Janus-faced influence of the Hund's rule coupling in strongly correlated materials

We show that in multi-band metals the correlations are strongly affected by the Hund's rule coupling, which depending on the filling promotes metallic, insulating or bad-metallic behavior. The quasiparticle coherence and the proximity to a Mott insulator are influenced distinctly and, away from single- and half-filling, in opposite ways. A strongly correlated bad-metal far from a Mott phase is found there. We propose a concise classification of 3d and 4d transition-metal oxides within which the ubiquitous occurrence of strong correlations in Ru- and Cr-based oxides, as well as the recently measured high N\'eel temperatures in Tc-based perovskites are naturally explained.