Nonlocal string order parameter in the S=12 Kitaev-Heisenberg ladder

We study the spin-12 Kitaev-Heisenberg (KJ) model in a two-leg ladder. Without a Heisenberg interaction, the Kitaev phase in the ladder model has Majorana fermions with local Z2 gauge fields, and is usually described as a disordered phase without any order parameter. Here we identify a long-range nonlocal string order parameter (SOP) in the Kitaev phase which survives with a finite Heisenberg interaction. The SOP is obtained by relating the …