General theory of constructing potential with bound states in the continuum

TL;DR

This paper develops a comprehensive theory for constructing potentials that support bound states at positive energies, highlighting the prevalence of nonlocal potentials in such states and providing methods for their practical realization.

Contribution

It introduces a general framework for designing nonlocal potentials that support positive energy bound states, a rare feature in local potentials, with practical construction methods and numerical demonstrations.

Findings

Nonlocal potentials frequently support positive energy bound states.

Local potentials rarely support positive energy bound states.

Constructed potentials can support arbitrary positive energy bound states.

Abstract

We present a general theory of potentials that support bound states at positive energies (bound states in the continuum). On the theoretical side, we prove that, for systems described by nonlocal potentials of the form $V(r,r')$, bound states at positive energies are as common as those at negative energies. At the same time, we show that a local potential of the form $V(r)$ rarely supports a positive energy bound state. On the practical side, we show how to construct a (naturally nonlocal) potential which supports an arbitrary normalizable state at an arbitrary positive energy. We demonstrate our theory with numerical examples both in momentum and coordinate spaces with emphasis on the important role played by nonlocal potentials. Finally, we discuss how to observe bound states at positive energies, and where to search for nonlocal potentials which may support them.