Carleman Estimates and Absence of Embedded Eigenvalues

Let L = −∆− W be a Schrödinger operator with a potential W ∈ L n+1 2 (R), n ≥ 2. We prove that there is no positive eigenvalue. The main tool is an L − Lp′ Carleman type estimate, which implies that eigenfunctions to positive eigenvalues must be compactly supported. The Carleman estimate builds on delicate dispersive estimates established in [7]. We also consider extensions of the result to variable coefficient operators with long range and short range potentials and gradient potentials.