Spohn H. Dynamics of Charged Particles and their Radiation Field

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Notes and references 323

(σ

+ 2J

+ 2S

)ψ

= ψ

, the g-factor is thus given by

g =



1 −

ψ

,(P

+ eA

)(H (0) − E(0))

−1

+ eA

)ψ



⊗F



−1



1 − 2ψ

,(J

+ S

)ψ



⊗F

−2Imψ

,(P

+ eA

)

(H (0) − E(0))

−1

+ eA

)



⊗F



(19.107)

in agreement with (19.104).

We recall that, to second order and with no cutoffs, the g-factor is computed to

g = 1 +



2π



O(α

), (19.108)

which is 0.2% away from the true value. For ﬁxed, small e both numerator and

denominator in (19.107) are expected to tend to 0 as  →∞in such a way that

their ratio is close to 1.

Notes and references

Section 19.1

The infrared behavior of radiative corrections to scattering was ﬁrst studied by

Bloch and Nordsieck (1937), Nordsieck (1937), and Pauli and Fierz (1938). Within

the framework of the massless scalar Nelson model of section 19.2, Fr

ohlich

(1973) constructs the one-particle shell and investigates the scattering theory.

Further progress in this direction is Pizzo (2000). In his thesis Chen (2001) estab-

lishes the infrared limit of the energy–momentum relation. In contrast to the Pauli–

Fierz model the scalar Nelson model is infrared divergent also at p = 0. The Gross

transformation (19.28) switches to the representation corresponding to p = 0. In

fact it implements the shift φ(x) − eV

ϕcoul

(x).Werefer to Arai (2001), L

orinczi,

Minlos and Spohn (2002b) and Hirokawa, Hiroshima and Spohn (2002). The quan-

tized Maxwell ﬁeld coupled to a classical current is a standard textbook example

(Kibble 1968; Thirring 1958). The representation theory for coherent states is de-

veloped by Klauder, McKenna and Woods (1966) with follow-ups within the alge-

braic framework (Emch 1972; Dubin 1974; Bratteli and Robinson 1987, 1997).

Section 19.2

The scalar ﬁeld model is studied in solid state physics and includes a large body of

experimental work. It describes an electron coupled to the optical mode of a polar

crystal and is known as a polaron (Landau 1933; Fr

ohlich 1954). In the standard

approximation the dispersion of the ﬁeld is ω(k) = ω

and the coupling function

324 Behavior at very large and very small distances

ϕ(k) =|k|

−1

. The ground state energy is well approximated by the variational

approach of Feynman (1955). Upper and lower bounds are proved by Lieb and

Yamazaki (1958). A large coupling theory is available (Pekar 1954). A rigorous

proof of the Pekar limit can be found in Donsker and Varadhan (1983) and Lieb and

Thomas (1997). The effective mass is studied in Spohn (1987) who also provides

an extensive list of references. The Pekar limit of the effective mass still remains

an open problem. Useful reviews are Devreese and Peeters (1984) and Gerlach and

owen (1991). Gross (1976) develops systematic corrections to the large coupling

theory. Nelson (1964a) studies the scalar model through functional integration; see

chapter 14. Nelson (1964b) uses the transformation of Gross (1962), itself inspired

by Lee, Low and Pines (1953), to control the removal of the ultraviolet cutoff.

Nelson’s analysis is pushed much further in Fr

ohlich (1973, 1974). The discussion

of chapter 14 transcribes word for word to the Nelson model with the welcome

simpliﬁcation that stochastic Ito integrals become Riemann integrals. We refer to

orinczi and Minlos (2001), L

orinczi et al. (2002a), and Betz et al. (2002). For suf-

ﬁciently small coupling the existence of a ground state for H of (19.18) is proved

in Hirokawa et al. (2002). On the one-particle level, H =



+ m

− e

/4π|x|

is not bounded from below for large e.Since for the Nelson model E( p) |p| for

large p, the same instability could be present for the Hamiltonian (19.18). Hainzl,

Hirokawa and Spohn (2003) provide upper and lower bounds on E

bin

which estab-

lish (19.52) with an error

O(e

log e).

