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Ann. I. H. Poincaré – AN 31 (2014) 639–654

www.elsevier.com/locate/anihpc

On linear instability of solitary waves for the nonlinear Dirac equation

Andrew Comech

a,b

, Meijiao Guan

c

, Stephen Gustafson

c,,1

aTexas A&M University, College Station, TX 77843, USA bInstitute for Information Transmission Problems, Moscow 101447, Russia

cUniversity of British Columbia, Vancouver V6T 1Z2, Canada

Received 5 September 2012; received in revised form 29 April 2013; accepted 3 June 2013 Available online 11 June 2013

Abstract

We consider the nonlinear Dirac equation, also known as the Soler model:

i∂tψ= −·ψ+mβψψβψk

βψ, m >0, ψ (x, t)∈CN, x∈Rn, k∈N.

We study the point spectrum of linearizations at small amplitude solitary waves in the limitωm, proving that ifk >2/n, then one positive and one negative eigenvalue are present in the spectrum of the linearizations at these solitary waves withωsufficiently close tom, so that these solitary waves are linearly unstable. The approach is based on applying the Rayleigh–Schrödinger perturbation theory to the nonrelativistic limit of the equation. The results are in formal agreement with the Vakhitov–Kolokolov stability criterion.

©2013 Elsevier Masson SAS. All rights reserved.

Résumé

Nous considérons l’équation de Dirac non linéaire, aussi connue comme modèle de Soler. Nous étudions le spectre ponctuel des linéarisations autour d’ondes solitaires de petite amplitude dans la limiteωm, et montrons que sik >2/nune valeur propre positive et une négative sont présentes dans le spectre des linéarisations autour de ces ondes solitaires lorsqueωest suffisamment proche dem, ce qui entraîne que ces ondes solitaires sont linéairement instables. L’approche est basée sur l’application de la théorie des perturbations de Rayleigh–Schrödinger à la limite non relativiste de l’équation. Les résultats sont en accord formel avec le critère de stabilité de Vakhitov–Kolokolov.

©2013 Elsevier Masson SAS. All rights reserved.

1. Introduction

A natural simplification of the Dirac–Maxwell system[23]is the nonlinear Dirac equation, such as the massive Thirring model[41]with vector–vector self-interaction and the Soler model[36]with scalar–scalar self-interaction

* Corresponding author.

E-mail addresses:comech@math.tamu.edu(A. Comech),meiji.g@stat.ubc.ca(M. Guan),gustaf@math.ubc.ca(S. Gustafson).

1 Research supported by an NSERC Discovery Grant.

0294-1449/$ – see front matter ©2013 Elsevier Masson SAS. All rights reserved.

http://dx.doi.org/10.1016/j.anihpc.2013.06.001

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(known in dimensionn=1 as the massive Gross–Neveu model[21,27]). These models with self-interaction of local type have been receiving a lot of attention in particle physics (see e.g.[34]), as well as in the theory of Bose–Einstein condensates[26,29].

There is an enormous body of research devoted to the nonlinear Dirac equation, which we cannot cover compre- hensively here. The existence of standing waves in the nonlinear Dirac equation was studied in[36,17,28,20]. The question of stability of solitary waves is of utmost importance: perturbations ensure that we only ever encounter stable configurations. Recent attempts at asymptotic stability of solitary waves in the nonlinear Dirac equation[8,9,32,5,16]

rely on the fundamental question ofspectral stability:

Consider the Ansatzψ (x, t )=ω(x)+ρ(x, t))eiωt, withφω(x)eiωt a solitary wave solution. Let∂tρ=Aωρ be the linearized equation forρ. DoesAω have eigenvalues in the right half-plane?

Definition 1.1.Ifσ (Aω)iR, we say that the solitary waveφωeiωt is spectrally stable. Otherwise, we say that the solitary wave is linearly unstable.

Remark 1.2.Let us note that the “spectral stability”σ (Aω)iRdoes not guarantee stability. One of the possibilities which may still lead to instability is the presence of eigenvalues of higher algebraic multiplicity. This occurs e.g. at λ=0 in the casedQ(ω)/dω=0 (whereQ(ω)is the charge ofφω– see below) at a particular value ofω, leading to instability of the corresponding solitary wave; see[13]. In[15], to reflect this situation in the context of the nonlinear Schrödinger equation, linear instability is defined as either the presence of eigenvalues with positive real part or the presence of particular eigenvalues with higher algebraic multiplicity.

There is a very clear picture of the spectral stability for nonlinear Schrödinger and Klein–Gordon equations[43,35, 44,37]and general results for abstract Hamiltonian systems withU(1)symmetry[24], which stimulated many attempts at spectral stability in the nonlinear Dirac context. We mention the numerical simulations[1]and the analysis of the energy minimization under charge-preserving dilations and similar transformations[7,2,39,11]. In spite of this, the question of spectral stability of solitary waves of nonlinear Dirac equation is still completely open. Numerical results [3]show that in the 1D Soler model (cubic nonlinearity) all solitary waves are spectrally stable. We also mention the related numerical results in[14,10].

