Universal relationship between low-energy antiferromagnetic fluctuations and superconductivity in BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$

Abstract

To identify the key parameter for optimal superconductivity in iron pnictides, we measured the ${}^{31}$P-NMR relaxation rate on BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ ($x=0.22$ and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For $x=0.22$, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4~GPa, whereas the superconducting transition temperature $T_{\mathrm{c}}$ increases to twice its former value. In contrast, for $x=0.28$ near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and $T_{\mathrm{c}}$ reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the $T_{\mathrm{c}}$ dome as a function of Weiss temperature $\hat{I}¸$, which measures the distance to the QCP. Moreover, the $T_{\mathrm{c}}$-$\hat{I}¸$ curve under pressure precisely coincides with that with chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$.

5 pages, 6 figures