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The spectrum of nonlocal discrete Schrödinger operators with a δpotential
Pacific Journal of Mathematics for Industry volume 6, Article number: 7 (2014)
Abstract
The behaviour of the spectral edges (embedded eigenvalues and resonances) is discussed at the two ends of the continuous spectrum of nonlocal discrete Schrödinger operators with a δpotential. These operators arise by replacing the discrete Laplacian by a strictly increasing C^{1}function of the discrete Laplacian. The dependence of the results on this function and the lattice dimension are explicitly derived. It is found that while in the case of the discrete Schrödinger operator these behaviours are the same no matter which end of the continuous spectrum is considered, an asymmetry occurs for the nonlocal cases. A classification with respect to the spectral edge behaviour is also offered.
Introduction
1.1 Nonlocal discrete Schrödinger operators on lattice
The spectrum of discrete Schrödinger operators has been widely studied for both combinatorial Laplacians and quantum graphs; for some recent summaries see [3,4,7,9,11,16,19] and the references therein. Specifically, eigenvalue behaviours of discrete Schrödinger operators on $l^{2}\left (\mathbb {Z}^{d}\right)$ are discussed in e.g. [2,8,10,12,13,15]. However, for discrete nonlocal Schrödinger operators only few results are known. Typical examples include discrete fractional Schrödinger operators.
In this paper we define generalized discrete Schrödinger operators which include discrete fractional Schrödinger operators and others whose counterparts on $L^{2}\left (\mathbb {R}^{d}\right)$ are currently much studied [5,6,17,18]. In [14] we have introduced a class of generalized Schrödinger operators whose kinetic term is given by so called Bernstein functions of the Laplacian. These operators are nonlocal and via a FeynmanKac representation generate subordinate Brownian motion killed at a rate given by the potential. Their discrete counterparts studied in this paper also have a probabilistic interpretation in that they generate continuous time random walks with jumps on $\mathbb {Z}^{d}$ .
In the present paper we consider a class of Schrödinger operators obtained as a strictly increasing C^{1}function of the discrete Laplacian and a δpotential. This includes, in particular, Bernstein functions (see below) of the discrete Laplacian. In the presence of a δpotential the above probabilistic picture then describes free motion with a “bump” which can be interpreted as an impurity in space. Our aim here is to investigate the spectrum of such operators, specifically, embedded eigenvalues and resonances at the edges of the continuous spectrum.
Let d≥1 and L be the standard discrete Laplacian on $l^{2}\left (\mathbb {Z}^{d}\right)$ defined by
We give a remark on the definition of Laplacians for the reader’s convenience. In the previous paper [15] we defined the discrete Laplacian by $L_{0}\psi (x)=\frac {1}{2d}\sum _{xy=1}\psi (y)$ , and found that the spectrum of L_{0} is the closed interval [−1,1]. In this paper we define the negative Laplacian by $\psi (x)\to \frac {1}{2d} \sum _{xy=1}\left (\psi (y) \psi (x)\right)$ , and flipping the signature, we define the positive Laplacian (1.1). Thus the spectrum of L is positive, i.e., σ(L)=[0,2]. Hence we can consider the Bernstein functions of L. Also, let V(x)=vδ_{x,0} be δpotential with mass v concentrated on x=0, i.e., Vψ(x)=0 for x≠0 and Vψ(x)=vψ(x). Then the operator
is the discrete Schrödinger operator with δpotential. In order to define a nonlocal version of h, we use Fourier transform on $l^{2}\left (\mathbb {Z}^{d}\right)$ . Let $\mathbb {T}^{d} =\left [\pi, \pi \right ]^{d}$ be the ddimensional torus, and set
The scalar product on is denoted by $(\,f,g)=\int _{\mathbb {T}^{d}}\bar {f}(\theta) g(\theta) d\theta $ . The Fourier transform is then defined by for $\theta = \left (\theta _{1},\ldots,\theta _{d}\right)\in \mathbb {T}^{d} $ . Then the discrete Laplacian L transforms as
where
i.e., the right hand side above is a multiplication operator on . In this paper we use a nonlocal discrete Laplacian Ψ(L) defined for a suitable function Ψ by applying Fourier transform.
