  Phase transitions at a finite (i.e. non-zero) temperature are typically
dominated by classical correlations, in contrast to zero temperature
transitions where quantum mechanics plays an essential role. Therefore, it is
natural to ask if there are any signatures of a finite temperature phase
transition in measures that are sensitive only to quantum correlations. Here we
study one such measure, namely, entanglement negativity, across finite
temperature phase transitions in several exactly solvable Hamiltonians and find
that it is a singular function of temperature across the transition. As an
aside, we also calculate the entanglement of formation exactly in a related,
interacting model.
