Schroedinger equations with indefinite effective mass
  The consistency of the concept of quantum (quasi)particles possessing effective mass which is both position- and excitation-dependent is analyzed via simplified models. It is shown that the system may be stable even when the effective mass m=m(x,E) itself acquires negative values in a limited range of coordinates x and energies E. 
Miloslav Znojil and G\'eza L\'evai
quant-ph math-ph math.MP