Polarization and localization in insulators: generating function approach

Details Polarization and localization in insulators: generating function approach Polarization and localization in insulators: generating function approach

1999-11-02

arxiv.org/abs/cond-mat/9911007v2

Physics

Condensed Matter

25 pages and 3 postscript figures, to appear in Phys. Rev. B. Manuscript was made more concise and some new results were added

Scientific paper

10.1103/PhysRevB.62.1666

We develop the theory and practical expressions for the full quantum-mechanical distribution of the intrinsic macroscopic polarization of an insulator in terms of the ground state wavefunction. The central quantity is a cumulant generating function which yields, upon successive differentiation, all the cumulants and moments of the probability distribution of the center of mass X/N of the electrons, defined appropriately to remain valid for extended systems obeying twisted boundary conditions. The first moment is the average polarization, where we recover the well-known Berry phase expression. The second cumulant gives the mean-square fluctuation of the polarization, which defines an electronic localization length xi_i along each direction i. It follows from the fluctuation-dissipation theorem that in the thermodynamic limit xi_i diverges for metals and is a finite, measurable quantity for insulators. It is possible to define for insulators maximally-localized ``many-body Wannier functions'', which for large N become localized in disconnected regions of the high-dimensional configuration space, establishing a direct connection with Kohn's theory of the insulating state. Interestingly, the expression for xi_i^2, which involves the second derivative of the wavefunction with respect to the boundary conditions, is directly analogous to Kohn's formula for the ``Drude weight'' as the second derivative of the energy.

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