LaFe$_{0.6}$Sb$_{2}$: Strongly to weakly correlated system with Ni doping*

Since the discovery of superconducting Ca$_{\mathrm{1-x}}$La$_{\mathrm{x}}$FeAs$_{2}$ with a T$_{\mathrm{c}}$ of 34 K [1], there has been an increasing interest in growing 112 iron pnictides in the search for high T$_{\mathrm{c}}$ superconductivity. We have grown large single crystals of LaFe$_{0.6}$Sb$_{2}$, which form in a tetragonal 112 structure with a significant amount of Fe vacancies, confirmed via single crystal x-ray diffraction. We present a doping study utilizing Ni which replaces both the Fe and vacancies while transforming the material from strongly to weakly correlated, as determined by low temperature heat capacity measurements. The Sommerfeld coefficient $\gamma $ of the undoped crystal is 50 mJ/mol Fe K$^{2}$, indicating a large mass enhancement, while LaNiSb$_{2}$ is 5 mJ/mol Ni K$^{2}$ with no vacancies and up to 18{\%} interstitial Ni according to energy-dispersive x-ray spectroscopy. When doping LaFeSb$_{2}$ with Ni, $\gamma $ remains constant when normalized per transition metal, possibly indicating a constant density of states. A divergence appears in C/T vs. T$^{2}$ once the vacancies are filled, at 89{\%} Ni, and the divergence remains until the LaNiSb$_{2}$ sample, which is a weakly correlated 1 K superconductor. [1] Katayama, et al. arXiv:1311.1303v1 (2013).