Definition1.
For a given Ψ∈C^{1}((0,∞)) such that dΨ(x)/dx>0, x∈(0,∞), we define the nonlocal discrete LaplacianΨ(L) by
Also, we call
nonlocal discrete Schrödinger operator with δpotential.
An example of such a function is Ψ(u)=u^{α/2}, 0<α<2, which describes a discrete Laplacian of fractional order α/2. Other specific choices will be given in Example 1 below.
Under Fourier transform (1.7) is mapped into
where .
Since σ(L)=[0,2] and Ψ is strictly increasing, it is immediate that σ(Ψ(L))=Ψ([0,2])=[Ψ(0),Ψ(2)]. In what follows we consider the spectrum of H_{ v } instead of h. Note that the map Φ↦v(Ω,Φ)Ω is a rankone operator, and thus the continuous spectrum of the rankone perturbation H_{ v } of L is [Ψ(0),Ψ(2)], for every $v\in \mathbb {R}$ . See e.g. [1,20] for rankone perturbations.
1.2 Ψ(∗)resonances and Ψ(∗)modes
As it will be seen below, for a sufficiently large value of −v>0 there exists an eigenvalue E_{−}(v) of H_{ v } strictly smaller than Ψ(0). Suppose that E_{−}(v)↑Ψ(0) as v↑v_{0} with some v_{0}≠0. If Ψ(0) is an eigenvalue of $H_{v_{0}}$ , we call the eigenvector associated with Ψ(0) a Ψ(0)mode. If Ψ(0) is not an eigenvalue of $H_{v_{0}}$ , we call it a Ψ(0)resonance. Similarly, for a sufficiently large v>0 it will be seen that there exists an eigenvalue E_{+}(v) strictly larger than Ψ(2). Suppose that E_{+}(v)↓Ψ(2) as v↓v_{2} with some v_{2}≠0. If Ψ(2) is an eigenvalue of $H_{v_{2}}$ , we call the eigenvector associated with Ψ(2) a Ψ(2)mode, and a Ψ(2)resonance whenever Ψ(2) is not an eigenvalue of $H_{v_{2}}$ .
For the discrete Schrödinger operator L+V these modes and resonances were studied in e.g. [15], in particular, their dependence on the dimension d. For d=1,2, there is no 0mode, 2mode, 0resonance or 2resonance, for d=3,4 there are 0 and 2resonances, and for d≥5 there are 0 and 2modes. This shows that the eigenvalue behaviour at both edges (0 and 2) is the same. See Table 1.
As it will be seen below, for the case of the fractional Laplacian we have the remarkable fact that the edge behaviours are in general different at the two sides. See Table 2 and note that $\sigma \left (\sqrt L\right)= \left [0,\sqrt 2\right ]$ .
Eigenvalues
2.1 A criterion for determining the eigenvalues
Consider the eigenvalue equation
or, equivalently,
We introduce the functions:
The following result gives an integral test to spot the eigenvalues of H_{ v }.
Lemma1.
E is an eigenvalue of H_{ v } for a given v if and only if I(E)<∞ and J(E)≠0. Furthermore, if E is an eigenvalue of H_{ v }, then the coupling constant v satisfies
Proof.
To show the necessity part, suppose that E is an eigenvalue and Φ an associated eigenvector. Assuming (Ω,Φ)=0, we have H_{ v }Φ=Ψ(L(θ))Φ=EΦ. Since Ψ(L(θ)) has no point spectrum, this is a contradiction. This gives (Ω,Φ)≠0 and E−Ψ(L(θ))≠0 for almost every $\theta \in \mathbb {T}^{d}$ with
Thus I(E)<∞ follows, and (Ω,Φ)≠0 implies J(E)≠0.
For the sufficiency part, suppose now that I(E)<∞ and J(E)≠0 hold. Define the $L^{2}\left (\mathbb {T}^{d}\right) $ function
with a chosen c. It is straightforward to see that Φ satisfies H_{ v }Φ=EΦ whenever for v
holds. By J(E)≠0 it follows that there exists v such that (2.5) is satisfied, hence E is an eigenvalue of H_{ v }.
In order to investigate Ψ(∗)resonances and Ψ(∗)modes we use Lemma 1 and estimate the two integrals I(E) and J(E) at the two ends E=Ψ(∗) of the interval [Ψ(0),Ψ(2)].
2.2 The location of eigenvalues
Lemma2.
Let $E\in \mathbb {R}\setminus \left [\Psi (0),\Psi (2)\right ]$ . Then there exists v≠0 such that E is an eigenvalue of H_{ v }.
Proof.
In this case it is easily seen that I(E)<∞ and J(E)≠0. Then E is an eigenvalue and v is given by (2.4).
Lemma3.
Let σ_{ p }(H_{ v }) be the point spectrum of H_{ v }. Then σ_{ p }(H_{ v })∩(Ψ(0),Ψ(2))=∅, for every v≠0.
Proof.
Due to monotonicity of Ψ, there is a unique x∈(0,2) such that Ψ(E)=Ψ(x). Thus
with some C>0. Hence
It is directly seen that the right hand side diverges, and thus the lemma follows.
Next consider the cases E=Ψ(2) and E=Ψ(0). For a systematic discussion of the eigenvalue behaviour of H_{ v } we introduce the following concept.
Definition2.
We say that Ψ is of (a,b)type whenever
Lemma4.
Let Ψ be of (a,b)type. Then we have the following behaviour.

