Two topics in one talk
We describe a variational formulation of Monte Carlo renormalization group. The process of solving the variational problem greatly alleviates many sampling difficulties related to the Monte Carlo simulations, in both pure and quench-disordered systems. We demonstrate the utility of the method in both classical and quantum, pure and disordered systems.
We provide numerical evidence that after a local quench in an isolated infinite quantum spin chain, the quantum state locally relaxes to the ground state of the post-quenched Hamiltonian, i.e. dissipates. This is a consequence of the unitary quantum dynamics. A mechanism similar to the eigenstate thermalization hypothesis is shown to be responsible for the dissipation observed. We also demonstrate that integrability obstructs dissipation. The numerical simulations are done directly in the thermodynamic limit with a time-evolution algorithm based on matrix product states. The area law of entanglement entropy is observed to hold after the local quench. As a result, the simulations can be performed for long times with small bond dimensions. Various local quenches on the Ising chain and the three-state Potts chain are studied.