%0 Journal Article %T A spectral isoperimetric inequality for cones %A Pavel Exner %A Vladimir Lotoreichik %J Mathematics %D 2015 %I arXiv %X In this note we investigate three-dimensional Schr\"odinger operators with $\delta$-interactions supported on $C^2$-smooth cones, both finite and infinite. Our main results concern a Faber-Krahn-type inequality for the principal eigenvalue of these operators. The proofs rely on the Birman-Schwinger principle and on the fact that circles are unique minimisers for a class of energy functionals. %U http://arxiv.org/abs/1512.01970v1