  In this paper, we prove a limiting absorption principle for high-order
Schr\"odinger operators with a large class of potentials which generalize some
results by A. Ionescu and W. Schlag. Our main idea is to handle the boundary
operators by the restriction theorem of Fourier transform. Two key tools we use
in this paper are the Stein--Tomas theorem in Lorentz spaces and a sharp trace
lemma given by S. Agmon and L. H\"ormander
