The authors identify four space-time regimes controlled by a propagating wave front X_front(t) and present numerical evidence that both the front and the mean distance X(t) grow sub-diffusively as t^β with β(W) ≈ 1 − W/W_AT for 0.4 W_AT ≲ W ≲ 0.7 W_AT. They also demonstrate a space-time factorization Π(x,t) − Π(x,∞) = g(x)[Π(0,t) − Π(0,∞)] after the front passes, connect the return probability to the front position through an exponential relation, and argue that the RRG can serve as a proxy for many-body localization dynamics. The numerical results are for L up to 2^20, with the energy-shell fraction f = 1/8, and are supported by appendices on finite-size and energy-window dependence.

Significance. If the claims hold, this paper would establish that the RRG Anderson model hosts a genuine dynamical subdiffusive phase over a finite disorder interval, in contrast to Anderson models on Z^d with d>2 where subdiffusion is restricted to the critical point. The four-regime description of Π(x,t) and the factorization property are potentially useful organizing principles for slow dynamics near the MBL transition, and the connection to the Thouless time scaling is suggestive. The paper's strengths include the use of a well-defined observable (the radial distribution), consistency checks on the energy window and system size in Appendix A, and an explicit statement of the residual uncertainty about a thermodynamic-limit crossover to diffusion. However, the central exponent β(W) is taken from the same authors' previous return-probability work (Ref. [73]) rather than measured here, the numerical collapses use that value as an input, and the paper provides no error bars, realization counts, or code/data release. These issues make the quantitative phase claim weaker than the qualitative observation of sublinear spreading.

major comments (4)
  1. [§Model and methods, Eq. (5); §Results, Eqs. (10)–(11); Figs. 4(b), 5(b), 11] The exponent β(W) is not measured in this paper. It is imported from Ref. [73] as Eq. (5), and then used as the time-rescaling exponent in the collapses of Figs. 4(b), 5(b), and 11. Consequently, those collapses demonstrate consistency with the assumed β(W), not an independent extraction of β from X(t) or X_front(t). Since the central quantitative claim is X_front(t) ~ Γ(W)t^β and X(t) ~ t^β, the authors should fit β directly from the new data, report confidence intervals and collapse-quality metrics, and compare the fitted values with Eq. (5). Without this, the subdiffusive-phase exponent remains an inherited assumption rather than a result of the present analysis.
  2. [§Results, Figs. 2–5 and Appendix A] No statistical information is reported: the numbers of disorder realizations, graph samples, and initial states are not given, and no error bars appear on Π(x,t), X(t), or the collapse curves. For a numerical claim about a scaling exponent and a collapse, this is load-bearing: the apparent agreement with the proposed functional forms cannot be distinguished from finite-sampling scatter or systematic drift. Please provide uncertainties (for example, bootstrap over samples) and state the number of realizations for each W and L. Releasing the data-processing code would also allow the collapses to be checked.
  3. [Conclusions, final paragraph before Acknowledgments] The paper explicitly concedes that a crossover to β = 1 in the thermodynamic limit cannot be ruled out. Because the RRG diameter grows only as ln L and the pre-saturation time window is t < t_Th ~ (ln L)^{1/β}, all data shown are intrinsically finite-time. The abstract's claim of an "entire subdiffusive phase" is therefore stronger than the evidence presented. Either soften the claim to finite-time subdiffusive dynamics or add a scaling analysis in L showing that the exponent extracted in the available window is stable as L increases.
  4. [Abstract and Appendix D] The abstract states that the numerical results are supported by a "self-consistent semiclassical picture" relating β to the relaxation rate of the return probability, but no derivation of β is given. Appendix D shows only that the two collapse forms, Eqs. (7) and (8), are mutually consistent if the return probability decays exponentially with the front position, δR(x) ~ e^{−λx}; it does not determine β(W). The wording should be revised to describe this as a consistency condition or an ansatz, not a self-consistent derivation.
minor comments (4)
  1. [§Introduction, Fig. 1] The term "subdiffusive" is used with a nonstandard definition (X(t) ~ t^β with β < 1 on a hierarchical graph, where diffusive motion would be X(t) ~ t). This is explained in the text, but the abstract and Fig. 1 could benefit from an early clarification to avoid confusion with the standard mean-square-displacement terminology.
  2. [Appendix A, Fig. 6] The figure axis is labeled X^{−1}(0) while the text discusses X(0); please check the label or define the plotted quantity explicitly.
  3. [Footnote 79] The sentence "Xfront(t) it is given by the value of x for which Π(x,t) has a maximum" contains an extra "it is"; please correct to "Xfront(t) is given by...".
  4. [§Results, Fig. 4(a) inset] The inset is said to show all four stages of the evolution, but the caption does not explicitly mark the four stages; adding labels or arrows would make the claimed four-regime structure easier to verify.

