  We show that the nearest neighbour entanglement in a mixture of ground and
first excited states - a subjacent state - of the J1-J2 Heisenberg quantum spin
chain can be used as an order parameter to detect the phase transition of the
chain from a gapless spin fluid to a gapped dimer phase. We study the
effectiveness of the order parameter for varying relative mixing probabilities
between the ground and first excited states in the subjacent state for
different system sizes, and extrapolate the results to the thermodynamic limit.
We observe that the nearest neighbour concurrence can play a role of a good
order parameter even if the system is in the ground state, but with a small
finite probability of leaking into the first excited state. Moreover, we apply
the order parameter of the subjacent state to investigate the response to
separate introductions of anisotropy and of glassy disorder on the phase
diagram of the model, and analyse the corresponding finite-size scale exponents
and the emergent tricritical point in the former case. The anisotropic J1-J2
chain has a richer phase diagram which is also clearly visible by using the
same order parameter.
