Effects of alternating interactions and boundary conditions on quantum entanglement of three-leg Heisenberg ladder

The spin-12 three-leg antiferromagnetic Heisenberg spin ladder is studied under open boundary condition (OBC) and cylinder boundary condition (CBC), using the density matrix renormalization group and matrix product state methods, respectively. Specifically, we calculate the energy density, entanglement entropy, and concurrence while discussing the effects of interleg interaction J2 and the alternating coupling parameter gamma on these quantities. It is found that the introduction of gamma can completely reverse the concurrence distribution between odd and even bonds. Under CBC, the generation of the interleg concurrence is inhibited when gamma=0, and the introduction of gamma can cause interleg concurrence between chains 1 and 3, in which the behavior is more complicated due to the competition between CBC and gamma. Additionally, we find that gamma induces two types of long-distance entanglement (LDE) in the system under OBC: intraleg LDE and inter-leg one. When the system size is sufficiently large, both types of LDE reach similar strength and stabilize at a constant value. The study indicates that the three-leg ladder makes it easier to generate LDE compared with the two-leg system. However, the generation of LDE is inhibited under CBC which the spin frustration exists. In addition, the calculated results of energy, entanglement entropy and concurrence all show that there are essential relations between these quantities and phase transitions of the system. Further, we predict a phase transition point near gamma=0.54 under OBC. The present study provides valuable insights into understanding the phase diagram of this class of systems.