Janus-Faced Influence of Hund’s Rule Coupling in Strongly Correlated Materials

We show that in multiband metals the correlations are strongly affected by 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 Néel temperatures in Tc-based perovskites are naturally explained.

Figure 1

Quasiparticle weight $Z$ vs $U$ for $N=1$, 2, 3 electrons in $M=3$ orbitals. The grey arrows indicate the influence of an increasing Hund’s rule coupling $J/U$.

Figure 2

Quasiparticle weight $Z$ in a model with 3 orbitals, for $J/U=0.15$, as a function of the interaction strength $U$ and the number of electrons—from empty (0) to full (6). Darker regions correspond to good metals and lighter regions to bad metals. The black bars signal the Mott-insulating phases. One notes that, among integer fillings, the case of 2 electrons (2 holes) displays bad-metal behavior in an extended range of coupling. Specific materials are schematically placed on the diagram (see text).