Non local parity order in the two-dimensional Mott insulator

TL;DR

This paper investigates the behavior of non-local parity order parameters in the two-dimensional Mott insulator phase of the Bose Hubbard model, revealing how a modified parity operator can serve as an experimental signature of the insulating state.

Contribution

It introduces a modified brane parity operator that remains non-zero in the 2D Mott insulator, extending the understanding of non-local order parameters beyond one dimension.

Findings

Standard brane parity decays to zero in 2D insulator

Modified parity remains non-zero in the insulator phase

Results are accessible to experimental detection

Abstract

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 non-zero value of a non-local "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 LxM ladders, we explore the behavior of "brane" parity operators for L going to infinity from one dimension (i.e., M=1) to two dimensions (i.e., M going to infinity). 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. (N.Y.) 334, 256 (2013)]. Upon introducing a further phase in the brane parity, we show that its expectation value becomes non-zero 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.