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Quantum mechanics of an electron in a homogeneous magnetic field and a singular magnetic flux tube

Thienel, H P

Published Sep 17, 1998• 5 citations
OpenAlex

Abstract

The eigenvalue problem of the Hamiltonian of an electron confined to a plane and subjected to a perpendicular time-independent magnetic field which is the sum of a homogeneous field and an additional field contributed by a singular flux tube, i.e. of zero width, is investigated. Since both a direct approach based on distribution-valued operators and a limit process starting from a non-singular flux tube, i.e. of finite size, fail, an alternative method is applied leading to consistent results. An essential feature is quantum mechanical supersymmetry at g=2 which imposes, by proper representation, the correct choice of "boundary conditions". The corresponding representation of the Hilbert space in coordinate space differs from the usual space of square-integrable 2-spinors, entailing other unusual properties. The analysis is extended to g different from 2 such that supersymmetry is explicitly broken. Finally, for equal fluxes in the singular tube the Hilbert space has to be the same as for the singular Aharonov-Bohm system and this problem is analyzed accordingly.

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The eigenvalue problem of the Hamiltonian of an electron confined to a plane and subjected to a perpendicular time-independent magnetic field which is the sum of a homogeneous field and an additional field contributed by a singular flux tube, i.e. of zero width, is investigated. Since both a direct approach based on distribution-valued operators and a limit process starting from a non-singular flux tube, i.e. of finite size, fail, an alternative method is applied leading to consistent results. An essential feature is quantum mechanical supersymmetry at g=2 which imposes, by proper representation, the correct choice of "boundary conditions". The corresponding representation of the Hilbert space in coordinate space differs from the usual space of square-integrable 2-spinors, entailing other unusual properties. The analysis is extended to g different from 2 such that supersymmetry is explicitly broken. Finally, for equal fluxes in the singular tube the Hilbert space has to be the same as for the singular Aharonov-Bohm system and this problem is analyzed accordingly.
Invariant operator method for discrete or continuous spectrum eigenvalue and unitary transformation approach are employed to study the two-dimensional time-dependent Pauli equation in presence of the Aharonov–Bohm effect (AB) and external scalar potential. For the spin particles the problem with the magnetic field is that it introduces a singularity into wave equation at the origin. A physical motivation is to replace the zero radius flux tube by one of radius R, with the additional condition that the magnetic field be confined to the surface of the tube, and then taking the limit R → 0 at the end of the computations. We point that the invariant operator must contain the step function θ(r – R). Consequently, the problem becomes more complicated. In order to avoid this difficulty, we replace the radius R by ρ(t)R, where ρ(t) is a positive time-dependent function. Then at the end of calculations we take the limit R → 0. The qualitative properties for the invariant operator spectrum are described separately for the different values of the parameter C appearing in the nonlinear auxiliary equation satisfied by ρ(t), i.e., C > 0, C = 0, and C < 0. Following the C's values the spectrum of quantum states is discrete (C > 0) or continuous (C ≤ 0).
The ((2+1)-dimensional) Aharonov–Bohm effect is analyzed for a spin-1/2 particle in the case that a 1/rpotential is present. Scalar and vector couplings are each considered. It is found that the approach in which the flux tube is given a finite radius that is taken to zero only after a matching of boundary conditions does not give physically meaningful results. Specifically, the operations of taking the limit of zero flux tube radius and the Galilean limit do not commute. Thus there appears to be no satisfactory solution of the relativistic Aharonov–Bohm–Coulomb problem using the finite radius flux tube method.
In this work, we review two methods used to approach singular Hamiltonians in (2+1) dimensions. Both methods are based on the self-adjoint extension approach. It is very common to find singular Hamiltonians in quantum mechanics, especially in quantum systems in the presence of topological defects, which are usually modelled by point interactions. In general, it is possible to apply some kind of regularization procedure, as the vanishing of the wave function at the location of the singularity, ensuring that the wave function is square integrable and then can be associated with a physical state. However, an analysis based on the self-adjoint extension approach can lead to more general boundary conditions that still gives acceptable physical states. We exemplify the methods by exploring the bound and scattering scenarios of a spin 1/2 charged particle with an anomalous magnetic moment in the Aharonov-Bohm potential in the conical space.
Expansions of a plane wave in Landau states are derived corresponding to an adiabatic and to a sudden switching on of the magnetic field. Two earlier solutions given by Faisal (1982) and by Ohsaki (1983) are found to be valid alternative expansions in the adiabatic case, which differ only in the choice of normalisations of the basis states used. A result identical to Ohsaki's is reobtained, since the Landau states are confined by conservation of energy to a large but finite area L x L y approximately 1/ gamma , perpendicular to the field. This energy conservation requires the corresponding sum over the degenerate Landau states to be restricted within a finite range. The upper and lower limits of this range are derived. The completeness relation of Faisal's expansion shows that it is also effectively and automatically confined to the same range of states. In the limit gamma to 0 both sets of expansion coefficients become identical and the results go over to the desired plane-wave state. In the sudden case, when the field switches instantaneously from zero to a finite value, the expansion no longer consists of Landau states of a fixed energy, and the result is new. The two results correspond to different limiting cases of any experimental situation, and the sudden expansion is shown to be invalid at the highest laboratory field ( approximately 20 kG) in current use. The experimental situations corresponding to the two different expansions are discussed.
We show that the Hamiltonian H and the helicity operator Λ of a Dirac particle moving in two dimensions in the presence of an infinitely thin magnetic flux tube each admit a four-parameter family of self-adjoint extensions. Each extension is in one-to-one correspondence with the boundary conditions (BCs) to be satisfied by the eigenfunctions at the origin. Although the actions of these two operators commute before specification of BCs, to ensure helicity conservation it is not sufficient to take the same BCs for both operators. We show that, given certain relations between the parameters of the extensions, it is possible to write down the most general domain where both operators H and Λ are self-adjoint with helicity conservation and also Aharonov-Bohm symmetry (ϕ→ϕ + 1) preserved, where ϕ is the magnetic flux in natural units. The continuity of the dynamics is also obtained. Our results imply that neither helicity conservation nor Aharonov-Bohm symmetry by itself solves the problem of choosing the `physical' BCs for this system.