DMFT-QE Symposium: March 3rd

Talk 1:

Quantum Thermalization in Infinite Dimensions

Marco Schiro, Institute of Physics at College de France

Isolated quantum many-body systems which thermalize under their own dynamics are expected to act as their own thermal baths, thereby loosing memory of initial conditions and bringing their local subsystems to thermal equilibrium. Here we show that the infinite-dimensional limit of a quantum lattice model, as described by Dynamical Mean-Field theory (DMFT), provides a natural framework to understand this self-consistent thermalization process. Using the Fermi-Hubbard model as working example, we demonstrate that the emergence of a self-consistent bath occurs via a sharp thermalization front, moving balistically and separating the initial condition from the long-time thermal fixed point. We characterize the full DMFT dynamics through an effective temperature for which we derive a travelling-wave equation of the Fisher-Kolmogorov-Petrovsky-Piskunov (FKPP) type. Our results provide a new angle to understand the onset of quantum thermalisation in closed isolated systems.

Talk 2:

Dynamical properties of the spin-boson model using real-time quantum Monte Carlo

Olga Goulko, UMass Boston

We discuss how the real-time dynamics of the spin-boson model (a two-state system coupled to a bath of non-interacting harmonic modes) can be studied using the numerically exact inchworm Monte Carlo algorithm. In particular, we consider the population difference between the two states for the sub-Ohmic spin-boson model across a range of system-bath couplings, sub-Ohmic exponents, and temperatures. From short- and intermediate-time dynamics starting from an initially decoupled state, we extract signatures of the zero-temperature quantum phase transition between localized and delocalized states. We show that the dynamical phase diagram thus obtained differs from the equilibrium phase diagram in both the values of critical couplings, and the associated critical exponents. We also identify and quantitatively analyze two competing mechanisms for the crossover between coherent oscillations and incoherent decay. Deep in the sub-Ohmic regime, the crossover is driven by the damping of the oscillation amplitude, while closer to the Ohmic regime the oscillation frequency itself tends to zero at large coupling. Finally, we discuss the effect of the bath temperature on the dynamics and the localization and coherence properties of the system.