  An induced fractional zero-point angular momentum of charged particles by the
Bohm-Aharonov (B-A) vector potential is realized via a modified combined trap.
It explores a "spectator" mechanism in this type of quantum effects: In the
limit of the kinetic energy approaching one of its eigenvalues the B-A vector
potential alone cannot induce a fractional zero-point angular momentum at
quantum mechanical level in the B-A magnetic field-free region; But when there
is a "spectator" magnetic field the B-A vector potential induces a fractional
zero-point angular momentum. The "spectator" does not contribute to such a
fractional angular momentum, but plays essential role in guaranteeing
non-trivial dynamics at quantum mechanical level in the required limit. This
"spectator" mechanism is significant in investigating the B-A effects and
related topics in both aspects of theory and experiment.
