title: Fine structure of the entanglement entropy in the O(2) model.
abstract: We compare two calculations of the particle density in the superfluid phase of the O(2) model with a chemical potential μ in 1+1 dimensions. The first relies on exact blocking formulas from the Tensor Renormalization Group (TRG) formulation of the transfer matrix. The second is a worm algorithm. We show that the particle number distributions obtained with the two methods agree well. We use the TRG method to calculate the thermal entropy and the entanglement entropy. We describe the particle density, the two entropies and the topology of the world lines as we increase μ to go across the superfluid phase between the first two Mott insulating phases. For a sufficiently large temporal size, this process reveals an interesting fine structure: the average particle number and the winding number of most of the world lines in the Euclidean time direction increase by one unit at a time. At each step, the thermal entropy develops a peak and the entanglement entropy increases until we reach half-filling and then decreases in a way that approximately mirrors the ascent. This suggests an approximate fermionic picture.
authors: Department of Physics, Chongqing University, Chongqing 401331, China.
authors: Li-Ping
authors: LP
authors: Yang
authors: Department of Physics, University of Colorado, Boulder, Colorado 80309, USA.
authors: Yuzhi
authors: Y
authors: Liu
authors: Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
authors: Haiyuan
authors: H
authors: Zou
authors: Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China.
authors: Z Y
authors: ZY
authors: Xie
authors: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, USA.
authors: Y
authors: Y
authors: Meurice