Section 19.3

The estimates of the ground state energy are taken from Lieb and Loss (2000,

2002), who study in addition the case of many particles and semirelativistic mod-

els. The scaling (19.80) follows also from a perturbative one-loop renormalization

(Chen 1996; Bugliaro et al. 1996). The effective mass to order α

seems to be

novel. Details of the perturbative computation leading to (19.82) can be found

in Hiroshima and Spohn (2003). Fr

ohlich argues that the effective mass depends

nonanalytically on α and therefore the interchange of limits, α → 0 and  →∞,

leads to erroneous results. In a more proper treatment one should successively

eliminate the interaction at high momenta. The resulting renormalization group

ﬂow equations yield a plausible outcome and, indeed, reﬂect the nonanalytic de-

pendence in α. Moniz and Sharp (1974, 1977) and Grotch et al. (1982) claim

cutoff dependences of the effective mass which are in contradiction to our ﬁnd-

ings. The bound for γ



is from Lieb and Loss (2002), which is based on the lower

operator bound (19.95) for the Coulomb potential as proved in Lieb, Loss and

Siedentop (1996). The famous calculation of the Lamb shift by Bethe (1947) is

based on the dipole approximation and has a divergence as log .Aspointed out

immediately (Kroll and Lamb 1949), the shift becomes ultraviolet convergent in a

Notes and references 325

relativistic theory which necessarily includes positrons. The role of retardation is

mentioned by Kroll (1965). An accurate calculation is Au and Feinberg (1974), ig-

noring spin however. It is included in Grotch (1981), from which our numbers are

taken. Quantum electrodynamic effects are most dominant for the 1S Lamb shift,

which experimentally is determined with higher accuracy than the 2S

1/2

− 2P

1/2

splitting. We refer to Weitz et al. (1994). For small coupling quantitative estimates

on the binding energy are available. One constructs upper and lower bounds with

the leading terms given by formal perturbation theory, which is not directly ap-

plicable because of the missing spectral gap; see Catto and Hainzl (2004), Chen,

Voulgater and Vulgater (2003), Hainzl (2002, 2003), Hainzl, Seiringer (2002), and

Hainzl, Voulgater, Vulgater (2003). At present, one obstacle is that no correspond-

ing result for the effective mass is available. Since physically energies are cali-

brated through m

eff

,such a bound is mandatory. The g-factor as based on the

shift in energy is computed in Grotch and Kazes (1977) to second order in e. The

derivation of the nonperturbative expression (19.107) seems to be new.

Many charges, stability of matter

In the low-energy sector, to an excellent approximation, the world consists of pho-

tons, electrons, and nuclei. To simplify the forthcoming discussion, let us consider

only one species of nuclei with charge eZ, Z = 1, 2,....In fact, we also assume

that the nuclei are inﬁnitely heavy and located at positions r

,... ,r

∈ R

. This

is hardly realistic, but not of central importance for the stability issues studied here.

We also ignore nuclear spins. To include them would require yet another layer of

considerations. With these assumptions we have an arbitrary number of photons,

N electrons, and K nuclei governed by the Hamiltonian

H =



j=1



· ( p

− eA

)



+ H

+ V

ϕcoul

, (20.1)

compare with (13.39). σ

are the Pauli spin matrices for the j-th electron. Since

electrons are fermions, the corresponding Hilbert space is

H = P



, C

)

⊗N



⊗

F (20.2)

with P

denoting the projection onto the subspace of antisymmetric wave func-

tions. V

ϕcoul

is the smeared Coulomb potential, cf. (13.17), which in the case con-

sidered here is given through

ϕcoul

,... ,x

) = e



k|ϕ(k)|

|k|

−2





1≤i< j≤N

ik·(x

−x

)

−Z



i=1



j=1

ik·(x

−r

)

+ Z



1≤i< j≤K

ik·(r

−r

)



. (20.3)

One of the most basic facts about nature, which the Hamiltonian (20.1) should

better explain, is the apparent stability of ordinary matter over extremely long pe-

riods of time. It has become customary to divide the issue roughly into

326

Many charges, stability of matter 327

(i)

atomic stability,

(ii) energy stability (or H-stability),

(iii) thermodynamic stability.

