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American Journal of Physics

Illustrating gauge invariance in quantum mechanics through the free electron in a uniform time-varying electric field

C. Leubner

Published Aug 1, 1981• 9 citations
Publisher

Abstract

The nature of gauge transformations and gauge invariance in quantum mechanics about which there exists so much confusion in the literature is simply and pedagogically illuminated by first carefully investigating in various gauges the elementary problem of a free electron subject to a spatially uniform time-dependent electric field, and by then utilizing the findings from this particular example to clarify some general misconceptions.

DOI
10.1119/1.12420

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The nature of gauge transformations and gauge invariance in quantum mechanics about which there exists so much confusion in the literature is simply and pedagogically illuminated by first carefully investigating in various gauges the elementary problem of a free electron subject to a spatially uniform time-dependent electric field, and by then utilizing the findings from this particular example to clarify some general misconceptions.
This article is the second of a trilogy that addresses the perturbative response of general quantum systems, with possibly nontrivial ground state geometry, beyond linear order. Here, we establish concise, general formulae for second order response to a spatially uniform, time-varying electric field in the velocity gauge that are manifestly\textit{manifestly}manifestly free of static limit spurious divergences. We first discuss general quantum evolution in a curved space, then detail how such a situation is a natural byproduct of Hilbert space truncation, and point out crucial subtleties associated with the resulting finite curvatures. We then present a geometric perspective of the two popular gauges often used in quantum transport theories, the velocity gauge and the length gauge, and discuss how they, taking truncation-induced curvature effects into account, naturally lead to the same results in spite of the truncation. We highlight subtle formal discrepancies in the literature. Finally, we provide a general scheme for removing static limit spurious divergences in the velocity gauge without\textit{without}without frequency expansions and present concise and comprehensive Green's function formulae for responses up to second order. As an application of specific aspects of our theory, second order charge current responses in selected cases are analyzed in parts I and III.
The quantal motion of a charged nonrelativistic particle in a uniform but time-varying electric field is discussed. It is pointed out that states of definite but variable momentum exist in this case and are, in a way, the analog of the coherent states of the harmonic oscillator. The gauge invariance of the theory is illustrated, and the results are related to the description of the motion in an accelerated reference frame. A brief discussion of the propagation function and of the evolution operator is also presented.
The transient picture for a Bloch electron accelerating in an arbitrarily time-dependent homogeneous electric field is developed. The temporal sequence for the analysis includes the instant after electron injection, followed by the time required for a small change in electron wavenumber away from initial injection, leading to the final time evolution over many Bloch periods. The time-dependent behavior is studied using the properties of the Schrödinger equation. The electric field is described through the vector potential gauge, and the instantaneous eigenstates of the Bloch, electric-field-dependent Hamiltonian are used as basis states in describing the Bloch dynamics in the electric field. For each temporal sequence considered, the solution to the Schrödinger equation is established and comparatively discussed. The expectation value of the momentum is obtained for the special case of first order in a constant electric field; the resulting velocity derived is a field-dependent generalization of the natural Zitterbewegung-like behavior discussed in the recent literature. The early-time and long-time limits of the momentum expectation value and its time derivative demonstrate that the resistance to Bloch acceleration after initial band injection varies from real mass to effective mass dynamics as the electron accelerates through the band under the influence of electric field. This changing inertia from early injection of a free-mass electron is the result of the {\it real mass} electron {\it dressing-up} into the states of the crystal to become {\it an effective mass} electron. The ramifications of this temporal {\it dressing} behavior are discussed in considering the general dynamics of Bloch electrons subject to ultrastrong electric fields.