(1)
J(E)≠0 for both E=Ψ(0) and E=Ψ(2).

(2)
For E=Ψ(2) we have that I(E)<∞ if and only if d≥1+4a, and J(E)<∞ if and only if d≥1+2a.

(3)
For E=Ψ(0) we have that I(E)<∞ if and only if d≥1+4b, and J(E)<∞ if and only if d≥1+2b.
Proof.
Since Ψ is strictly increasing, the first statement follows directly.
Let Ψ be of (a,b)type. Then we have at θ≈(0,…,0),
and at θ≈(π,…,π),
Hence
and similarly
Thus the lemma follows for E=Ψ(2). For the case of E=Ψ(0) the proof is similar.
From these lemmas we can derive the spectral edge behaviour of H_{ v }. The next theorem is the main result in this paper.
Theorem1.
Assume that Ψ is of (a,b)type. Let
and
The spectral edge behaviour of H_{ v } is as follows.

(1)
Suppose that v>0. Then the following cases occur:

(i)
Let d<1+2b. Then for all v>0 there exists an eigenvalue E>Ψ(2).

(ii)
Let 1+2b≤d<1+4b. Then for v>v_{2} there exists an eigenvalue E>Ψ(2), while for v≤v_{2} there is no eigenvalue.

(iii)
Let 1+4b≤d. Then for v>v_{2} there exists an eigenvalue E>Ψ(2), for v=v_{2} the value E=Ψ(2) is an eigenvalue, while v<v_{2} there is no eigenvalue.

(i)

(2)
Suppose that v<0. Then the following cases occur:

(i)
Let d<1+2a. Then for all v<0 there exists an eigenvalue E<Ψ(0).

(ii)
Let 1+2a≤d<1+4a. Then for v<v_{0} there exists an eigenvalue E<Ψ(0), while for v≥v_{0} there is no eigenvalue.

(iii)
Let 1+4a≤d. Then for v<v_{0} there exists an eigenvalue E<Ψ(0), for v=v_{0} the value E=Ψ(0) is an eigenvalue, while for v>v_{0} there is no eigenvalue.

