Non-Linear Methods in Strongly Correlated Electron Systems

We analyze the structure of the group of (local) non-linear canonical transformations that exist in a system with n fermionic modes. To perform our study we develop an alternative framework to represent the generators of these canonical transformations; indeed we show how their definition, understanding and control is significantly improved using the Majorana fermion representation. These canonical transformations have the structure of a Lie group and we provide a representation for the elements of the Lie algebra that is very convenient both conceptually and practically (computationally): indeed our framework yields as side product an extremely effective tool to handle and work with SU(2n) Lie groups and algebras. Granting an enhanced control of the group of non-linear canonical transformation, our framework can be helpful in the study of strongly correlated electron systems, since it allows to easily identify fermionic degrees of freedom able to capture part of the correlations and thus may give a simpler representation of the Hamiltonian. Thanks to our analysis, also symmetry-based studies of the quantum Hamiltonians can be improved, since the simple representation of the generators of the canonical group permits to identify and understand otherwise hidden symmetries of difficult interpretation. The main aim of this work is to provide a comprehensive, general and scrupulous analysis of this framework, that we already applied in some circumstances. Therefore specific applications will not be presented in detail, since they can be found elsewhere, while only the formal and conceptual aspects will be developed thoroughly.