According to[43], ifφω(x)eiωt is a family of solitary wave solutions to the nonlinear Schrödinger equation iu˙= − 1

2muf

|u|2

u, u(x, t )∈C, x∈Rn, n1,

wheref is smooth and real-valued, and ifφω have no nodes (such solitary waves are calledground states), then the linearization at the solitary wave corresponding to a particular value ofωhas a positive eigenvalue if and only if at this value ofωone hasdQ(ω)/dω >0, whereQ(ω)= φω2L2 is the charge (ormass) of the solitary wave. The opposite condition,

d

dωQ(ω) <0, (1.1)

is called the Vakhitov–Kolokolov stability criterion; it ensures the absence of eigenvalues with positive real part. In the case of the nonlinear Dirac equation, the condition(1.1)gives a less definite answer about the spectral stability. All we know is that at the value ofωwhereωQ(ω)vanishes, withQ(ω)being the charge of the solitary waveφωeiωt, two eigenvalues of the linearized equation collide atλ=0, but we do not know where these eigenvalues are located whenωQ(ω)=0 (see e.g.[12]).

Yet, it is natural to expect that the condition(1.1)remains meaningful in the nonrelativistic limit, as it was suggested in[11]. While it is a common practice to obtain solitary wave solutions for relativistic equations as bifurcations from the solitary waves to the equation corresponding to the nonrelativistic limit (see e.g. the review[19]), we show that the spectrum of the linearization at a solitary wave could also be learned from the nonrelativistic limit. More precisely, we develop the idea that the family of real eigenvalues of the linearization at a solitary wave of the nonlinear Dirac equation bifurcating fromλ=0 is a deformed family of eigenvalues of the linearization of the corresponding nonlinear Schrödinger equation. As a result, we prove that if the Vakhitov–Kolokolov stability criterion[43]guarantees linear instability for the NLS, then the same conclusion also holds for solitary waves withωmin the nonlinear Dirac

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equation. Let us mention that our results only apply to the solitary wave solutions which we obtain from the solitary waves of the nonlinear Schrödinger equation in the nonrelativistic limit of the nonlinear Dirac.

The model and the main results are described in Section 2. The necessary constructions in the context of the nonlinear Schrödinger equation are presented in Section3. The existence and asymptotics of solitary waves of the nonlinear Dirac equation is covered in Section4. The main result (Theorem 2.3) follows fromLemma 4.2(existence of solitary wave solutions and their asymptotics) andProposition 5.2(presence of a positive eigenvalue in the spectrum of the linearized operator), which we prove using the Rayleigh–Schrödinger perturbation theory.

2. Main result

We consider the nonlinear Dirac equation i∂tψ= −·ψ+mβψf

ψβψ

βψ, ψ (x, t)∈CN, x∈Rn, (2.1)

withm >0 andψ being the Hermitian conjugate ofψ. We assume that the nonlinearityf (s) is smooth and real- valued, and that

f (0)=0. (2.2)

Above,α·=n

j=1αj∂x

j, and the Hermitian matricesαj andβ are chosen so that (·+βm)2=

+m2 IN,

whereINis theN×N unit matrix. That is,αj andβare to satisfy

αjαk+αkαj=2δj kIN, β2=IN; αjβ+βαj=0. (2.3)

The generalized massive Gross–Neveu model (the scalar–scalar case withk >0 in the terminology of[11]) corre- sponds to the nonlinearityf (s)= |s|k.

According to the Dirac–Pauli theorem (cf.[18,42,30]and[40, Lemma 2.25]), the particular choice of the matrices αjandβdoes not matter:

Lemma 2.1(Dirac–Pauli theorem). Letn∈N. For any sets of Dirac matricesαj andα˜j˜of the same dimen- sionN, with1jn, there is a unitary matrixSsuch that

˜

αj=S1αjS, 1jn, β˜=S1βS (2.4)

ifnis odd, and such that

˜

αj=σ S1α˜jS, 1jn, β˜=σ S1βS, σ= ±1, (2.5)

ifnis even.

For more details, see[4].

Lemma 2.1allows one, by a simple change of variable, to transform the nonlinear Dirac equation, changing the set of Dirac matrices. Thus, when studying the spectral stability, we can choose the Dirac matrices at our convenience.

