A Transmission Framework for Stark Operators with $δ$ Interactions on a Compact Lipschitz Interface

Abstract

We study an elliptic transmission problem associated with a Stark operator and a $δ$ interaction on a compact Lipschitz interface in $\mathbb R^d$. The background differential expression contains the unbounded coefficient $-Fx_1$, and the interface strength is an arbitrary real function $α\in L^\infty(Σ)$. We introduce a transmission class with piecewise $H^1$ regularity near the interface and an $L^2$ action away from it. This setting gives Dirichlet traces in $H^{1/2}(Σ)$ and weak normal derivatives in $H^{-1/2}(Σ)$ without assuming smoothness of $Σ$. We prove that the transmission conditions define a self-adjoint realization of the formal operator $H_{F,0}+αδ_Σ$. We also obtain a boundary resolvent formula in terms of the free Stark resolvent and a bounded operator from $H^{-1/2}(Σ)$ to $H^{1/2}(Σ)$. The formula implies that the resolvent difference is compact on $L^2(\mathbb R^d)$. Consequently, if $F\ne0$, the essential spectrum of the interacting operator is $\mathbb R$. The result supplies a direct boundary reduction for an interface problem whose background potential is not bounded and is not translation invariant.