Wavefunction transformation due to changing group and phase velocities: an expose

TL;DR

This paper explains how the geometric and dynamic phases in quantum wavefunctions originate from changes in phase and group velocities, respectively, using Pancharatnam's phase concept, and relates these to photon polarization during electronic transitions.

Contribution

It provides a microscopic explanation linking Berry's phase to phase velocity changes and dynamic phase to group velocity, clarifying their physical origins.

Findings

Berry's phase arises from phase velocity changes.

Dynamic phase is linked to group velocity.

Photon polarization changes are due to electron wavefunction velocity shifts.

Abstract

If a time-dependent Hamiltonian is allowed to evolve adiabatically, and if it returns to its original form, then the ground state wavefunction must have picked up the dynamic or(and) the geometric phase factor(s) due to some interaction during the above evolution. Here, (i) we invoke the Pancharatnam's notion of phase retardation to show that the microscopic origin of Berry's phase is due to a change in the phase velocity (of a wavefunction), and (ii) the dynamic phase factor is shown to have its origin in the group velocity of a wavefunction. We also expose that the emitted photon's polarization during an electronic transition has its origin in the changes of the phase velocity of the electron wavefunction. Here, we prove the above statements, (i) and (ii), such that they are not due to some interpretational issues, hence, an expos${\rm \acute{e}}$.