Circularity Check

2 steps flagged · score 6.0 of 10

The subdiffusive phase claim inherits its exponent β(W)=1−W/W_AT from the same authors' prior return-probability fit; the collapses use that β as an input rather than extracting it.

  1. self citation load bearing [Model and methods, paragraph following Eq. (5)]
    "In Ref. [73] we have shown that for small values of W (0<W<0.16WAT≃3) the return probability Π(0,t) is consistent with the result of Eq. (3), confirming that the system is in a fully ergodic phase. For larger disorder, W∈[0.4WAT,0.7WAT]≃[8,13], Π(0,t) decays as a stretched exponential ∼e^{−Γt^{β(W)}}, where the exponent is well approximated by β(W)≃1−W/WAT, 0.4WAT≲W≲0.7WAT (5)."

    The central quantitative exponent β(W) is not derived in this paper; it is quoted from Ref. [73], a return-probability study by the same group (Bera, De Tomasi, Khaymovich, Scardicchio). That prior result is a fitted, numerical form, not an externally established theorem. The present paper then makes this β the load-bearing input for the claimed front propagation and X(t) growth, so the phase claim's exponent reduces to a self-citation of an earlier fit rather than being independently established here.

  2. fitted input called prediction [Results, Eqs. (10)-(11) and Fig. 5; also Fig. 4(b) and Appendix C]
    "This collapse allows us to extract the following subdiffusive wave-front evolution Xfront(t)≃Γ(W)t^{β(W)}, β(W)<1. (10) ... Figure 5(a) shows the algebraic growth of X(t) in time for several W X(t)∼t^{β(W)}, (11) with the same subdiffusive exponent β(W), Eq. (5), as in the wave-front propagation Xfront(t)."

    The collapse in Fig. 4(b) and the analogous collapses in Fig. 5(b) and Appendix C rescale the time axis with β(W) taken from Eq. (5); the data are therefore only tested for consistency with that assumed β, not used to extract it. Eq. (10) is presented as an 'extraction' even though β was fed into the rescaling, and Eq. (11) explicitly reuses Eq. (5) as the same exponent. Thus Eqs. (10) and (11) are, at the level of the exponent, restatements of the prior fitted input rather than independent predictions. The raw log-log growth of X(t) does provide some independent qualitative evidence of sublinear spreading, which prevents the entire claim from being purely circular.

full rationale

The paper contains substantial self-contained numerical content: the four-regime space-time structure, the space-time factorization for x<Xfront, the finite-size and energy-window checks, and the direct observation that X(t) grows sublinearly before saturation. These parts are not circular. However, the quantitative centerpiece, β(W)=1−W/WAT, is imported as Eq. (5) from Ref. [73] by the same authors and is then used to rescale time in all the collapses supporting Xfront(t)∼t^β and X(t)∼t^β. Consequently, the claimed exponent and the width of the claimed subdiffusive phase are inherited from a prior fitted result rather than independently extracted in this work; the collapses demonstrate self-consistency with the assumed β, not a free determination of it. The paper is also honest about the main limitation, stating: 'We do not report any crossover to diffusivity for our available system sizes and time scales. Although we cannot completely rule out this possibility in the thermodynamic limit.' That limitation weakens the phase claim but is not itself a circularity. The absence of error bars or collapse-quality metrics further limits the strength of the claimed confirmation. Overall, this is partial circularity: the central quantitative claim reduces, at the exponent level, to a self-cited fit, while the qualitative subdiffusive spreading has independent grounding. Hence a score of 6 is appropriate.

Assumptions & free parameters 3 free parameters · 6 assumptions · 0 invented entities

The paper introduces no new particles or forces. Its load-bearing input is the fitted stretched-exponential exponent beta(W) from the same authors' previous work, plus phenomenological factorization assumptions that are verified numerically rather than derived. The free parameters are the exponent, the decay rate, and the front-profile decay constant.