(i)
Proof.
Consider the case v>0 and let d<1+2b. Then for all E>Ψ(2) we have I(E)<∞ and J(E)≠0. Thus there exists v such that E is an eigenvalue of H_{ v }.
Let 1+2b≤d<1+4b. Then for all E>Ψ(2) we have that I(E)<∞ and J(E)≠0. Thus E is an eigenvalue of H_{ v }. Since J(E)<∞, it follows that E↓Ψ(2) as v↓v_{2}>0. However, E=Ψ(2) is not an eigenvalue since I(E)=∞.
Let d≥1+4b. Then for all E>Ψ(2) we have I(E)<∞ and J(E)≠0. Thus E is an eigenvalue of H_{ v }. Since J(E)<∞, we obtain E↓Ψ(2) as v↓v_{2}>0. Since I(E)<∞, we have that E=Ψ(2) is also an eigenvalue. The cases for v<0 can be dealt with similarly.
Remark1.
Note that in general −v_{0}≠v_{2}.
Remark2.
From the above it is seen that the spectral edge behaviour of H_{ v } depends on the dimension d as well as on the parameters a and b, and the result is different according to which edge is considered. For a summary see Table 3.
It is worthwhile to see the implications more closely for some specific choices of function Ψ.
Example 1.

(1)
Discrete Schrödinger operator: Let Ψ(u)=u. Then Ψ is of (1,1)type and H_{ v }=L+V. See Table 1.

(2)
Fractional discrete Schrödinger operator: Let Ψ(u)=u^{α/2} for 0<α<2. Then Ψ is of (α/2,1)type and H_{ v }=L^{α/2}+V.

(3)
Relativistic fractional discrete Schrödinger operator: Let Ψ(u)=(u+m^{2/α})^{α/2}−m for 0<α<2 and m>0. Then Ψ is of (1,1)type.

(4)
Discrete jumpdiffusion operator: Let Ψ(u)=u+bu^{α/2} with 0<α<2. Then Ψ is of (α/2,1)type.

(5)
Rotationally symmetric geometric discrete αstable operator: Let Ψ(u)= log (1+u^{α/2}) for 0<α<2. Then Ψ is of (α/2,1)type.

(6)
Higher order discrete Laplacian: Let Ψ(u)=u^{β} for β>1. Then Ψ is of (β,1)type.

(7)
Bernstein functions of the discrete Laplacian: Let Ψ be a Bernstein function with vanishing right limits, i.e., $\Psi : \mathbb {R}^{+} \to \mathbb {R}^{+}$ which can be represented in the form $\Psi (u)=bu+\int _{0}^{\infty } \left (1e^{uy}\right)\nu (dy)$ , where b≥0 and ν is a Lévy measure with mass on (0,∞) satisfying $\int _{0}^{\infty } (1\wedge y) \nu (dy) < \infty $ . Then it follows that $\Psi ^{\prime }(2) =b+\int _{0}^{\infty } y e^{2y} \nu (dy) \not =0$ . Furthermore, since Ψ is concave, we have a=α/2 with some α≥2. Hence Ψ is of (α/2,1)type with some 0≤α≤2. Note that the first five examples above are specific cases of Bernstein functions.
A classification of spectral edge behaviour
The functions Ψ of the discrete Laplacian can be classified according to the behaviour of the eigenvalues at the two ends of the interval [ Ψ(0),Ψ(2)].
Definition3.
We call Ψnormal type if Ψ is (1,1)type, and fractional type if Ψ is (α/2,1)type with 0<α<2.
The two types show qualitatively different behaviour and we discuss them separately.
3.1 Normal type
Let Ψ be of normal type. In this case the spectral edge behaviour is the same as that of the discrete Schrödinger operator L+V. The following result has been obtained in [15].
Proposition1.
Let Ψ be normal type. We have the following cases.

(1)
Let d=1 or 2. For every v>0 there exists an eigenvalue E>Ψ(2), and for every v<0 an eigenvalue E<Ψ(0).

(2)
Let d=3 or 4. If v>0, then there exists v_{2}>0 such that for all v>v_{2} an eigenvalue E>Ψ(2) exists, and for v≤v_{2} no eigenvalue exists. If v<0, then there exists v_{0}<0 such that for all v<v_{0} an eigenvalue E<Ψ(0) exists, and for v<v_{0} no eigenvalue exists.

