Search for: All records

We study the chaotic properties of a large-spin XXZ chain with onsite disorder and a small number of excitations above the fully polarized state. We show that while the classical limit, which is reached for large spins, is chaotic, enlarging the spin suppresses quantum chaos features. We examine ways to facilitate chaos by introducing additional terms to the Hamiltonian. Interestingly, perturbations that are diagonal in the basis of product states in the z-direction do not lead to significant enhancement of chaos, while off-diagonal perturbations restore chaoticity for large spins, so that only three excitations are required to achieve strong level repulsion and ergodic eigenstates. 

We provide bounds on temporal fluctuations around the infinite-time average of out-of-time-ordered and time-ordered correlators of many-body quantum systems without energy gap degeneracies. For physical initial states, our bounds predict the exponential decay of the temporal fluctuations as a function of the system size. We numerically verify this prediction for chaotic and interacting integrable spin-1/2 chains, which satisfy the assumption of our bounds. On the other hand, we show analytically and numerically that for the XX model, which is a noninteracting system with gap degeneracies, the temporal fluctuations decay polynomially with system size for operators that are local in the fermion representation and decrease exponentially in the system size for non-local operators. Our results demonstrate that the decay of the long-time temporal fluctuations of correlators cannot be used as a reliable metric of chaos or lack thereof. 

We analyze and discuss convergence properties of a numerically exactalgorithm tailored to study the dynamics of interacting two-dimensionallattice systems. The method is based on the application of the time-dependentvariational principle in a manifold of binary and quaternary TreeTensor Network States. The approach is found to be competitive withexisting matrix product state approaches. We discuss issues relatedto the convergence of the method, which could be relevant to a broaderset of numerical techniques used for the study of two-dimensionalsystems. 

We numerically investigate the minimum number of interacting particles,which is required for the onset of strong chaos in quantum systemson a one-dimensional lattice with short-range and long-range interactions.We consider multiple system sizes which are at least three times largerthan the number of particles and find that robust signatures of quantumchaos emerge for as few as 4 particles in the case of short-rangeinteractions and as few as 3 particles for long-range interactions,and without any apparent dependence on the size of the system.