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arXiv:quant-ph/9702025

Quantum Interference of Polarized Electrons in the Presence of Magnetic Strings

Silviu Olariu

Published Feb 10, 1997• 0 citations
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Abstract

This work analyzes the effects of shielding on the Aharonov-Bohm scattering of electrons endowed with spin. The interaction of polarized electrons with bare and shielded magnetic strings is studied with the aid of the Dirac equation. It is found that the difference between the amplitudes for the scattering by bare and shielded strings of incident wave packets of width $δ$ and impact parameter $d$ is proportional to $\exp(-d^2/2δ^2)$.

DOI
10.48550/arxiv.quant-ph/9702025

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•Jan 1, 1983•1 citation
This work analyzes the effects of shielding on the Aharonov-Bohm scattering of electrons endowed with spin. The interaction of polarized electrons with bare and shielded magnetic strings is studied with the aid of the Dirac equation. It is found that the difference between the amplitudes for the scattering by bare and shielded strings of incident wave packets of width δδδ and impact parameter ddd is proportional to exp⁡(−d2/2δ2)\exp(-d^2/2δ^2)exp(−d2/2δ2).
In this paper we consider the scattering of charged particles by magnetic strings for several configurations of the incident state and of the enclosed flux. We analyze the scattering of a plane wave by an infinite magnetic string and show that the scattering contribution arises from flux-dependent phase shifts followed by quantum diffusion. Then we report that the amplitude for the scattering of a wave packet of finite extension by a magnetic string becomes exponentially small as the width of the packet goes to zero. At the same time the phase at the center of the incident wave packet is progressively shifted by an amount proportional to the enclosed flux, as predicted on the basis of the quasiclassical approximation. We consider further the scattering by a pair of parallel strings carrying opposite fluxes and show that the probability pattern is affected by the magnetic fluxes mainly in the vicinity of a finite surface spanning the two strings. We finally discuss the relevance for the Aharonov-Bohm effect of the multivalued gauge transformations which eliminate the potentials from the field-free region. The results reported in this paper confirm the reality of the quantum effects of remote fluxes, and prove that the state of the electromagnetic field is not completely determined by the local knowledge of the field strengths.
The Aharonov-Bohm scattering of relativistic spin-1∕21∕21∕2 particles with an anomalous magnetic moment are studied. The scattering cross sections for unpolarized and polarized particles are obtained by solving the Dirac-Pauli equation. It is somewhat unexpected that the results are in general the same as those for particles without an anomalous magnetic moment. However, when the incident energy takes some special values, the cross section for polarized particles is dramatically changed. In these cases the helicity of scattered particles is not conserved. In particular, the helicity of particles scattered in the backward direction is all reversed. In the nonrelativistic limit, a very simple relation between the polarized directions of the incident and scattered particles is found, for both general and special incident energies. For particles without an anomalous magnetic moment this relation can be drawn from previous results but it appears to be unnoticed.
The scattering of relativistic spin-1/2 neutral particles with a magnetic dipole moment by a long straight charged line and a magnetic flux line at the same position is studied. The scattering cross sections for unpolarized and polarized particles are obtained by solving the Dirac-Pauli equation. The results are in general the same as those for pure Aharonov-Casher scattering (by the charged line alone) as expected. However, in special cases when the incident energy, the line charge density, and the magnetic flux satisfy some relations, the cross section for polarized particles is dramatically changed. Relations between the polarization of incident particles and that of scattered ones are presented, both in the full relativistic case and the nonrelativistic limit. The characteristic difference between the general and special cases lies in the backward direction: in the general cases the incident particles are simply bounced while in the special cases their polarization is turned over simultaneously. For pure Aharonov-Casher scattering there exist cases where the helicities of all scattered particles are reversed. This seems to be remarkable but appears unnoticed previously. Two mathematical approaches are employed to deal with the singularity of the electric and magnetic field and it turns out that the physical results are essentially the same.
Both classical and quantum wave interference phenomena rely on the notion of phase. In both classical as well as quantum electrodynamics the phases are acquired locally; however, the quantum phases have the additional property that they can be transmitted even when the classical value of the degree of freedom relevant for the transfer (such as the value of the electric field at one point) are zero. Here we use a covariant formalism that is capable of explaining the electric and magnetic versions of the Aharonov-Bohm effect, as well as the Aharonov-Casher effect, through local interactions of charges and currents with the quantum electromagnetic field. We show that Aharonov-Bohm effect, which is just another instance of interference based on a quantum phase, is present even if the solenoid generating the magnetic field is shielded by a superconductor. This is because even though the classical value of the magnetic field is zero in the superconductor (the Meissner effect), the quantum operators pertaining to the magnetic field are non-zero and are furthermore responsible for transmitting the phase information between the solenoid and the electron. The conclusion of our work is that quantum phases cannot be shielded and we believe that this, among other consequences, finally resolves the last outstanding mystery in the Aharonov-Bohm effect.
Aharonov-Bohm (AB) effect is discussed from a viewpoint which stresses the influence of the fields whose source is the scattered particle. Speculations are presented to suggest that AB scattering may be influenced by subtle properties of superconducting surfaces which may shield the particle's fields away from the external magnetic field. Experiments are required to test the reality of such speculations. If such shielding effects can be measured, AB effect may become a sensitive probe to search for unusual surface states which display off diagonal long range order.