Complex phase diagram of Ba 1-x Na x Fe 2 As 2 : a multitude of phases striving for the electronic entropy

The low-temperature electronic phase diagram of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, obtained using high-resolution thermal-expansion and specific-heat measurements, is shown to be considerably more complex than previously reported, containing nine different phases. Besides the magnetic ${C}_{2}$ and reentrant ${C}_{4}$ phases, we find evidence for an additional, presumably magnetic, phase below the usual spin-density-wave transition, as well as a possible incommensurate magnetic phase. All these phases coexist and compete with superconductivity, which is particularly strongly suppressed by the ${C}_{4}$-magnetic phase due to a strong reduction of the electronic entropy available for pairing in this phase.