Sr${}_2$NbO${}_4$: A $4d$ analogue of the layered perovskite Sr${}_2$VO${}_4$

This work focuses on the layered perovskite Sr${}_2$NbO${}_4$, a 4$d$ analogue of Sr${}_2$VO${}_4$, which remains an unsolved puzzle with a possible intriguing hidden magnetic order. Using density functional theory (DFT) calculations, we demonstrate the robust thermodynamic stability and exfoliability of Sr${}_2$NbO${}_4$, suggesting potential applications as a 2D material. Imperfect Fermi surface nesting indicates instabilities that may drive symmetry lowering, charge/orbital density waves, or superconductivity. Dynamical mean-field theory (DMFT) calculations reveal moderate mass renormalization $(m^{\ast }\mathrm{/}m\sim 1.3)$ and an itinerant character of magnetism with strong longitudinal spin fluctuations. The exchange interaction is dominated by in-plane ferromagnetic coupling with much weaker interlayer antiferromagnetic exchange.