Pavel Exner and Martin Fraas
A remark on helical waveguides
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ABSTRACT.  Motivated by a proposal to create an 
optical helix-shaped waveguides for cold atoms and molecules, we 
discuss local perturbations which can create bound states in such 
a setting. This is known about a local slowdown of the twist; we 
show that a similar effect can result from a local tube protrusion 
or a change of the helix radius in correlation with its pitch 
angle.