We use the standard Pauli matrices, σ1=

0 1 1 0

, σ2=

0 −i i 0

, σ3= 1 0

0 −1

, (2.6)

to make the following choice:

n=1: N=2, α= −σ2; (2.7)

n=2: N=2, αj=σj, 1j2; (2.8)

n=3: N=4, αj=

0 σj

σj 0

, 1j3; (2.9)

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and in all these cases we choose β=

IN/2 0 0 −IN/2

. (2.10)

Remark 2.2.If nis even, Lemma 2.1may only allow us to transform Eq.(2.1)with a particular set of the Dirac matrices to the set of the Dirac matrices as in(2.7), (2.8), (2.9), and (2.10), but with the opposite signs (this corresponds toσ = −1 in(2.5)). In this case, the signs ofαj are flipped by taking the spatial reflections, whileβ being opposite to(2.10)corresponds to considering the nonrelativistic limitω→ −m, with appropriate changes toTheorem 2.3.

For a large class of nonlinearitiesf (s), there are solitary wave solutions of the form ψ (x, t )=φω(x)eiωt, φωH1

R,CN

, |ω|< m. (2.11)

In dimensionn=1, one can take φω(x)=

v(x, ω) u(x, ω)

, (2.12)

withv(x, ω)positive and even andu(x, ω)real-valued and odd; under these conditions, the solitary waveφω(x)is unique (see Section4.1for details).

Forn=2 andn=3, respectively,

φω(x)=

v(r, ω) ieu(r, ω)

, φω(x)=

⎢⎢

v(r, ω) 1

0

iu(r, ω)

cosθ esinθ

⎥⎥

, (2.13)

wherev(r, ω)andu(r, ω)are real-valued, radially symmetric functions;(r, φ)are standard polar coordinates inR2, and (r, θ, φ)are standard spherical coordinates inR3. The existence of solitary waves of this form is proved, for example, in [17]. The particular solutions we consider here, however, are those constructed forω close to m, as outlined in Section4.2, from the nonrelativistic limitωm.

Due to theU(1)-invariance, for solutions to(2.1)the value of the charge functional Q(ψ )=

Rn

ψ (x, t )2dx

is formally conserved. For brevity, we also denote byQ(ω)the charge of the solitary waveφω(x)eiωt: Q(ω)=

Rn

φω(x)2dx. (2.14)

We are interested in the spectrum of linearization of the nonlinear Dirac equation(2.1)at a solitary wave solu- tion(2.11).

Theorem 2.3.Letn3. Assume thatf (s)=sk, wherek∈Nsatisfiesk >2/n(andk <2forn=3). Then there is ω1< msuch that the solitary wave solutionsφωeiωt to(2.1)described above, are linearly unstable forω1, m).

More precisely, letAω be the linearization of the nonlinear Dirac equation at a solitary waveφω(x)eiωt. Then for ω1, m)there are eigenvalues

±λωσp(Aω), λω>0, λω=O(mω).

SeeFig. 1.

Remark 2.4.The existence of an eigenvalue with positive real part in the linearization at a particular solitary wave generally implies the dynamic, ornonlinear, instability of this wave. We expect that this could be proved following the argument of[22]given in the context of the nonlinear Schrödinger equation.

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Fig. 1. Main result: The point spectrum of the linearization of the nonlinear Dirac equation inRn,n3, withf (s)=sk,k >2n, at a solitary wave withωmcontains two nonzero real eigenvalues,±λω, withλω=O(mω). SeeTheorem 2.3. Also plotted on this picture is the essential spectrum, with the edges atλ= ±i(mω)and with the embedded threshold points (branch points of the dispersion relation) atλ= ±i(m+ω).

Remark 2.5.Theorem 2.3extends easily to nonlinearitiesfC2(R)of the form f (s)=ask+O

sk+1

, a >0.

Remark 2.6.InTheorem 2.3, the valueω1< mcould be taken to be the smallest point such that there is aC1family of solitary wavesωφω forω1, m)and moreoverωQ(ω)does not vanish on1, m). Indeed, by[12], the positive and negative eigenvalues remain trapped on the real axis, not being able to collide atλ=0 forω1, m)as long asωQ(ω)does not vanish on this interval. These eigenvalues cannot leave into the complex plane, either, since they are simple, while the spectrum of the operator is symmetric with respect to the real and imaginary axes.

Remark 2.7.If the family of solitary wavesωφω is defined forω0, m)withω0< ω1andωQ(ω)vanishes atω1, we do not know what happens forωω1. IfωQ(ω)changes the sign atω1, then, generically, either the pair of real eigenvalues, having collided atλ=0 whenω=ω1, turn into a pair of purely imaginary eigenvalues (linear instability disappears), or instead two purely imaginary eigenvalues, having met atλ=0, turn into the second pair of real eigenvalues (linear instability persists). IfωQ(ω)vanishes atω1but does not change the sign, then generically the eigenvalues touch and separate again, remaining onR+andR. More details are in[12].

Remark 2.8. Theorem 2.3 is in formal agreement with the Vakhitov–Kolokolov stability criterion[43], since for ωmone hasQ(ω) >0 fork >2n. Let us mention that the sign of the stability criterion,Q(ω) <0, differs from [43]because of their writing the solitary waves in the formϕ(x)e+iωt.

