General principles of statistical mechanics, P.A. Pearce the connection between statistical mechanics and quantum field theory, B.M. McCoy two-dimensional models in statistical mechanics, R.J. Baxter topics in conformal field theory, C. Itzykson quantum gravity as dynamical triangulation, A.A. Migdal lattice gauge theory, C.J. Hamer quantum groups for physicists, M. Gould chaos versus order in Hamiltonian dynamical systems, G.R.W. Quispel quantum and classical integrable systems and statistical mechanics, R.K. Bullough and J.T. Timonen introduction to quantum field theory, R.J. Crewther.
We investigate the presence of quantum chaos in the spectrum of the bidimensional Fermi liquid by means of analytical and numerical methods. This model is integrable in a certain limit by bosonization of the Fermi surface. We study the effect on the level statisticsof the momentum cutoff
Λ present in the bidimensional bosonization procedure. We first analyse the level spacing statistics in the
Λ-restricted Hilbert space in one dimension. With
g2 and
g4 interactions, the level statistics are found to be Poissonian at low energies, and G.O.E. at higher energies, for a given cut-off
Λ. In order to study this cross-over, a finite temperature is introduced as a way of focussing, for a large inverse temperature
β, on the low energy many-body states, and driving the statistics from G.O.E. to Poissonian. As far as two dimensions are concerned, we diagonalize the Fermi liquid Hamiltonian with a small number of orbitals. The level spacing statistics are found to be Poissonian in the
Λ-restricted Hilbert space, provided the diagonal elements are of the same order of magnitude as the off-diagonal matrix elements of the Hamiltonian.
Krylov complexity has recently emerged as a new paradigm to characterize quantum chaos in many-body systems. However, which features of Krylov complexity are a prerogative of quantum chaotic systems and how they relate to more standard probes, such as spectral statistics or out-of-time-order correlators (OTOCs), remain open questions. Recent insights have revealed that in quantum chaotic systems Krylov state complexity exhibits a distinct peak during time evolution before settling into a well-understood late-time plateau. In this work we propose that this Krylov complexity peak (KCP) is a hallmark of quantum chaotic systems and suggest that its height could serve as an order parameter for quantum chaos. We demonstrate that the KCP effectively identifies chaotic-integrable transitions in two representative quantum-mechanical models at both infinite and finite temperature: the mass-deformed Sachdev-Ye-Kitaev model and the sparse Sachdev-Ye-Kitaev model. Our findings align with established results from spectral statistics and OTOCs while introducing an operator-independent diagnostic for quantum chaos, offering more universal insights and a deeper understanding of the general properties of quantum chaotic systems.