Investigating the Spectral Geometry of a Soft Wall
  The idealized theory of quantum vacuum energy density is a beautiful application of the spectral theory of differential operators with boundary conditions, but its conclusions are physically unacceptable. A more plausible model of a reflecting boundary that stays within linear spectral theory confines the waves by a steeply rising potential function, which can be taken as a power of one coordinate, z^\alpha. We report investigations of this model with considerable student involvement. An exact analytical solution with some numerics for \alpha=1 and an asymptotic (semiclassical) analysis of a related problem for \alpha=2 are presented. 
J. D. Bouas, S. A. Fulling, F. D. Mera, K. Thapa, C. S. Trendafilova,   and J. Wagner
quant-ph hep-th math-ph math.MP