The excitation energy spectrum for a system with electron pairs tunneling in a two-leg ladder has a doping depended gap

Since the time of the first high temperature superconductor was discovered, there is no yet a commonly accepted explanation of this phenomenon. Many publications on the matter start from some plausible reasoning leading to establishing of the model Hamiltonian and a discussion of the structure of the ground state. That plausible reasoning represents the physical view of the authors and, as long as the Hamiltonian and the ground state are set, the next step is using various mathematical methods to analyze the properties of the model. Many approaches are based on the Hubbard model. The reason for using the Hibbard model is the fact that the parent state of a HTSC is an antiferromagnetic, which, when doped, exhibits many peculiar properties, including HTSC. However, the search for new models is continuing and might lead to new insights on the matter and help to advance understanding of the nature of HTSC.