An atom is the special case of (20.1) with K = 1 (hence r

= 0) and N , Z

arbitrary. Atomic stability means that the ground state for H looks like what we

know from real atoms in nature. In particular, provided that N < Z + 1, or per-

haps N ≤ Z + 1 admitting a negatively charged ion, H has a ground state eigen-

vector with an exponentially localized electronic density. Also the ultraviolet cutoff

should not have to be ﬁne-tuned. Our understanding of the stability of atoms and

molecules within nonrelativistic QED has advanced spectacularly over the past

few years. An overview is provided in section 20.1.

Energy stability and thermodynamic stability refer to the property that matter at

the human scale is (volume) extensive: Adding two buckets of water of 10 liters

each merely results in 20 liters of water. Since now many molecules are involved,

(20.1) is to be considered for large N with N

∼

KZ, Z ≤ 100. For an energy

stable system, the volume of the combined system in its ground state is at least

as large as the sum of the volumes of the subsystems. It is more convenient to

re-express this property in energetic terms. If E(N ; K, r

,... ,r

) denotes the

ground state energy of H in (20.1), then for an H-stable system

E(N ; K , r

,... ,r

) ≥−c

(N + K ) (20.4)

with suitable c

≥ 0 independent of the location of the nuclei. In fact, such a bound

obviously holds, since

H ≥ V

ϕcoul

≥−





k|ϕ(k)|

|k|

−2



N + e

K ). (20.5)

While correct, (20.5) teaches us little about the physics involved, since the bound

is cutoff-dependent and is not of the order of one Rydberg, as expected.

The condition (20.4) overlooks the fact that even when the electrons are stripped

off to inﬁnity they still carry a self-energy. Denoting as before the self-energy of a

single electron by E

self

, the sharper stability condition is

E(N ; K , r

,... ,r

) − NE

self

≥−c

(N + K ) (20.6)

with some suitable constant c

independent of the location of the nuclei. Hopefully

is of order of a Rydberg and less sensitive to the cutoff than c

. Energy stability,

as far as aspects of the quantized radiation ﬁeld are involved, is discussed in section

20.3.

As the name indicates, thermodynamic stability means that the thermodynamic

potentials are volume extensive. In particular, the thermodynamic pressure, i.e.

the force per unit area on the conﬁning container, is in essence size independent.

328 Many charges, stability of matter

For proper statistical mechanics also the nuclei should have a ﬁnite mass. In our

context a natural model would be to assume charge neutrality, i.e. N = KZ, and

that the nuclei form a regular crystal lattice. Then one aspect of thermodynamic

stability is a ground state energy proportional to the number of particles, which

requires (20.6) to be augmented by an upper bound linear in N + K .Werefer to

the notes at the end of the chapter for further details.

No surprise, energy and thermodynamic stability are best understood in the case

when the interaction with the radiation ﬁeld is neglected. This raises the question:

In what sense is the standard N -body Coulomb Hamiltonian a good approximation

to (20.1)? In the classical context we discussed this problem rather exhaustively in

section 11.2. Quantum mechanics adds a layer of difﬁculty, as will be explained in

section 20.2.

20.1 Stability of atoms and molecules

The number of electrons, N ,isregarded as ﬁxed and the goal is to understand

under what conditions, in their lowest-energy state, they are all bound to the nuclei.

For this purpose the interaction between nuclei can be dropped. We also ignore

the smearing of the Coulomb potential. On the other hand, we want to allow a

variation in the nucleon charge, i.e. the j-th nucleus is located at r

and has charge

, Z

> 0, j = 1,...,K .With these modiﬁcations, the Hamiltonian reads

(N ) =



j=1



· ( p

− eA

))



+ H



1≤i< j≤N

(4π|x

− x

−1

−



i=1



j=1

(4π|x

−r

−1

. (20.7)

The form factor ensures a smooth cutoff at large k,butϕ(0) = (2π)

−3/2

as it

should. The bottom of the spectrum for H

(N ) is

(N ) = inf σ(H

(N )) = inf

ψ,ψ

ψ, H

(N )ψ

. (20.8)

We will have to compare with free electrons whose Hamiltonian is

(N ) =



j=1



· ( p

− eA

))



+ H



1≤i< j≤N

(4π|x

− x

−1

(20.9)

20.1 Stability of atoms and molecules 329

Its lowest energy is denoted by E

(N ).Itisunlikely that the effective interaction

induced by the photon cloud overrules the combined Coulomb repulsion and Fermi

exclusion. Thus E

(N ) = NE

(1) is expected, but will not be assumed here.

