Engineering geometrically flat Chern bands with Fubini-Study K\"ahler structure

TL;DR

This paper introduces a systematic method to construct geometrically flat K"ahler Chern bands with coinciding Berry curvature and quantum volume form, useful for fractional Chern insulators, and analyzes their properties and limitations.

Contribution

It presents a novel construction of geometrically flat K"ahler Chern bands using K"ahler quantization and Bergman kernel asymptotics, including explicit realizations and numerical validation.

Findings

Constructed K"ahler bands become flatter with more bands.

No-go theorem for perfect flatness with finite bands.

Finite-range hoppings do not significantly disrupt flatness.

Abstract

We describe a systematic method to construct models of Chern insulators whose Berry curvature and the quantum volume form coincide and are flat over the Brillouin zone; such models are known to be suitable for hosting fractional Chern insulators. The bands of Chern insulator models where the Berry curvature and the quantum volume form coincide, and are nowhere vanishing, are known to induce the structure of a K\"ahler manifold in momentum space, and thus we are naturally led to define K\"ahler bands to be Chern bands satisfying such properties. We show how to construct a geometrically flat K\"ahler band, with Chern number equal to minus the total number of bands in the system, using the idea of K\"ahler quantization and properties of Bergman kernel asymptotics. We show that, with our construction, the geometrical properties become flatter as the total number of bands in the system is increased; we also show the no-go theorem that it is not possible to construct geometrically perfectly flat K\"ahler bands with a finite number of bands. We give an explicit realization of this construction in terms of theta functions and numerically confirm how the constructed K\"ahler bands become geometrically flat as we increase the number of bands. We also show the effect of truncating hoppings at a finite length, which will generally result in deviation from a perfect K\"ahler band but does not seem to seriously affect the flatness of the geometrical properties.