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.

Consider the one-dimensional Anderson model, i.e., the operator H acting on a dense subset of 2(Z) via
(1)
where Vx are i.i.d. random variables. We assume that the distribution of V0 is bounded and not concentrated at one point (in most of the discussion below, the first assumption can be relaxed to the existence of a finite fractional moment E|V0|η<). Carmona–Klein–Martinelli5 showed that under these assumptions H exhibits Anderson localisation, i.e., almost surely H has pure point spectrum, and moreover
(2)
where E=E[H] is the collection of eigenpairs of H (the spectrum is almost surely simple, so |ψ| is well-defined), and γ(λ) is the Lyapunov exponent of H at energy λ. Under more restrictive assumptions on H, the pure point nature of the spectrum was first proved by Goldsheid–Molchanov–Pastur8 and by Kunz–Souillard;12 the exponential decay of the eigenfunctions was first established by Molchanov.15 

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.

A stronger notion of Anderson localisation involves the notion of eigenfunction correlator, introduced by Aizenman.1 Denote
where the supremum is taken over Borel functions. If H has pure point spectrum, the correlator takes the form
(3)
Then there exists γ > 0 such that for any x,
(4)
In fact, in the current setting (4) holds with γ = γinf, where
(5)
and σ(H) is the spectrum of H (a deterministic set)—see Ref. 11. This strong form of (4) implies (2), as well as dynamical localisation, decay of the Fermi projection as well as other properties of relevance in quantum dynamics.

Ge and Zhao built on the work11 and proved the following:

Theorem 1.1
(Ge–Zhao). For the operator H of (1) with V0 bounded and not concentrated at one point, one has, for any xZ,
(6)

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 γE 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:

Theorem 1.2.

For a > 0, consider the operator Ha = H0 + aV with Vx being a bounded random variable having an atom at 0. Then γE 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  γinfc log a for sufficiently large a. Therefore, by the above theorem, γE(Ha)γI(Ha) for a large enough.

Furthermore, the above theorem remains true for any bounded random potential satisfying mild conditions, at sufficiently high disorder:

Theorem 1.3.

Let V={Vi}iZ be a nondeterministic, bounded, i.i.d. random potential, and let HaH0 + aV.

Then, for any a large enough

For a,bZ, denote by H[a,b] the restriction of H to [a, b] (with Dirichlet boundary conditions), and let GE[H[a,b]]=(H[a,b]E)1. Let τ > 0, ER and N ⩾ 1. A site xZ is called (τ, E, N)-nonresonant [x ∉ Res(τ, E, N)] if
Otherwise, x is called (τ, E, N)-resonant [x ∈ Res(τ, E, N)]. The proof of the theorem uses the following.

Claim 1.
Assume that V0 is bounded and not concentrated at one point. Then for any τ > 0 there exist C, c > 0 such that

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.

Next, we need a representation for the eigenfunction correlator as a singular integral [see (7.4) at p. 102 of Ref. 2]:
(7)
Having these two ingredients, we argue as follows. Without loss of generality we can assume that x = 0. Set τ=12minEσ(H)γ(E) and N=y10, and consider the event
According to Claim 1, P(R)CecN. On the complement Ω\R, we have R=AB, where
For EA,
whence
and an analogous bound can be deduced for B.
Thus
Finally,
Thus, γEmin(c,τ)>0.

Remark.
This proof can be extended to quasi-one-dimensional operator, such as the Anderson model on the strip of width W or the more general model studied in Ref. 13. A slightly weaker version of (7) is still true in this case (see Ref. 6):
and the argument above follows with minor modifications.

Let K > 0 be a large numerical constant (independent of any parameters), to be specified later. For x > 0, consider the event
We shall prove the following: for any ϵ > 0, one has on ΩK,x for sufficiently large x:
(8)
Since P(ΩK,x)=(1p)2Kx+1, this this would imply that
(9)
as claimed.
We now turn to the proof of (8). Since the argument is uniform in a, we will use H for Ha. Observe that for any δ > 0,
(10)
In fact,
Let T = H0 be the free Laplacian [obtained by setting Vx ≡ 0 in (1)], and let TK,x be the restriction of T to the finite volume [−Kx, (K + 1)x]. Then, by applying the resolvent identity and the reverse triangle inequality twice, we get
By the Combes–Thomas estimate (Ref. 2, Theorem 10.5), we deduce that for δ ∈ (0, 1),
Now,
(11)
Indeed, g(x) = G[T](0, x) is by definition the square-summable solution to the equation
Plugging in the ansatz g(x) = αeξ|x|, we find that this is indeed a solution provided that
hence |g(x)||α|e(Rξ)|x|, where Rξc2δ for δ > 0 small enough, as
in a neighbourhood of 0.