The general strategy is to introduce a suitable notion of the ionization energy

ion

(N ). Then the binding energy is deﬁned by

bin

(N ) = E

ion

(N ) − E

(N ). (20.10)

If E

bin

(N )>0, the energy interval  = [E

(N ), E

ion

(N )) is nonempty and states

with an energy distribution supported by  should be well localized near the nu-

clei. Amongst them there will be the stable ground state.

It seems clear how to proceed. If one electron is moved to inﬁnity it has en-

ergy E

(1) and the corresponding lowest-energy state of H

(N ) has the energy

(N − 1) + E

(1).Ofcourse it could be energetically more favorable to move

two electrons to inﬁnity, etc. Thus

ion

(N ) = min

1≤N



≤N



(N − N



) + E



)



(20.11)

with the convention E

(0) = 0. Note that if the interaction with the photon ﬁeld

is turned off, formally setting ϕ = 0, then E

(N ) = 0 and (20.11) agrees with the

standard deﬁnition of the ionization energy for the Coulomb Hamiltonian.

There is a more direct way of moving electrons to inﬁnity. As in chapter 16,

we regard ψ(x) as a

⊗ F -valued wave function, x = (x

,...,x

).Wedeﬁne

as the projection on the subspace of wave functions satisfying ψ(x) = 0 for

|x| < R. Then the alternative deﬁnition is

ion

(N ) = lim

R→∞

inf σ(P

(N )P

). (20.12)

As proved by Griesemer (2002) the deﬁnitions (20.11) and (20.12) of the ioniza-

tion energy agree in the context of the Pauli–Fierz Hamiltonian. Note that with

(20.12) it is obvious that E

bin

≥ 0. Also, if H

(N ) admits surplus electrons, nec-

essarily E

bin

= 0.

Let us denote by E

= E

(N )) the spectral resolution of H

(N ), i.e. E

is the projection corresponding to the energy interval (−∞,λ].

Theorem 20.1 (Exponential localization). Let E

bin

(N )>0 and let us choose

λ, β such that λ + (β

/2m)<E

ion

(N ),E

(N ) ≤ λ, β>0.IfE

ψ = ψ, then

e

β|x|

ψ

≤ c

ψ

. (20.13)

The proof is due to Griesemer (2004). In fact, the proof exploits only properties

of the Laplacian. As in chapter 16, we regard

H = L

, H

) = L

, d

x) ⊗

with some Hilbert space H

of “internal degrees of freedom”. The operator

330 Many charges, stability of matter

H with domain D(H ) is self-adjoint on H. Let f ∈ C

∞

, R) with f and ∇ f

bounded. The crucial assumption concerns the double commutator

[[H, f ], f ] =−2|∇ f |

(20.14)

with f regarded as a multiplication operator. Note that (20.14) holds in the case

F = C and H =−. But (20.14) holds also for H = H

(N ) setting n = 3N .As

before the ionization threshold for H is

ion

= lim

R→∞

inf σ(P

). (20.15)

Proposition 20.2 Let H satisfy (20.14) and let λ + β

< E

ion

, β>0. Then

e

β|x|

(H )

< ∞. (20.16)

Let us return to the existence of a ground state for H

(N ).IfE

bin

> 0, the

exponential localization is a favorable indication. But it could happen that more

and more photons are bound by the electrons. Thus we need a soft photon bound

of the type of Theorem 15.1. The proof is now considerably more demanding and

established by Griesemer, Lieb and Loss (2001).

Theorem 20.3 (Existence of a ground state). If E

bin

(N )>0, then H

(N ) has a

ground state.

Because of Pauli exclusion and spin, no obvious positivity is available which

would ensure uniqueness. Note that there is no restriction on the coupling strength

e. Also, by Proposition 20.2, the ground state is exponentially localized with length

less than 1/

√

2mE

bin

(N ).

The existence of a ground state is reduced to the issue of whether E

bin

(N )>0.

While the statement looks innocent and seems to require only the clever choice of

awave function, the actual construction is ingenious and has been achieved only

very recently by Lieb and Loss (2003). The main obstacle is the, in position space,

nonlocal nature of the photon kinetic energy.

