= Quantum step-reflection phase
{title2=$r=(k-i\kappa)/(k+i\kappa),\quad\delta=-2\arctan(\kappa/k)$}
A particle incident below a semi-infinite step has a unit-modulus <quantum reflection amplitude> but a nontrivial phase. Continuity of the oscillating and evanescent solutions yields the displayed coefficient, for the incident phase origin at the interface. Its decaying exterior wave has zero net <probability current>. The phase depends on the chosen scattering convention; the unit reflection <probability> does not.
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