paper

Existence and computation of generalized Wannier functions for non-periodic systems in two dimensions and higher

arXiv:2003.06676 · doi:10.1007/s00205-021-01721-9

Abstract

Exponentially-localized Wannier functions (ELWFs) are an orthonormal basis of the Fermi projection of a material consisting of functions which decay exponentially fast away from their maxima. When the material is insulating and crystalline, conditions which guarantee existence of ELWFs in dimensions one, two, and three are well-known, and methods for constructing the ELWFs numerically are well-developed. We consider the case where the material is insulating but not necessarily crystalline, where much less is known. In one spatial dimension, Kivelson and Nenciu-Nenciu have proved ELWFs can be constructed as the eigenfunctions of a self-adjoint operator acting on the Fermi projection. In this work, we identify an assumption under which we can generalize the Kivelson-Nenciu-Nenciu result to two dimensions and higher. Under this assumption, we prove that ELWFs can be constructed as the eigenfunctions of a sequence of self-adjoint operators acting on the Fermi projection. We conjecture that the assumption we make is equivalent to vanishing of topological obstructions to the existence of ELWFs in the special case where the material is crystalline. We numerically verify that our construction yields ELWFs in various cases where our assumption holds and provide numerical evidence for our conjecture.

arXiv comment64 pages, 15 figures. Many revisions: (1) Proof generalized WFs we construct are (a) WFs when system periodic (b) respectful of time-reversal, so gap conditions imply topological triviality (2) Proofs streamlined using integral kernel of P; generalized to more general position operators (3) Fixed error in constructive proof that PXP gaps implies analytic and periodic Bloch frame in 2d

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