Sharp bound for embedded eigenvalues of Dirac operators with decaying potentials

Abstract: We study eigenvalues of the Dirac operator with canonical form \begin{equation} L_{p,q} \begin{pmatrix} u \ v \end{pmatrix}= \begin{pmatrix} 0 & -1 \ 1 & 0 \end{pmatrix}\frac{d}{dt} \begin{pmatrix} u \ v \end{pmatrix}+\begin{pmatrix} -p & q \ q & p \end{pmatrix}\begin{pmatrix} u \ v \end{pmatrix},\nonumber \end{equation} where $ p$ and $q$ are real functions. Under the assumption that \begin{equation} \limsup_{x\to \infty}x\sqrt{p^2(x)+q^2(x)}<\infty,\nonumber \end{equation} the essential spectrum of $L_{p,q}$ is $(-\infty,\infty)$. We prove that $L_{p,q}$ has no eigenvalues if $$\limsup_{x\to \infty}x\sqrt{p^2(x)+q^2(x)}<\frac{1}{2}.$$ Given any $A\geq \frac{1}{2}$ and any $\lambda\in\R$, we construct functions $p$ and $q$ such that $\limsup_{x\to \infty}x\sqrt{p^2(x)+q^2(x)}=A$ and $\lambda$ is an eigenvalue of the corresponding Dirac operator $L_{p,q}$. We also construct functions $p$ and $q$ so that the corresponding Dirac operator $L_{p,q}$ has any prescribed set {(finitely or countably many)} of eigenvalues.