Remark 2.9.It has been shown that in the 1D case withk=1, the small amplitude solitary waves are spectrally stable[4].

Remark 2.10.We expect that in the 1D case withk=2 (“quintic nonlinearity”) the small solitary wave solutions of the nonlinear Dirac equation in 1D are spectrally stable. For the corresponding nonlinear Schrödinger equation (quintic nonlinearity in 1D), the charge is constant, thus the zero eigenvalue of a linearized operator is always of higher algebraic multiplicity. For the Dirac equation, this degeneracy is “resolved”: using the expression for the charge Q(ω)from[11, Section 2A], one can see that the charge is now a decaying function forωm(with nonzero limit as ωm), suggesting that there are two purely imaginary eigenvalues±λωin the spectrum ofAω, withλω=o(mω), but no eigenvalues with nonzero real part.

Remark 2.11.Let us notice that in the 3D case for the cubic nonlinearityf (s)=s (this is the original Soler model from[36]), based on the numerical evidence from[36,1], the chargeQ(ω)has a local minimum atω1≈0.936m, suggesting that the solitary waves withω1< ω <1 are linearly unstable, but then atω=ω1 the real eigenvalues

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collide atλ=0, and there are no nonzero real eigenvalues in the spectrum forωω1. Incidentally, this agrees with the “dilation-stability” results of[39](one studies whether the energy is minimized or not under the charge-preserving dilation transformations).

Remark 2.12.We cannot rule out the possibility that the eigenvalues with nonzero real part could bifurcate directly from the imaginary axis into the complex plane. Such a mechanism is absent for the nonlinear Schrödinger equation linearized at a ground state, for which the point eigenvalues always remain on the real or imaginary axes. At present, though, we do not have examples of such bifurcations in the context of nonlinear Dirac equation.

Remark 2.13.Forn=3, we only consider the casek=1, and we do not consider dimensionsn >3. This is because of the fact that the equation−u+u= |u|2kuinRnhas nontrivial solutions inH1(Rn)if and only if 0< k <2/(n−2), as follows from the virial identities; see[31]and[6, Example 1]. This is why our method does not allow us to construct solitary wave solutions to the nonlinear Dirac equation(2.1)inRn,n3, withk2/(n−2).

Remark 2.14. We consider here only integer powers k (and only dimensionsn=1, 2, and 3), being physically the most important cases. Mathematically, this merely avoids some minor technical complications associated with a non-smooth nonlinearity, and the instability argument can be extended to handle the corresponding equation with f (s)= |s|k, in any dimensionn1, under the conditionk >2/n. (The restrictionk <2/(n−2)is needed so that there are nontrivial solitary waves for the NLS; see the previous remark.)

3. Nonlinear Schrödinger and its solitary waves

We are going to use the fact that the nonrelativistic limit of the nonlinear Dirac equation yields the nonlinear Schrödinger equation,

i∂tψ= − 1

2mψ− |ψ|2kψ, ψ (x, t )∈C, x∈Rn, k >0, n∈N. (3.1) 3.1. Solitary waves

The properties of solitary wave solutions ψ (x, t )=φω(x)eiωt, φωH1

Rn ,

with the amplitudeφω(x)satisfying the stationary equation

− 1

2mφ(x)− |φ|2kφ=ωφ, x∈Rn, (3.2)

are well-known[38,6]. For anyk >0 whenn2 and for 0< k <2/(n−2)whenn3, for eachω(−∞,0), there is a unique positive, radially symmetric solution

φω(x)=φω

|x|

>0,

which decays exponentially. This family of solitary waves, known as the ground states, is generated by rescaling a single amplitude function:

φω(x)= |ω|2k1 F

2m|ω|x

, ω <0, (3.3)

whereF (x)=F (|x|) >0 solves

FF2k+1= −F, x∈Rn. (3.4)

In one space dimension (n=1), fork >0,F (x)is given by the explicit formula F (x)=

k+1 cosh2kx

2k1 .

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3.2. Linearization at a solitary wave

To derive the linearization of the nonlinear Schrödinger equation(3.1)at a solitary waveψ (x, t )=φω(x)eiωt, we use the Ansatz

ψ (x, t )=

φω(x)+ρ(x, t )

eiωt, ρ(x, t )∈C, x∈Rn, and arrive at the linearized equation

tρ=jl(ω)ρ, ρ(x, t)=

Reρ(x, t ) Imρ(x, t )

, (3.5)

where j=

0 1

−1 0

, l=

l 0 0 l+

, (3.6)

withl±self-adjoint Schrödinger operators l(ω)= − 1

2m− |φω|2kω, l+(ω)=l(ω)−2k|φω|2k. (3.7)

Since the solitary wave amplitudesφω(x)=φω(|x|)we take here are radially symmetric, we may consider the opera- tors

lrad,l±,rad:=l,l±restricted to radially symmetric functions. (3.8) The linear stability theory of NLS ground states is well understood, and can be summarized, in terms of their charge(2.14), as follows:

Lemma 3.1(Vakhitov–Kolokolov stability criterion). (See[43].) For the linearization(3.5)at a ground state solitary waveφω(x)eiωt, there are real nonzero eigenvalues±λσd(jl),λ >0, if and only if d Q(ω) >0at this value of ω. If so, then±λσd(jlrad)are simple eigenvalues, and moreoverkerl+,rad= {0}.

