Crystal Growth and Electronic Phase Diagram of 4 d -doped Na 1- δ Fe 1- x Rh x As in Comparison to 3 d -doped Na 1- δ Fe 1- x Co x As

Single crystals of ${\mathrm{Na}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{T}}_{x}\mathrm{As}$ 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 $(0\ensuremath{\le}x\ensuremath{\le}0.013)$ is fully suppressed in the optimally doped sample. The superconducting transition temperature $({T}_{c})$ is enhanced from 10 K in ${\mathrm{Na}}_{1\ensuremath{-}\ensuremath{\delta}}\mathrm{FeAs}$ to 21 K in the optimally doped sample $(x=0.019)$ of the ${\mathrm{Na}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}\mathrm{As}$ 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 ${\mathrm{Na}}_{1\ensuremath{-}\ensuremath{\delta}}\mathrm{FeAs}$, suggesting a generic phase diagram for both dopants.