We study the influence of Hund's coupling on the nature of electronic correlations in ${\mathrm{Sr}}_2{\mathrm{CoO}}_4$ within dynamical mean-field theory. Our results suggest strong signatures of Hund's metal physics in this compound. The Co $3d$ states show a large orbital differentiation in the degree of correlations and mass enhancement. The imaginary-time correlation functions suggest the presence of spin-orbital separation and large local charge fluctuations in the system. A breakdown of the Fermi-liquid picture is observed at the lowest calculated temperature for various strengths of Hund's coupling, suggesting a Fermi-liquid coherence scale lower than $\sim 100$ K. Interestingly, a sudden emergence of a gapped state is noted for $e_g$ orbitals in its spectral density of states at $\sim 200$ K in the vicinity of the Fermi level. Among the Co $3d$ states, $3d_{z^2}$ and $3d_{x^2-y^2}$ foster enlarged correlations. This study conclusively identifies ${\mathrm{Sr}}_2{\mathrm{CoO}}_4$ as a single-layered $3d$ transition metal oxide to be classified as a Hund's metal.

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• Electronic structure
• Strongly correlated systems
• DFT+DMFT
• First-principles calculations
• Many-body techniques