Using(3.3), we compute:

Q(ω)=

Rn

φω(x)2dx= |ω|1k

Rn

F2

2m|ω|x

dx=C|ω|1kn2, ω <0, (3.9)

whereC=

RnF2(

2my) dy >0. We see from(3.9)that forω <0 one hasQ(ω) <0 fork <2/n,Q(ω)=0 for k=2/n, andQ(ω) >0 fork >2/n. Thus:

Lemma 3.2. Letn∈N. If k > n/2 (and k <2/(n−2)if n3)then σp(jlrad)λ}, for some λ >0, and in particular the NLS ground states are linearly unstable.

4. Nonlinear Dirac and its solitary waves

Solitary waves are solutions to(2.1)of the form ψ (x, t )=φω(x)eiωt; φωH1

R,CN , ω∈R and as such the amplitudeφω(x)must satisfy

ωφω= −·ψ+mβψf ψβψ

βψ, x∈Rn. (4.1)

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4.1. Solitary waves in one dimension

We first give a simple demonstration of the existence and uniqueness of solitary waves in one dimension, following the article[3], and allowing for more general nonlinearitiesf (s).

Lemma 4.1.Letf (0)=0. Denoteg(s)=mf (s), and letG(s)be the antiderivative ofg(s)such thatG(0)=0.

Assume that there isω0< msuch that for givenω0, m)there existsΓω>0such that

ωΓω=G(Γω), ω=g(Γω), and ωs < G(s) fors(0, Γω). (4.2) Then there is a solitary wave solutionψ (x, t )=φω(x)eiωt to(2.1), where

φω(x)=

v(x, ω) u(x, ω)

, (4.3)

with bothvandureal-valued, belonging toH1(R)as functions ofx,vbeing even anduodd.

More precisely, forx∈Randω0, m), let us defineX(x, ω)andY(x, ω)by

X =v2u2, Y =vu. (4.4)

ThenX(x, ω)is the unique positive symmetric solution to

x2X = −X

−2G(X)2+2ω2X2

, lim

x→±∞X(x, ω)=0, (4.5)

andY(x, ω)= −1xX(x, ω). This solution satisfiesX(0, ω)=Γω. Proof. Substitutingφω(x)eiωt, withφωfrom(4.3), into(4.1), we obtain:

ωv=xu+g

|v|2− |u|2 v, ωu= −xvg

|v|2− |u|2

u. (4.6)

Since we assume that bothvanduare real-valued, we may rewrite(4.6)as the following Hamiltonian system:

xu=ωvg

v2u2

v=vh(v, u),

xv=ωu+g

v2u2

u=uh(v, u), (4.7)

where the Hamiltonianh(v, u)is given by h(v, u)=ω

2

v2+u2

−1 2G

v2u2

. (4.8)

The solitary wave with a particularω0, m)corresponds to a trajectory of this Hamiltonian system such that

x→±∞lim v(x, ω)= lim

x→±∞u(x, ω)=0,

hence limx→±∞X =0. SinceG(s)satisfiesG(0)=0, we conclude thath(v(x), u(x))≡0, which leads to ω

v2+u2

=G

v2u2

. (4.9)

We conclude from(4.9)that solitary waves may only correspond to|ω|< m,ω=0.

The functionsX(x, ω)andY(x, ω)introduced in(4.4)are to solve

⎧⎨

xX = −4ωY,

xY = −

v2+u2

g(X)+ωX = −1

ωG(X)g(X)+ωX, (4.10)

and to have the asymptotic behavior lim|x|→∞X(x)=0, lim|x|→∞Y(x)=0. In the second equation in(4.10), we used the relation(4.9). The system(4.10)can be written as the following equation onX:

x2X = −X

−2G(X)2+2ω2X2

=4

G(X)g(X)ω2X

. (4.11)

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This equation describes a particle in the potential−2G(s)2+2ω2s2. The condition(4.2)is needed so thats=Γωis the turning point for the zero energy trajectory in this potential. The existence of a positive solutionX(x, ω)follows.

This solution is unique up to a translation, and it will be made symmetric inxby requiringX(0, ω)=Γω. 2 4.2. Solitary waves in the nonrelativistic limit

In dimensionsn=1, 2 and 3, we consider solitary wave amplitudesφω(x)of the forms given in(2.12) and (2.13).

