New possibilities for supersymmetry breakdown in quantum mechanics and second order irreducible Darboux transformations

New types of irreducible second order Darboux transformations for the one dimensional Schroedinger equation are described. The main feature of such transformations is that the transformation functions have the eigenvalues grater then the ground state energy of the initial (or reference) Hamiltonian. When such a transformation is presented as a chain of two first order transformations, an intermediate potential is singular and therefore intermediate Hamiltonian can not be Hermitian while the final potential is regular and the final Hamiltonian is Hermitian. Second derivative supersymmetric quantum mechanical model based on a transformation of this kind exhibits properties inherent to models with exact and broken supersymmetry at once.