Recent experiments on two iron-pnictide families suggest the existence of a single quantum phase transition (QPT) inside the superconducting dome despite the fact that two separate transition lines - magnetic and nematic - cross the superconducting dome at Tc. Here we argue that these two observations are actually consistent. We show, using a microscopic model, that each order coexists with superconductivity for a wide range of parameters, and both transition lines continue into the superconducting dome below Tc. However, at some Tmerge<Tc, the two transitions merge and continue down to T=0 as a single simultaneous first-order nematic/magnetic transition. We show that superconductivity has a profound effect on the character of this first-order transition, rendering it weakly first-order and allowing strong fluctuations to exist near the QPT.
@article{arxiv.1305.4670,
title = {How many quantum phase transitions exist inside the superconducting dome of the iron pnictides?},
author = {Rafael M. Fernandes and Saurabh Maiti and Peter Wölfle and Andrey V. Chubukov},
journal= {arXiv preprint arXiv:1305.4670},
year = {2013}
}