free parameters (3)
  • beta(W) = 1 - W/W_AT = Range from about 0.56 (W=8) to about 0.23 (W=14), using W_AT about 18.1
    Imported from the same authors' prior fit of the return probability (Ref [73], Eq. 5). It is the central exponent used for X_front(t) and X(t), so the new spatial claim inherits it.
  • Gamma(W) = not tabulated
    Decay rate in Pi(0,t) ~ exp(-Gamma t^beta), stated in Eqs. (9) and (10). It is fitted to return-probability data in the previous work and sets the amplitude of X_front(t).
  • lambda = not tabulated
    Decay rate of the front profile f(z) = exp(-lambda z), Eq. (7) and Eq. (9). It is determined by the collapse analysis, but no numerical value is reported.
assumptions (6)
  • standard math Standard unitary time evolution and spectral decomposition of H with the projector P_DeltaE, Eqs. (1) and (2).
    The whole definition of Pi(x,t) rests on this standard quantum-mechanical framework.
  • domain assumption The RRG is locally tree-like with N(x) ~ K^x up to the diameter D = ln L / ln K.
    Used for the saturation value Pi(x,infinity) = N(x)/L and for the Thouless-time scaling. It is valid only until loops become important.
  • domain assumption The RRG is a valid proxy for the Fock-space structure of many-body localized systems.
    The mapping to MBL motivates the study and the final implications, but it is not proven in this paper.
  • ad hoc to paper The stretched-exponential return probability Pi(0,t) ~ exp(-Gamma t^beta) with beta(W) approximately 1 - W/W_AT holds for 0.4 W_AT < W < 0.7 W_AT.
    Assumed from Ref [73] without derivation. This is the quantitative foundation for the front and width exponents.
  • ad hoc to paper Space-time factorization Pi(x,t) - Pi(x,infinity) = g(x)[Pi(0,t) - Pi(0,infinity)], Eq. (8), and the front profile Eq. (7) with f(z) = exp(-lambda z).
    Phenomenological ansatz verified by collapse, not derived from the Hamiltonian. It is used to connect the return probability to X_front(t).
  • domain assumption The energy window deltaE = f E_BW with f = 1/8 at band center is small enough to define a microcanonical wave packet and does not affect the dynamics.
    Checked for f = 1/8, 1/16, and 1/32 in Appendix A, but only for X(t) and mainly at W = 8.

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Cite this review

Pith. "Pith review of Sub-diffusion in the Anderson model on random regular graph." pith.science (2026). https://pith.science/paper/IKLER7J5

@misc{pith2026190811388,
  author       = {Pith},
  title        = {Pith review of: Sub-diffusion in the Anderson model on random regular graph},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/IKLER7J5}},
  note         = {Machine review of arXiv:1908.11388}
}
abstract

We study the finite-time dynamics of an initially localized wave-packet in the Anderson model on the random regular graph (RRG). Considering the full probability distribution $\Pi(x,t)$ of a particle to be at some distance $x$ from the initial state at time $t$, we give evidence that $\Pi(x,t)$ spreads sub-diffusively over a range of disorder strengths, wider than a putative non-ergodic phase. We provide a detailed analysis of the propagation of $\Pi(x,t)$ in space-time $(x,t)$ domain, identifying four different regimes. These regimes in $(x,t)$ are determined by the position of a wave-front $X_{\text{front}}(t)$, which moves sub-diffusively to the most distant sites $X_{\text{front}}(t) \sim t^{\beta}$ with an exponent $\beta < 1$. We support our numerical results by a self-consistent semiclassical picture of wavepacket propagation relating the exponent $\beta$ with the relaxation rate of the return probability $\Pi(0,t) \sim e^{-\Gamma t^\beta}$. Importantly, the Anderson model on the RRG can be considered as proxy of the many-body localization transition (MBL) on the Fock space of a generic interacting system. In the final discussion, we outline possible implications of our findings for MBL.

Figures

Figures reproduced from arXiv: 1908.11388 by the authors.

Figure 1
Figure 1. FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figures from the paper (7 more)
Figure 5
Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p004_5.png]
Figure 7
Figure 7. Figure 7: FIG. 7. Left panel: Wavepacket size [PITH_FULL_IMAGE:figures/full_fig_p008_7.png]
Figure 8
Figure 8. Figure 8: FIG. 8. Left panel: Π( [PITH_FULL_IMAGE:figures/full_fig_p009_8.png]
Figure 9
Figure 9. Figure 9: FIG. 9. Left panel: Π( [PITH_FULL_IMAGE:figures/full_fig_p009_9.png]
Figure 11
Figure 11. Figure 11: FIG. 11. The figures show the collapse of the maximum in [PITH_FULL_IMAGE:figures/full_fig_p010_11.png]
Figure 13
Figure 13. Figure 13: The wave-front collapse, the lower panel of [PITH_FULL_IMAGE:figures/full_fig_p010_13.png]
Figure 12
Figure 12. Figure 12: ) the probability distribution is well localized close to the initial state, with 1 . X(0) . 2. As time evolves a wave-front transfers most of the weight of Π(x, t) to the most distant sites x ≈ D where D ' ln(L)/ ln K is the diameter of the graph. It is clear from th…

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