In Exact dynamical decay rate for the almost Mathieu operator by Jitomirskaya et al. [Math. Res. Lett. 27(3), 789–808 (2020)], the authors analysed the dynamical decay in expectation for the supercritical almost-Mathieu operator in function of the coupling parameter, showing that it is equal to the Lyapunov exponent of its transfer matrix cocycle, and asked whether the same is true for the 1d Anderson model. We show that this is never true for bounded potentials when the disorder parameter is sufficiently large.
I. INTRODUCTION
While the proof of Ref. 5 employs multi-scale analysis, single-scale proofs have recently been found by Bucaj et al.,4 Gorodetski–Kleptsyn,9 and Jitomirskaya–Zhu.11 Generalisations to models with off-diagonal disorder and to matrix-valued potentials are studied in Refs. 13 and 16.
Ge and Zhao built on the work11 and proved the following:
In Sec. II we give another, arguably, simpler, proof of this result, adopting an argument from Ref. 6.
Jitomirskaya et al. 10 studied the validity of (6) in the almost-periodic setting, namely, for the supercritical almost-Mathieu operator with Diophantine frequency, and showed that in that setting can be taken to be equal to γinf. They asked whether the same is true for the Anderson model. We show that this is not the case. A first counterexample comes from the Anderson–Bernoulli model:
For a > 0, consider the operator Ha = H0 + aV with Vx being a bounded random variable having an atom at 0. Then is bounded from above uniformly in a.
In particular, if Vx is a Bernoulli random variable with parameter p, by a result of Martinelli and Micheli,14 γinf ⩾ c log a for sufficiently large a. Therefore, by the above theorem, for a large enough.
Furthermore, the above theorem remains true for any bounded random potential satisfying mild conditions, at sufficiently high disorder:
Let be a nondeterministic, bounded, i.i.d. random potential, and let Ha ≔ H0 + aV.
II. PROOF OF THEOREM 1.1
See Ref. 13, Proposition 2.1 for this formulation (in the more general case of matrix potentials) and Ref. 11, Theorem 4.1, for a similar statement in the pure one-dimensional case.
III. PROOF OF THEOREM 1.2
Having set c = c2δ and 2K + 1 = ⌈100 c2/c1⌉, we obtain (8).
IV. PROOF OF THEOREM 1.3: LOGARITHMIC DIVERGENCE FOR GENERAL POTENTIALS
A version of the Martinelli–Micheli bound the 1d Anderson model with absolutely continuous, bounded potential has been proven in 1983 by Avron et al. in Ref. 3.
We strongly believe that the main result of this section (and thus Theorem 1.3 as a whole) can be extended to the most general setting for which 1-d localisation has been proven (nondeterministic potential with any finite fractional moment). However, generalising (13) to the case where one only has a generic fractional moment appears to be nasty.
An alternative to this approach could be extending the proof of the logarithmic divergence for the Anderson–Bernoulli model in Ref. 14 to the general case; however, even if this seems to be doable and should not present major technical difficulties, additional estimates would be needed to make Martinelli’s and Micheli’s already six page long proof work for generic potentials, and many formulas would get much longer and nastier.
In conclusion, the length of the present paper would likely get doubled by such an attempt, therefore we avoid it to keep the paper short and more readable while keeping the result reasonably general.
V. PROOF OF THEOREM 1.3: GREEN FUNCTION ESTIMATES
Since the above argument uses crucially the fact that a Bernoulli random variable is 0 with positive probability, one might suspect that the presence of an atom at zero is required for the annealed dynamical decay to be non-Lyapunov. However, in this section we will use a simple trick to eliminate the atom at 0. The trick relies on the observation that it is possible to decompose any dilated random variable aX having an atom as a sum of two (not necessarily independent) random variables, one of which is bounded in a, and the other has basically the same distribution as aX with the difference that the atom has been subtracted some mass. If we subtract in this way mass from the atom at a sufficient rate, and control the error given by the bounded addend, then we can show that the growth of the annealed decay rate in a is much slower than the logarithmic lower bound prescribed by the results of Avron–Craig–Simon and Shubin–Vakilian–Wolff.
In order to exploit the case of a potential with an atom at 0, we will make use of the following observation.
This observation basically asserts that we can remove (or, by extension, subtract mass to) an atom from the distribution of a random variable at the cost of adding another (dependent) random variable uniformly bounded in the coupling.
It follows by simply observing that if has the same distribution of X and is chosen to take the same values as (so that would retain its usual law and its atom at , but becoming totally dependent on ), then .
Without loss of generality, we can take R = δ/10.
In fact, δ is by construction always positive and we can always multiply the potential by any finite constant and incorporate such constant into the disorder parameter a.
We now use these two observations to prove the general result. Take V such that supp(V) ⊆ [−R, R] and set R = δ/10. Apply Observation 5.1 to the potential V with ϵ = ϵ(a) ≫ a−β for all β > 0, and decompose , where , and is a random variable having the same distribution as V except for having an atom at 0 of mass ϵ(a) (with the necessary renormalisation). Then , where .
Notice that this time we chose δ instead of iδ as a spectral parameter. The reason for this choice is that we need to use the negativity of the shifted Laplacian to compare its Green’s function to that of the (negative) shifted operator.
ACKNOWLEDGMENTS
I am deeply indebted to my former Ph.D. advisor, Sasha Sodin, for suggesting the topic of this paper and for giving major contributions to its development. I believe he should have been a co-author of this paper, but for reasons I do not understand he asked me to erase his name from it.
I am also grateful to Alexander Elgart for many useful comments on a preliminary version of this paper, and to the anonymous referee for useful suggestions and for pointing out a flaw in the original argument.
This work has been supported by the Grant No. EPSRC EP∖T004290∖1.
This work was started when the author was a Ph.D. student at Roma Tre University.
AUTHOR DECLARATIONS
Conflict of Interest
The author has no conflicts to disclose.
Author Contributions
Davide Macera: Conceptualization (equal); Investigation (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal).
DATA AVAILABILITY
Data sharing is not applicable to this article as no new data were created or analyzed in this study.