Nonlocal Parity Order in the Two-Dimensional Mott Insulator

The Mott insulator is characterized by having small deviations around the (integer) average particle density n, with pairs with n-1 and n + 1 particles forming bound states. In one dimension, the effect is captured by a nonzero value of a nonlocal string of parities, which instead vanishes in the superfluid phase where density fluctuations are large. Here, we investigate the interaction induced transition from the superfluid to the Mott insulator, in the paradigmatic Bose Hubbard model at n = 1. By means of quantum Monte Carlo simulations and finite size scaling analysis on L x M ladders, we explore the behavior of brane parity operators from one dimension (i.e., M = 1 and L -> 8) to two dimensions (i.e., M -> 8 and L -> 8). We confirm the conjecture that, adopting a standard definition, their average value decays to zero in two dimensions also in the insulating phase, evaluating the scaling factor of the perimeter law [S.P. Rath et al., Ann. Phys. (Berlin) 334, 256 (2013)]. Upon introducing a further phase in the brane parity, we show that its expectation value becomes nonzero in the insulator, while still vanishing at the transition to the superfluid phase. These quantities are directly accessible to experimental measures, thus providing an insightful signature of the Mott insulator.