Hole-doped ReFe₂As₂ (Re = Ba, Sr, Ca) exhibit much richer phase diagrams than the corresponding electron-doped systems. In particular, the phase diagram of Na-doped BaFe₂As₂ exhibits a small pocket of a double-Q reentrant C4 magnetic phase [1], as well as another yet unidentified magnetic phase [2]. In strong analogy with the charge order observed in underdoped cuprates [3], these additional phases compete strongly with the emerging superconducting order [2,4].

Here we present a detailed phase diagram of the Na-doped SrFe₂As₂ system using thermodynamic probes (heat capacity, thermal expansion and magnetization). The double-Q C4 reentrant phase is much more stable in this system, and our data demonstrates that the phase diagram of Na-doped SrFe₂As₂ exhibits even more complexity than the K- and Na-doped BaFe₂As₂ counterparts.

[1] J. M. Allred et al., Nature Physics 12, 493–498 (2016).
[2] L. Wang et al., Phys. Rev. B93, 014514 (2016).
[3] B. Keimer et al. Nature 518, 179 (2015).
[4] A. E. Boehmer et al., Nat. Commun. 6, 7911 (2015).