  Common wisdom asserts that bound excitons cannot form in high-dimensional
(d>1) metallic structures because of their overwhelming screening and
unavoidable resonance with nearby continuous bands. Strikingly, here we
illustrate that this prevalent assumption is not quite true. A key ingredient
that has been overlooked is that of viable decoherence that thwarts the
formation of resonances. As an example of this general mechanism, we focus on
an experimentally relevant material and predict bound excitons in twisted
bilayer graphene, which is a two-dimensional gapless structure exhibiting
metallic screening. The binding energies calculated by first-principles
simulations are surprisingly large. The low-energy effective model reveals that
these bound states are produced by a unique destructive coherence between two
alike subband resonant excitons. In particular, this destructive coherent
effect is not sensitive to the screening and dimensionality, and hence may
persist as a general mechanism for creating bound excitons in various metallic
structures, opening the door for excitonic applications based on metallic
structures.
