Magnetic and orbital orders coupled to negative thermal expansion in Mott insulators Ca ₂Ru _1-xM _xO ₄ (M = Mn and Fe)

Ca ₂RuO ₄ is a structurally driven Mott insulator with a metal-insulator transition at T _MI = 357K, followed by a well-separated antiferromagnetic order at T _N = 110 K. Slightly substituting Ru with a 3d transition metal ion M effectively shifts T _MI by weakening the orthorhombic distortion and induces either metamagnetism or magnetization reversal below T _N. Moreover, M doping for Ru produces negative thermal expansion in Ca ₂Ru _1-xM _xO ₄ (M = Cr, Mn, Fe, or Cu); the lattice volume expands on cooling with a total volume expansion ratio, ΔV/V, reaching as high as 1%. The onset of the negative thermal expansion closely tracks T _MI and T _N, sharply contrasting classic negative thermal expansion that shows no relevance to electronic properties. In addition, the observed negative thermal expansion occurs near room temperature and extends over a wide temperature interval up to 300 K. These findings underscore new physics driven by a complex interplay between orbital, spin, and lattice degrees of freedom.