  In this paper we explore the entanglement in orthogonal dimer-plaquette
Ising-Heisenberg chain, assembled between plaquette edges, also known as
orthogonal dimer plaquettes. The quantum entanglement properties involving an
infinite chain structure are quite important, not only because the mathematical
calculation is cumbersome but also because real materials are well represented
by infinite chain. Using the local gauge symmetry of this model, we are able to
map onto a simple spin-1 like Ising and spin-1/2 Heisenberg dimer model with
single effective ion anisotropy. Thereafter this model can be solved using the
decoration transformation and transfer matrix approach. First, we discuss the
phase diagram at zero temperature of this model, where we find five ground
states, one ferromagnetic, one antiferromagnetic, one triplet-triplet
disordered and one triplet-singlet disordered phase, beside a dimer
ferromagnetic-antiferromagnetic phase. In addition, we discuss the
thermodynamic properties such as entropy, where we display the residual
entropy. Furthermore, using the nearest site correlation function it is
possible also to analyze the pairwise thermal entanglement for both orthogonal
dimers, additionally we discuss the threshold temperature of the entangled
region as a function of Hamiltonian parameters. We find quite interesting thin
reentrance threshold temperature for one of the dimers, and we also discuss the
differences and similarities for both dimers.

Statistical Mechanics