Having set c = c2δ and 2K + 1 = ⌈100 c2/c1⌉, we obtain (8).

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.

Theorem 4.1
(Avron et al.3). Let Ha = H0 + aV be a random Schrödinger operator where V is a bounded random potential with absolutely continuous density. Then the Lyapunov exponent γa of Ha is such that
where K is a finite constant.

We will adapt the proof in Ref. 3 to any potential having finite first moment (not necessarily absolutely continuous).7 Avron et al.’s proof relies on the Thouless’ formula for the Lyapunov exponent of a Schrödinger operator H, stating that
(12)
where κH(E′) denotes the integrated density of states of the operator H. They proceed then to bound the negative part of the logarithm in 12 using the Wegner estimate: If H = H0 + V is a random Schrödinger operator with i.i.d. potential, then
which is unfortunately proven true only when the distribution ρV of the potential is absolutely continuous.
Fortunately, Shubin et al. proved in Ref. 17 a slightly weaker bound for the IDS of a random Schrödinger operator whose potential satisfies the conditions of Theorem 1.3: if E[|V0|]< (condition that we have automatically since the distribution of V0 is bounded), then
(13)
for some α ∈ (0, 1) and some constant C > 0. This bound is sufficient to let the Avron–Craig–Simon argument work in the present generality.
By applying the Thouless formula to Ha and splitting the logarithm into its positive and negative parts, we get that
We claim that
(14)
for some C > 0 uniform in a, and that
(15)
for some positive constant c.
The first bound (14) is proven by using inequality (13):

Remark.

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.

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.

Proof.

In order to exploit the case of a potential with an atom at 0, we will make use of the following observation.

Observation 5.1.
Let X be a bounded random variable, and let x̄supp(X)[R,R]. Suppose that X is absolutely continuous in a neighborhood of x̄ and denote by X̃ϵ the random variable having the same density as X, except for the fact that it has an atom at x̄ of mass ϵ, suitably renormalised. Then there exists a bounded random variable η̃ (not necessarily independent on X) such that
for some a′ > 0.

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 η̃ has the same distribution of X and X̃ϵ is chosen to take the same values as η̃ (so that X̃ϵ would retain its usual law and its atom at x̄, but becoming totally dependent on η̃), then aX̃ϵ+η̃=d(a+1)X.

Observation 5.2.

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) ≫ aβ for all β > 0, and decompose aV=aṼϵ(a)+η̃, where η̃=dV, and Ṽϵ(a) 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 Ha=TR+aṼϵ(a), where TR=T0+η̃.

Thus again, if Q(0, x) ⩾ ec|x| we get, as in (9),
(16)
Furthermore, we call TK,xR the restriction of TR to the box [−Kx, (K + 1)x], analogously as before. We will shift TR by −2 · 1 so that the spectrum of the resulting operator lies below δ/2. A double resolvent expansion analogue to the one performed in the Proof of Theorem 1.2 and the Combes-Thomas bound yield
on the event

Remark.

Notice that this time we chose δ instead of 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.

Eventually, the only thing left to us to show is that
By writing down the Neumann series for Gδ[T − 2 · 1], we get the following inequalities:
We can compute Gδ/2[T0 − 2 · 1](0, x) explicitly via the same method used in the proof of 1.2. In this case, we get that
In particular, when δ is small, Gδ/2[T0 − 2 · 1](x, y) decays exponentially with rate of order δ and
Setting, again, K large enough so that 2K+1=100c̃/c3, and setting c=c̃δ2, we finally conclude that
This, combined with (16) and the Avron–Craig–Simon bound for general bounded potentials proven in Paragraph 4, implies the thesis.□

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.

The author has no conflicts to disclose.

Davide Macera: Conceptualization (equal); Investigation (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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