Substituting these into the nonlinear Dirac equation(2.1), a straightforward calculation results in the system

⎧⎨

ωv=ru+n−1

r u+mvf

v2u2 v, ωu= −rvmu+f

v2u2 u

(4.12) for the pair of real-valued functions v =v(r, ω), u=u(r, ω). Notice that Eq. (4.12) includes the 1-dimensional case(4.6)if we interpretr=x.

Recalling thatf (s)=sk, we arrive at m)v=ru+n−1

r uf v, +m)u= −rv+f u, with

f :=

v2u2k

.

To consider the nonrelativistic limit, we set m2ω2=2, 0< m,

and rescalev(r, ω)andu(r, ω)as follows:

v(r, ω)=1kV (r, ), u(r, ω)=1+1kU (r, ).

ThenV,Ushould satisfy

⎧⎪

⎪⎩

m)1kV =2+1k

RU+n−1 R U

1/ kf V , +m)1+1kU= −1+1kRV +1+1kf U,

whereR=r denotes the “rescaled variable”. Usingω=m2m1 2+O(4), and taking into account that f =

2kV22+2kU2k

=2V2k+4O

U2k+V2k , we rewrite the system as

⎧⎨

− 1 2m+O

2

V =RU+n−1

R UV2k+1+2O

U2k+V2k

|V| , 2m+O

2

U= −RV +2V2kU+4O

U2k+V2k

|U| .

(4.13) The rescaled system(4.13)has an obvious limit as→0. Formally (for now) setting

V (r)ˆ =lim

0V (r, ), U (r)ˆ =lim

0U (r, ), we arrive at

− 1

2mVˆ =RUˆ +n−1

R Uˆ − ˆV2k+1, 2mUˆ = −RV .ˆ (4.14)

Substituting the second equation into the first one yields

− 1 2m

R2+n−1 R R

Vˆ− ˆV2k+1= − 1

2mV ,ˆ Uˆ = − 1 2mRV .ˆ

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This equation forV (r)ˆ is precisely Eq.(3.2)for NLS solitary wave amplitudesφω withω= −2m1 . Thus we letV (r)ˆ be the (unique) NLS ground state:

V (r)ˆ :=(2m)2k1F (r), U (r)ˆ := −(2m)2k11F (r), (4.15) withF (r)the unique positive spherically symmetric solution to(3.4). We can use this nonrelativistic limit to construct nonlinear Dirac solitary waves for2=m2ω2m2:

Lemma 4.2.There isω0< msuch that forω=√

m220, m), there are solutions of (4.12)of the form v(r, ω)=1kV (r)ˆ + ˜V (r)

, u(r, ω)=1+1kU (r)ˆ + ˜U (r) , ˜VH2+ ˜UH2=O

2 .

Remark 4.3.In the one-dimensional case, since the solitary waves are unique (up to symmetries), it follows that these asymptotics describeeverysolitary wave forωclose tom, or equivalently every small amplitude solitary wave.

Proof. The argument parallels that of[25], where the (more general) nonlinearityf (s)= |s|θ, 0< θ <2 is considered forn=3. Writing

V (R, )= ˆV (R)+ ˜V (R, ), U (R, )= ˆU (R)+ ˜U (R, ), and subtracting equations(4.13) and (4.14), we arrive at

− 1

2mV˜+O 2

V =

R+n−1 R

U˜ −(2k+1)Vˆ2kV˜

+O

| ˆV|2k+1V˜2+ | ˜V|2k+1

+2O

U2k+V2k

|V| , 2mU˜ +O

2

U= −RV˜+2V2kU+4O

U2k+V2k

|U| , which, setting

Ξ (R, ):=

V (R, )˜ U (R, )˜

, we may rewrite as

=OH1

2 +O

2|Ξ| + |Ξ|2+ |Ξ|2k+1 , where

H:=

2m1 +(2k+1)Vˆ2k(∂R+nR1)

R 2m

. (4.16)

Since ξ η

∈kerH ⇐⇒ ξ∈kerl+, η= − 1 2mRξ,

and kerl+,rad= {0}(cf. definitions(3.7), (3.8)), we see that kerH= {0}. It then follows from the fact thatl+1,radis bounded fromL2r(Rn,C)toH2(Rn,C), thatH1is bounded fromHr1(Rn,C2)toH2(Rn,C2)(hereL2r,Hr1are the corresponding subspaces of spherically symmetric functions). Hence

Ξ =H1 OH1

2 +O

2|Ξ| + |Ξ|2+ |Ξ|2k+1

, (4.17)

and sinceΞLH2 (recall thatn3), we arrive easily at R.H.S.(4.17)

H2COH1

2 +O

2|Ξ| + |Ξ|2+ |Ξ|2k+1

H1

C

2+ΞH2+ Ξ2H2+ Ξ2kH2+1

,

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and so we see that for small enough, the map on the r.h.s. of(4.17) maps the ball of radiusinH2into itself.

