Asymptotics of the Schrödinger operator levels for a crystal film with a nonlocal potential

We consider a three-dimensional Schrödinger operator for a crystal film with a nonlocal potential, which is a sum of an operator of multiplication by a function, and an operator of rank two (“separable potential”) of the form $V=W (x) +\lambda _1(\cdot,\phi _1)\phi _1+\lambda _2(\cdot,\phi _2)\phi _2$. Here the function $W(x)$ decreases exponentially in the variable $x_3$, the functions $\phi _1(x)$, $\phi _2(x)$ are linearly independent, of Bloch type in the variables $x_1,\,x_2$ and exponentially decreasing in the variable $x_3$. Potentials of this type appear in the pseudopotential theory. A level of the Schrödinger operator is its eigenvalue or resonance. The existence and uniqueness of the level of this operator near zero is proved, and its asymptotics is obtained.