Exact propagating Dirac wave packets in an attractive Coulomb-like potential

We construct exact, positive-energy, normalizable wave-packet solutions of the Dirac equation in the axisymmetric potential $V=-v_0\mathrm{/}\rho$ – to our knowledge, the first such solutions in any external potential. Remarkably, one family comprises only elementary functions whose longitudinal profiles reproduce the free-Schrödinger Hermite–Gauss wave packets in the nonrelativistic limit. All packets share two striking features: (i) a probability density that is pointwise decoupled from spin orientation – despite the inherent spin-orbit coupling of the Dirac equation – and (ii) a complete freezing of their time evolution at the critical coupling $v_0\to \mathrm{\hslash }c\mathrm{/}2$. We also present a simple scheme that maps solutions of the 2D Helmholtz equation to further exact Dirac wave packets.