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Gauge transported propagator for the bounded-coefficient magnetic operator

Statement

Assume the Axiom of Choice and the Axiom of Countable Choice (The Axiom of Choice, The Axiom of Countable Choice (ACω)). Fix ℏ>0, m>0, q∈R, admissible magnetic data (A,Φ) on R3 and a gauge function χ:R3×I→R as in Gauge multiplication preserves H2 and transports the magnetic momentum, with underlying open interval I⊆R. Put Aj′(⋅,t):=Aj+Djχ(⋅,t), Φ′(⋅,t):=Φ−∂tχ(⋅,t) and, for t∈I, H′(t):=KA′(t),Φ′(t),G(t):=multiplication by eiqℏχ(⋅,t), so that (A′(t),Φ′(t)) is admissible for every t and H′(t) is self-adjoint with domain W2,2(R3;C) by Self-adjointness of the bounded-coefficient magnetic Schrodinger operator. Let K:=KA,Φ and U(τ):=e−iτK/ℏ for τ∈R, the strongly continuous unitary group generated by −iK/ℏ (A self-adjoint operator generates a strongly continuous unitary group, Integral of a measurable function against a projection-valued measure).

(a) For every t∈I one has the operator identity on W2,2 H′(t)=G(t) K G(t)−1−q ∂tχ(⋅,t), where the last term is multiplication by −q ∂tχ(⋅,t); explicitly, G(t)KG(t)−1=KA′(t),Φ is the magnetic operator with the transported vector potential and the original scalar potential Φ.

(b) For every u0∈W2,2 and s∈I the curve u(t):=G(t) U(t−s) G(s)−1u0(t∈I) lies in W2,2 for every t, is differentiable into L2 on I, and satisfies u(s)=u0 and iℏ u′(t)=H′(t)u(t) for every t∈I. It is the unique such curve: if v:I→W2,2 is continuous into W2,2(R3;C), differentiable into L2, with iℏ v′(t)=H′(t)v(t) for all t and v(s)=u0, then v=u.

(c) The two-parameter family U′(t,s):=G(t) U(t−s) G(s)−1(t,s∈I) consists of unitary operators mapping W2,2 onto W2,2, satisfies U′(t,t)=I and the composition law U′(t,r)U′(r,s)=U′(t,s) for all t,r,s∈I, and u(t)=U′(t,s)u0 is the solution of (b).

The theorem provides dynamics for the transformed coefficient class only; it asserts no general well-posedness for arbitrary time-dependent unbounded generators.

Facts & Assumptions

Given: ℏ,m>0, q∈R, admissible (A,Φ), the gauge function χ, the transported data A′(t),Φ′(t), the operators K=KA,Φ, H′(t)=KA′(t),Φ′(t) and the multipliers G(t).

[F1]

K is self-adjoint with domain D(K)=W2,2, and U(τ)=e−iτK/ℏ is a strongly continuous unitary group with U(τ)D(K)=D(K), KU(τ)=U(τ)K on D(K), and lim⁡τ→01τ(U(τ)x−x)=−iℏKx exactly for x∈D(K) (A self-adjoint operator generates a strongly continuous unitary group, Integral of a measurable function against a projection-valued measure).

[F2]

For each t, G(t) is unitary, preserves W1,2 and W2,2, and (Pj−qAj′(⋅,t))G(t)u=G(t)(Pj−qAj)u for u∈W1,2; moreover t↦G(t)h is continuous for h∈L2 and differentiable with ddtG(t)h=iqℏ(∂tχ(⋅,t))G(t)h (Gauge multiplication preserves H2 and transports the magnetic momentum). The same assertions hold for −χ in place of χ (Gauge multiplication preserves H2 and transports the magnetic momentum).

[F3]

For each t, (A′(t),Φ′(t)) is admissible: DkAj′=DkAj+DkDjχ(⋅,t) and Φ′=Φ−∂tχ(⋅,t) are bounded with the uniform bounds of the hypotheses; and H′(t)=KA′(t),Φ′(t) is self-adjoint with domain W2,2 (Self-adjointness of the bounded-coefficient magnetic Schrodinger operator, Bounded-coefficient magnetic Schrodinger operator).

