Unified Phase Diagram for Iron-Based Superconductors

High-temperature superconductivity is closely adjacent to a long-range antiferromagnet, which is called a parent compound. In cuprates, all parent compounds are alike and carrier doping leads to superconductivity, so a unified phase diagram can be drawn. However, the properties of parent compounds for iron-based superconductors show significant diversity and both carrier and isovalent dopings can cause superconductivity, which casts doubt on the idea that there exists a unified phase diagram for them. Here we show that the ordered moments in a variety of iron pnictides are inversely proportional to the effective Curie constants of their nematic susceptibility. This unexpected scaling behavior suggests that the magnetic ground states of iron pnictides can be achieved by tuning the strength of nematic fluctuations. Therefore, a unified phase diagram can be established where superconductivity emerges from a hypothetical parent compound with a large ordered moment but weak nematic fluctuations, which suggests that iron-based superconductors are strongly correlated electron systems.

Figure 1

Temperature dependence of ${|{\chi }_n|}^{-1}$ for various iron-based superconductors. The data include $R\mathrm{FeAsO}$ ($R=\mathrm{La}$, Ce, Pr, Nd), ${\mathrm{NaFe}}_{1-x}{\mathrm{Ni}}_x\mathrm{As}$ ($x=0$, 0.015), FeSe, ${\mathrm{BaFe}}_2({\mathrm{As}}_{0.69}{\mathrm{P}}_{0.31}{)}_2$, ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{Fe}}_2{\mathrm{As}}_2$, and ${\mathrm{Ca}}_{0.82}{\mathrm{La}}_{0.18}{\mathrm{FeAs}}_2$. Only one set of data is shown for each kind of materials. For FeSe, ${\mathrm{NaFe}}_{0.985}{\mathrm{Ni}}_{0.015}\mathrm{As}$, ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{Fe}}_2{\mathrm{As}}_2$, and ${\mathrm{Ca}}_{0.82}{\mathrm{La}}_{0.18}{\mathrm{FeAs}}_2$, the values of ${\chi }_n$ are negative. The solid lines are linear fitting results.

Figure 2

(a) Scaling between ordered moment $M$ and ${|A_n|}^{-1}$. The straight dashed line in the front panel is guided to the eye according to the data of ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$. Superconductivity (SC) tends to appear with sufficient small ${|A_n|}^{-1}$. The values and error bars of magnetic moments are from previous measurements by neutron diffraction [5, 6, 7, 8, 9, 10, 11, 39, 54]. The vertical error bars are the standard deviations of multiple measurements. The values of $A_n$ for ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$ are obtained from previous works [34] assuming Fermi velocities change little with Ni doping. The data points of ${\mathrm{BaFe}}_2({\mathrm{As}}_{0.69}{\mathrm{P}}_{0.31}{)}_2$, ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{Fe}}_2{\mathrm{As}}_2$, FeSe, and ${\mathrm{NaFe}}_{1.985}{\mathrm{Ni}}_{0.015}\mathrm{As}$ overlap each other. (b) Schematic phase diagram of iron-based superconductors where superconductivity (SC) is achieved by tuning ${|A_n|}^{-1}$ to suppress the AFM order in a hypothetical parent compound (HPC). It should be pointed out that the values of $T_N$ and $T_c$ are material dependent.