(3)
Let d≥5. If v>0, then there exists v_{2}>0 such that for all v>v_{2} an eigenvalue E>Ψ(2) exists, for v=v_{2} the value E=Ψ(2) is an eigenvalue, and for v<v_{2} no eigenvalue exists. If v<0, then there exists v_{0}<0 such that for all v<v_{0} an eigenvalue E<Ψ(0) exists, for v=v_{0} the value E=Ψ(0) is an eigenvalue, and for v>v_{0} no eigenvalue exists.
Thus the spectral edge behaviour for positive and negative v is qualitatively the same, and the details only depend on the dimension d.
3.2 Fractional type
In the fractional type case we have the following spectral edge behaviour.
Theorem2.
Let Ψ be of fractional type. The following cases occur.

(1)
If v>0, then the spectral edge behaviour is the same as for normal type Ψ with v>0.

(2)
If v<0, then we have the following cases:

(i)
Let d<1+α. Then for every v<0 there exists an eigenvalue E<0.

(ii)
Let 1+α≤d<1+2α. There exists v_{0}<0 such that for all v<v_{0} an eigenvalue E<0 exists, while for v≤v_{0} no eigenvalue exists.

(iii)
Let d≥1+2α. There exists v_{0}<0 such that for all v<v_{0} an eigenvalue E<0 exists, for v=v_{0} the value E=0 is an eigenvalue, and for v>v_{0} no eigenvalue exists.

(i)
In the fractional case it is seen that the edge behaviours for positive and negative v are in general different from each other, in contrast with the normal type case.
3.2.1 The case of α=1
For α=1 the spectral edge behaviour of $H_{v}=\sqrt L+V$ is displayed for dimensions d=1,…,4 and d≥5 in Table 2. We have displayed the specific situations in Figures 1, 2, 3 and 4 below, where ⊕ denotes a resonance, ∙ an eigenvalue, and × denotes a value which is not an eigenvalue. For dimension d=1,5 the edge behaviours at 0 and $\sqrt 2$ are symmetric. See Figures 1 and 4. On the other hand for dimensions d=2,3,4 the edge behaviours at 0 and $\sqrt 2$ are again different. See Figures 2 and 3.
3.2.2 Massless and massive cases
Consider the Bernstein function $\Psi (u)=\sqrt {u+m^{2}}m$ with m≥0. This allows to define the relativistic discrete Schrödinger operator $\sqrt {L+m^{2}}m+V$ . Then it follows that Ψ(u) is (1,1)type for m>0, and (1/2,1)type for m=0. In particular, the edge behaviours of $\sqrt {L+m^{2}}m+V$ for m>0 are symmetric, and the edge behaviours of $\sqrt {L}+V$ and $\sqrt {L+m^{2}}m+V$ are different. More generally, consider the Bernstein function Ψ(u)=(u+m^{2/α})^{α/2}−m, with 0<α<2 and m≥0. This defines the rotationally symmetric relativistic αstable operator (L+m^{2/α})^{α/2}−m. We conclude that Ψ(u) is of (1,1)type for m>0 but of (α/2,1)type for m=0. Thus the edge behaviours of (L+m^{2/α})^{α/2}−m+V and L^{α/2}+V are different.
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Acknowledgements
FH is financially supported by GrantinAid for Science Research (B) 23340032 from JSPS. JL thanks Institut MittagLeffler, Stockholm, for the opportunity to organise the researchinpeace workshop “LiebThirringtype bounds for a class of Feller processes perturbed by a potential" during the period 25 July – 9 August 2013. JL also thanks London Mathematical Society for a travel grant to this workshop. FH thanks the invitation to this workshop, and we both express our gratitude to IML for the kind hospitality and inspiring work environment, where most of this paper has been prepared.
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Keywords
 Discrete Schrödinger operator
 Nonlocal Schrödinger operator
 Modes and resonances
 Rankone perturbation
 Bernstein function