[F4]

A real function continuous on a closed interval and differentiable in its interior with zero derivative is constant (The mean value theorem, as the case g(x)=x of Cauchy's: for f continuous on [a,b] with a<b and differentiable on (a,b) there is c∈(a,b) with f(b)−f(a)=f′(c)(b−a)). For an L2-differentiable curve z, expansion of the inner product gives (∥z∥2)′=2Re⁡⟨z′,z⟩. If T is self-adjoint and z∈D(T), then ⟨Tz,z⟩ is real, so 2Re⁡⟨−iTz/ℏ,z⟩=0.

Proof

1.1givenF2F3algebra

[Transported magnetic square] Let w∈W2,2 and put u:=G(t)w, so that u∈W2,2 by [F2] and w=G(t)−1u. By Self-adjointness of the bounded-coefficient magnetic Schrodinger operator clause (a) and [F2], PA,jw∈W1,2 and PA′,j(t)G(t)PA,jw=G(t)PA,j(PA,jw) for the transported momentum PA′,j(t)=Pj−qAj′(⋅,t); here the covariance identity is applied twice, the first time to w∈W1,2 and the second to the class PA,jw∈W1,2. Summing over j, 12m∑jPA′,j(t)2u=G(t)12m∑jPA,j2w, and adding the scalar potential term, which is multiplication by qΦ commuting with G(t), gives KA′(t),Φu=G(t)KA,Φw=G(t)KG(t)−1u. Since u∈W2,2 was arbitrary, G(t)KG(t)−1=KA′(t),Φ on W2,2.

2.1givenstep 1.1F3algebra

[Conclusion of (a)] By the definition of Φ′(t) and the linearity of K in the scalar potential, H′(t)=KA′(t),Φ′(t)=KA′(t),Φ−q ∂tχ(⋅,t) as operators on W2,2; combining with step 1.1 gives (a).

3.1givenstep 2.1F1F2algebra

[The curve solves the equation] Let u0∈W2,2 and s∈I, and put w0:=G(s)−1u0, w(t):=U(t−s)w0 and u(t):=G(t)w(t). By [F1]-[F2], both w(t) and u(t) lie in W2,2 at each time, and w′=−iKw/ℏ in L2. Decompose the difference quotient of u as G(t+h)(w(t+h)−w(t))/h+(G(t+h)−G(t))w(t)/h. The first term tends to G(t)w′(t) by strong L2 continuity and unitarity; the second tends to (iq/ℏ)(∂tχ)G(t)w(t) by [F2] on the fixed vector w(t). Thus iℏu′=−q(∂tχ)u+G(t)Kw=H′(t)u by step 2.1, and u(s)=G(s)w0=u0. No time continuity in the W2,2 norm is needed. This proves existence in (b).

4.1givenstep 3.1F3F4algebra

[Uniqueness] Let v be as in (b), and put z(t):=v(t)−u(t). At every t, z(t)∈W2,2=D(H′(t)), and z is differentiable into L2 with iℏz′=H′(t)z and z(s)=0. By self-adjointness from [F3] and the norm derivative identity of [F4], (∥z(t)∥2)′=2Re⁡⟨−iH′(t)z(t)/ℏ,z(t)⟩=0. Apply [F4] on the closed interval joining s and t, which lies in the open interval I: the norm squared is constant and equals zero. Hence v(t)=u(t) for every t, proving uniqueness without any assertion of W2,2 continuity for G(t)−1v(t).

5.1step 3.1step 4.1F1F2algebra∎

[Propagator properties] Each U′(t,s)=G(t)U(t−s)G(s)−1 is unitary as a product of unitaries and maps W2,2 onto W2,2 by [F1] and [F2]. The identity U′(t,t)=G(t)G(t)−1=I is immediate; the composition law follows from G(r)−1G(r)=I: U′(t,r)U′(r,s)=G(t)U(t−r)G(r)−1G(r)U(r−s)G(s)−1=G(t)U(t−r+r−s)G(s)−1=U′(t,s). By step 3.1, u(t)=G(t)U(t−s)G(s)−1u0=U′(t,s)u0 solves the equation, and by step 4.1 it is the unique solution. This is (c).

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