Order, disorder and phase transitions in quantum many body systems

In this paper, I give an overview of some selected results in quantum many body theory, lying at the interface between mathematical quantum statistical mechanics and condensed matter theory. In particular, I discuss some recent results on the universality of transport coefficients in lattice models of interacting electrons, with specific focus on the independence of the quantum Hall conductivity from the electron-electron interaction. In this context, the exchange of ideas between mathematical and theoretical physics proved particularly fruitful, and helped in clarifying the role played by quantum conservation laws (Ward Identities), together with the decay properties of the Euclidean current-current correlation functions, on the interaction-independence of the conductivity.Comment: 35 pages, 7 figures. These notes are based on a presentation given at the Istituto Lombardo, Accademia di Scienze e Lettere, in Milano (Italy) on May 5, 2016, as well as on the notes of a course given at the EMS-IAMP summer school in mathematical physics `Universality, Scaling Limits and Effective Theories', held in Roma (Italy) on July 11-15, 2016. Final version, accepted for publicatio

On Realism and Quantum Mechanics

A discussion of the quantum mechanical use of superposition or entangled states shows that descriptions containing only statements about state vectors and experiments outputs are the most suitable for Quantum Mechanics. In particular, it is shown that statements about the undefined values of physical quantities before measurement can be dropped without changing the predictions of the theory. If we apply these ideas to EPR issues, we find that the concept of non-locality with its 'instantaneous action at a distance' evaporates. Finally, it is argued that usual treatments of philosophical realist positions end up in the construction of theories whose major role is that of being disproved by experiment. This confutation proves simply that the theories are wrong; no conclusion about realism (or any other philosophical position) can be drawn, since experiments deal always with theories and these are never logical consequences of philosophical positions.Comment: 13 pages. Accepted for publication in Il Nuovo Cimento

High density limit of the two-dimensional electron liquid with Rashba spin-orbit coupling

We discuss by analytic means the theory of the high-density limit of the unpolarized two-dimensional electron liquid in the presence of Rashba or Dresselhaus spin-orbit coupling. A generalization of the ring-diagram expansion is performed. We find that in this regime the spin-orbit coupling leads to small changes of the exchange and correlation energy contributions, while modifying also, via repopulation of the momentum states, the noninteracting energy. As a result, the leading corrections to the chirality and total energy of the system stem from the Hartree-Fock contributions. The final results are found to be vanishing to lowest order in the spin-orbit coupling, in agreement with a general property valid to every order in the electron-electron interaction. We also show that recent quantum Monte Carlo data in the presence of Rashba spin-orbit coupling are well understood by neglecting corrections to the exchange-correlation energy, even at low density values.Comment: 11 pages, 5 figure

Quasi-periodic solutions for quasi-linear generalized KdV equations

We prove the existence of Cantor families of small amplitude, linearly stable, quasi-periodic solutions of quasi-linear autonomous Hamiltonian generalized KdV equations. We consider the most general quasi-linear quadratic nonlinearity. The proof is based on an iterative Nash-Moser algorithm. To initialize this scheme, we need to perform a bifurcation analysis taking into account the strongly perturbative effects of the nonlinearity near the origin. In particular, we implement a weak version of the Birkhoff normal form method. The inversion of the linearized operators at each step of the iteration is achieved by pseudo-differential techniques, linear Birkhoff normal form algorithms and a linear KAM reducibility scheme.Comment: arXiv admin note: substantial text overlap with arXiv:1404.3125, arXiv:1508.02007, arXiv:1602.02411 by other author