Abstract
The position operator (defined within the Schrödinger representation in the standard way) becomes meaningless when periodic boundary conditions are adopted for the wave function, as usual in condensed matter physics. I show how to define the position expectation value by means of a simple many-body operator acting on the wave function of the extended system. The relationships of the present findings to the Berry-phase theory of polarization are discussed.
- Received 15 September 1997
DOI:https://doi.org/10.1103/PhysRevLett.80.1800
©1998 American Physical Society