The excitation energy spectrum for a system with electron pairs   tunneling in a two-leg ladder has a doping depended gap
  A new model with a new Hamiltonian is offered as the means for studying properties of a system of strongly correlated electrons. Consideration of the simplest possible situation, namely a system on non-interacting electrons in a two-leg ladder, leads to an expression for the excitation energy spectrum with no energy gap at the half-filling and with an energy gap away from the half filling. 
Valentin Voroshilov
physics.gen-ph