Theorem 20.4 (Strictly positive binding energy). Let eZ

tot

be the total nuclear

charge, Z

tot



j=1

.If

N < Z

tot

+ 1 , (20.17)

then E

bin

(N )>0 .

In nature ions carrying one, or perhaps two, extra electrons are rather common.

Such ﬁne chemical features are difﬁcult to access. In fact, even on the level of the

Coulomb Hamiltonian the excess charge for stable ions is poorly understood.

20.2 Quasi-static limit 331

20.2 Quasi-static limit

We plan to investigate under what limiting conditions the many-particle Pauli–

Fierz Hamiltonian can be approximated by the Coulomb Hamiltonian with possi-

ble corrections. The implementation of the limit (11.8) on the quantum level has

not yet been attempted. Thus we have to be satisﬁed with the more down-to-earth

limit c →∞already discussed brieﬂy at the beginning of section 11.2. c →∞

means that the interaction between the charges becomes instantaneous, a princi-

ple on which the Coulomb Hamiltonian is built. To study this limit we had better

reintroduce the velocity of light, which amounts to

H(c) =



j=1



−

√

)



+ V

ϕcoul

+ cH

. (20.18)

(x), V

ϕcoul

, and H

do not depend on c. The prefactors as written result from

reintroducing ω(k) = c|k|. The masses and charges are arbitrary.

c has a dimension. So what we really mean is |v|/c → 0, where v is some

characteristic velocity of the charges. Thus either c →∞at ﬁxed |v| or |v|→0

at ﬁxed c. The latter can also be achieved by assuming the particles to be

heavy and, hence, by replacing in (20.18) m

by ε

−2

,ε 1. On the classi-

cal level the limits c →∞and ε → 0 are related through the time scale change

t to εt, and thus are completely equivalent. Quantum mechanically the two

Hamiltonians are not unitarily related, which reﬂects the additional scale coming

from

.

Let us ﬁrst study the limit c →∞. Except for normal order the Hamiltonian

(20.18) reads

H(c) =



j=1

+ V

ϕcoul

−

√



j=1

· A

)

−

√



j=1

· B

) +



j=1

: A

)

: +cH

(20.19)

H(c) should be compared with the weak coupling Hamiltonian (17.4), written for

the long-time scale λ

−2

τ and with the abbreviation H

int

Q · A

(0),

= λ

−2

+ λ

−1

int

+ λ

−2

. (20.20)

The interaction part H

int

satisﬁes , H

int



= 0, which holds also for (20.19),

since , A

)

= 0, , B

)

= 0, and , : A

)

: 

= 0.

The central insight of the weak coupling theory is that the correction to H

results

from balancing λ

−2

int

)

with the time averaging due to λ

−2

; compare with

332 Many charges, stability of matter

(17.22). Clearly this balance can be achieved also in (20.19) by considering the

long-time scale

t = c

τ with τ = O(1). (20.21)

Then H

has the prefactor c

, H

int

the prefactor c

3/2

with a subleading correction

of order c, and H

has the prefactor c

. The analog of (17.24) becomes

Kρ =−



∞

dte

−1

int

−i(c

−1

int



]ρ. (20.22)

In the limit c →∞the dependence on L

drops out. In particular, this implies

that the correction term must be nondissipative; compare with (17.35) which is

evaluated at ω = 0.

Let us ﬁrst write out the limiting objects. The analog of H

ϕcoul



j=1

+ V

ϕcoul

. (20.23)

It is corrected by

(−i)V

ϕdarw

= i

∞



dt , H

int

−itH

int



, (20.24)

which upon working out the integrals becomes

ϕdarw

=−



i, j=1



k|ϕ(k)|

ik·x

( p

· Q

⊥

(k) p

−ik·x

−



i, j=1

12m

· σ



k|ϕ(k)|

ik·(x

−x

)

(20.25)

which is the Darwin correction. We set

ϕdarw

= H

ϕcoul

+ c

−2

ϕdarw

. (20.26)

Note that the integrability condition (17.27) is satisﬁed, since the integrand in

(20.24) is bounded by (1 + t

)

−1

.Incontrast to section 17.2, H

int

has an un-

bounded factor acting on

,which necessitates a restriction on the initial wave

function. We summarize as