Hund’s Induced Fermi-Liquid Instabilities and Enhanced Quasiparticle Interactions

Hund’s coupling is shown to generally favor, in a doped half-filled Mott insulator, an increase in the compressibility culminating in a Fermi-liquid instability towards phase separation. The largest effect is found near the frontier between an ordinary and an orbitally decoupled (“Hund’s”) metal. The increased compressibility implies an enhancement of quasiparticle scattering, thus favoring other possible symmetry breakings. This physics is shown to happen in simulations of the 122 Fe-based superconductors, possibly implying the relevance of this mechanism in the enhancement of the critical temperature for superconductivity.

Figure 1

Degenerate 2-orbital (left-hand panel) and 3-orbital (right-hand panel) Hubbard model with semicircular DOS of half-bandwidth $D$ and Ising-type Hund’s coupling $J/U=0.25$: $\mu$ versus $n$ curves for different values of $U$. In the 2-orbital case for $U_c/D=1.96$ at half filling ($n=2$ in this model), the system undergoes a Mott transition. The curves for $U>U_c$ show a negative slope inside a spinodal line departing from the Mott transition that is marked with black circles. In the 3-orbital case most of the $\mu$ versus $n$ curves for $U>U_c=1.515$ show a double change of slope (the same happens in the 5-orbital model [13]), so that the instability zone extends between two spinodal lines (black squares), both at finite doping from half filling ($n=3$ here).

Figure 2

Left-hand panel: The colored areas are the zones of instability towards phase separation in the $U$-doping plane for the 2-, 3-, and 5-orbital models, with $J/U=0.25$. The low-$U$ frontier departs from the Mott transition point at half filling (symbols). For a growing number of orbitals the unstable zone extends to larger and larger doping, albeit narrowing in $U$. Right-hand panel: A plausible mechanism by which Hund’s coupling causes the charge instability. Hund’s coupling reduces the width of the Hubbard bands at half filling, through the quenching of orbital fluctuations. Upon doping the quenching is lifted and the Hubbard bands are expected to expand (going from $\sim W$ to some larger value $\tilde{W}$). This makes it possible to have a lower chemical potential at larger particle density, i.e., a charge instability.

Figure 3

Lower left-hand panel: Compressibility (color scale) in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ calculated within $\mathrm{DFT}+\mathrm{SSMF}$ ($J/U=0.25$) in the $U$-density plane. The saturated yellow color corresponds to the unstable region (the pixelation is due to numerical discretization and is unphysical), surrounded by an area of enhanced compressibility (red). Upper panels: Compressibility plotted along the cuts (dashed lines) for constant $U=2.7\mathrm{eV}$ and constant density $n=6$, relevant values for ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. Lower right-hand panel: Orbitally resolved mass enhancements, showing that the compressibility enhancement happens near the crossover, where correlations become orbitally selective.