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 $(m^*/m\sim1.3)$ 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.