Crystal growth and the electronic phase diagram of the 4d doped Na_(1-δ)Fe_(1-x)Rh_xAs in comparison with 3d doped Na_(1-δ)Fe_(1-x)Co_xAs

Single crystals of Na$_{1-\delta}$Fe$_{1-x}$T$_x$As with T = Co, Rh have been grown using a self-flux technique. The crystals were thoroughly characterized by powder X-ray diffraction, magnetic susceptibility and electronic transport with particular focus on the Rh-doped samples. Measurements of the specific heat and ARPES were conducted exemplarily for the optimally doped compositions. The spin-density wave transition (SDW) observed for samples with low Rh concentration ($0\,\leq\,x\,\leq\,0.013$) is fully suppressed in the optimally doped sample. The superconducting transition temperature ($T_c$) is enhanced from $10$~K in Na$_{1-\delta}$FeAs to $21$~K in the optimally doped sample ($x$ = 0.019) of the Na$_{1-\delta}$Fe$_{1-x}$Rh$_x$As series and decreases for the overdoped compounds, revealing a typical shape for the superconducting part of the electronic phase diagram. Remarkably, the phase diagram is almost identical to that of Co-doped Na$_{1-\delta}$FeAs, suggesting a generic phase diagram for both dopants.