A similar estimate shows that this map is a contraction, and hence has a unique fixed pointΞ in this ball. Finally, we see from(4.17)thatΞH2=O(2). 2

5. Linear instability of small amplitude solitary waves

Our first observation here is that on spinor fields of the form ψ (x, t )=

Ψ1(x, t) Ψ2(x, t)

, ψ (x, t )=

Ψ1(r, t) ieΨ2(r, t)

,

ψ (x, t )=

⎢⎣

Ψ1(r, t) 1

0

2(r, t)

cosθ esinθ

⎥⎦

in dimensionsn=1,n=2, andn=3 respectively, the nonlinear Dirac equation(2.1)reduces to the system

⎧⎨

i∂tΨ1=

r+n−1 r

Ψ2+1f

|Ψ1|2− |Ψ2|2 Ψ1, i∂tΨ2= −rΨ12+f

|Ψ1|2− |Ψ2|2 Ψ2

(5.1) (with the conventionr=x for n=1). The solitary waves considered in(2.12) and (2.13)lie in this class of fields, corresponding to

Ψ1(r, t)=v(r, ω)eiωt, Ψ2(r, t)=u(r, ω)eiωt. (5.2) To prove the instability of these solitary waves, it suffices to show that they are unstable as solutions of(5.1).

5.1. Linearization at a solitary wave

To derive the linearization of system(5.1)at a solitary wave(5.2)we consider solutions in the form of the Ansatz Ψ1(r, t)

Ψ2(r, t)

=

v(r, ω) u(r, ω)

+

ρ1(r, t) ρ2(r, t)

eiωt, (5.3)

where

ρ(r, t ):=

ρ1(r, t) ρ2(r, t)

∈C2.

Inserting this Ansatz into system(5.1)withf (s)=sk and recalling thatu(r, ω)andv(r, ω)are real-valued, we find that the linearized system forρis

i∂tρ=

mω(v2u2)k r+nr1

rmω+(v2u2)k

ρ

−2k

v2u2k1

v2uv

uv u2

Reρ. (5.4)

We note that the above equation isR-linear but notC-linear, due to the presence of the term with Reρ=

Reρ1

Reρ2 . We rewrite Eq.(5.4)in terms of Reρ∈R2and Imρ∈R2:

t Reρ

Imρ

=JL(ω) Reρ

Imρ

(5.5) whereJcorresponds to 1/ i:

J=

0 I2

I2 0

,

(12)

and the 4×4 matrix operatorL(ω)is defined by L(ω)=

L+(ω) 0 0 L(ω)

, where, writing

f :=f φωβφω

=

v2(r, ω)u2(r, ω)k

, f :=f φωβφω

, we have

L(ω)=

mωf r+nr1

rmω+f

(5.6) and

L+(ω)=L(ω)−2f

v2uv

uv u2

. (5.7)

Let us remind the reader thatv=v(r, ω)andu=u(r, ω)in(5.6)–(5.7)both depend onω.

Then Eq.(5.5)which describes the linearization of the reduced system(5.1)at the solitary waveφωeiωt, takes the form

t

Reρ Imρ

=JL(ω) Reρ

Imρ

=

0 L(ω)

−L+(ω) 0

Reρ Imρ

.

For the sake of completeness we record here the essential spectrum of the linearized operator:

Lemma 5.1.σess(JL(ω))=iR\i(|ω| −m, m− |ω|).

Proof. The proof follows from noticing that, due to the exponential spatial decay ofv(r, ω),u(r, ω)and the Weyl theorem on the essential spectrum[33, Theorem XIII.14, Corollary 2], which leads to

σess

JL(ω)

=σess

J(Dmω) , where

Dm=

Dm 0

0 Dm

, Dm=2r+σ3m.

At the same time, sinceD2m= −+m2,σess(Dm)=R\(m, m), whileJcommutes withDmandσ (J)= {±i}, one concludes that

σess

J(Dmω)

=σess

i(Dmω)

σess

i(Dmω)

=iR\i

|ω| −m, m− |ω|

. 2

5.2. Unstable eigenvalue ofJLforωm

Proposition 5.2.Letk∈Nsatisfyk >2/n. There isω1< m(which depends onnandk)such that forω1, m) there are two families of eigenvalues

±λωσp

JL(ω)

, withλω>0, λω=O(mω).

