On the semi-classical analysis of Schr\"odinger operators with purely imaginary electric potentials in a bounded domain

In this paper, we describe the leftmost eigenvalue of the non-selfadjoint operator $\mathcal{A}_h = -h^2\Delta+iV(x)$ with Dirichlet boundary conditions on a smooth bounded domain $\Omega\subset\mathbb{R}^n\,$, as $h\rightarrow0\,$. $V$ is assumed to be a Morse function without critical point at the boundary of $\Omega\,$. More precisely, we compare $\inf\Re\sigma(\mathcal{A}_h)$ with the minimum of the spectrum's real part for some model operator. In the case where $V$ has no critical point, the spectrum is determined by the boundary points where $\nabla V$ is orthogonal, and the model operator involves a $1$-dimensional complex Airy operator in $\mathbb{R}^+\,$. If $V$ is a Morse function with critical points in $\Omega\,$, the behavior of the operator near the critical points prevails, and the model operator is a complex harmonic oscillator. This question is related to the decay of associated semigroups. In particular, it allows to recover, in a simplified setting, some stability results by Almog in superconductivity theory.