Ultrarelativistic bound states in the shallow spherical well
  We determine approximate eigenvalues and eigenfunctions shapes for bound states in the $3D$ shallow spherical ultrarelativistic well. Existence thresholds for the ground state and first excited states are identified, both in the purely radial and orbitally nontrivial cases. This contributes to an understanding of how energy may be stored or accumulated in the form of bound states of Schr\"odinger - type quantum systems that are devoid of any mass. 
Mariusz Zaba, Piotr Garbaczewski
quant-ph hep-th math-ph math.MP math.SP