Proof. The relation

0 L

−L+ 0

ϕ ϑ

=λϕ

ϑ

, withϕ, ϑ∈C2, can be written explicitly as follows:

⎢⎢

⎢⎣

λ 0 mωf r+nr1

0 −λr fmω

ωm+f+2f v2r−2f vuλ 0

r−2f vu m+ωf+2f u2 0 −λ

⎥⎥

⎥⎦

⎢⎣ ϕ1 ϕ2 ϑ1 ϑ2

⎥⎦=0. (5.8)

(13)

We divide the first and the third rows by2=m2ω2, the second and the fourth rows by, and substituteR=r, ϕ2=Φ2,ϑ2=Θ2, to get

⎢⎢

⎢⎣

λ2 0 mω2f R+nR1

0 −λRmω

ωm+f+2f v2

2(∂R+nR1)2fvuλ2 0

R12f vu m+ωf +2f u2 0 −λ

⎥⎥

⎥⎦

⎢⎣ ϕ1 Φ2 ϑ1 Θ2

⎥⎦=0. (5.9)

Anticipating the→0 limit, formally setΛ=lim0 λ

2, and introduce the matrices

AΛ=

⎢⎢

⎢⎣

Λ 0 2m1 − ˆV2k(R) R+nR1

0 0 −R −2m

2m1 +(2k+1)Vˆ2k(R)(∂R+nR1)Λ 0

R 2m 0 0

⎥⎥

⎥⎦, (5.10)

K1=diag[1,0,1,0], K2=diag[0,1,0,1], (5.11)

whereV (R), the NLS ground state, was introduced inˆ (4.15). We write(5.9)in the form

AΛη= λ

2Λ

K1η+λK2η+W η, η=

⎢⎣ ϕ1 Φ2 ϑ1 Θ2

⎥⎦∈C4, (5.12)

whereW (R, )is a zero order differential operator withLcoefficients.

Lemma 5.3.W (·, )L(R+,C4→C4)O(2).

Proof. ByLemma 4.2(and the Sobolev inequality), one has v(r, ω)=1kV (r)=1kV (r)ˆ + ˜V (r, )

, u(r, ω)=1+1kU (r)=1+1kU (r)ˆ + ˜U (r, )

, ˜VL+ ˜UL=O

2 . Then

f

v2u2

=2

V2U2k

, f

v2u2

=k22k

V22U2k1

,

− 1

2m+mωf −2f v2

2 =O

2

V22U2k

−2kV2

V22U2k1

= −(1+2k)Vˆ2k+OL 2

, f vu

=k2U V

V2U2k1

=OL 2

, mω+f=O

2 +2

V22U2k

=OL 2

, f u2=k4U2

V2U2k1

=OL 4

, and

1

2m+ωm+f

2 = +O 2

+

V22U2k

= ˆV2k+OL 2

, and the lemma follows directly from this list of estimates. 2

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Lemma 5.4.dim kerAΛ=dim ker(jlradΛ), where jlrad=

0 l+,rad

−l,rad 0

, and where, we recall,

l,rad= − 1 2m

R+n−1 R

R+ 1

2m− ˆV (R)2k, l+,rad= − 1

2m

R+n−1 R

R+ 1

2m−(2k+1)V (R)ˆ 2k.

Moreover, ifk > n/2 (k=1ifn=3), there isΛ >0such that±Λσd(jH)are simple eigenvalues. HereHis the operator defined in(4.16).

Proof. An easy computation shows thatΦ= Φ1

Φ2 Φ3

Φ4

∈kerAΛif and only if

2mΦ2= −RΦ1, 2mΦ4= −RΦ3, (jlradΛ) Φ3

Φ1

=0,

and the first statement of the lemma follows from this observation. The second statement then follows from Lemma 3.2. 2

So we may assume that there isΛ >0 such that±Λσd(AΛ), with eigenfunctions

kerA±ΛΦ±Λ=

⎢⎢

⎢⎣

±Φ1

2m1 RΦ1 Φ3

2m1 RΦ3

⎥⎥

⎥⎦, l+,radΦ1= −ΛΦ3, l,radΦ3=ΛΦ1.

We will use the Rayleigh–Schrödinger perturbation theory to show that there are eigenvalues ±λσd(JL)with λ=2Λ+o(2).

Writing

AΛ=JL0ΛK1, L0=

L0,+ 0 0 L0,

, with

L0,+= 1

2m(2k+1)Vˆ2k R+nR1

R −2m

, L0,= 1

2m− ˆV2k R+nR1

R −2m

self-adjoint, we see that

AΛ= −L0JΛK1=FAΛF, F:=

0 I2

I2 0

.

Hence kerAΛis spanned byΦΛ :=Λ. LetPΛdenote the orthogonal projection ontoΦΛ. Seekingηandλin the form

η=ΦΛ+ζ, ζΦΛ, λ=2+μ), then(5.12)becomes

AΛζ=μK1Λ+ζ )+2+μ)K2Λ+ζ )+W (ΦΛ+ζ ). (5.13) ApplyingPΛand 1−PΛto(5.13), one has:

0=μ!

ΦΛ,K1Λ+ζ )"

+2+μ)!

ΦΛ,K2Λ+ζ )"

+!

ΦΛ, W (Φ+ζ )η"

, (5.14)

AΛζ=(1PΛ)

μK1+2+μ)K2+W

Λ+ζ ). (5.15)

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