Selective Mott Physics as a Key to Iron Superconductors

We show that electron- and hole-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ are strongly influenced by a Mott insulator that would be realized for half-filled conduction bands. Experiments show that weakly and strongly correlated conduction electrons coexist in much of the phase diagram, a differentiation which increases with hole doping. This selective Mottness is caused by the Hund’s coupling effect of decoupling the charge excitations in different orbitals. Each orbital then behaves as a single-band doped Mott insulator, where the correlation degree mainly depends on how doped is each orbital from half filling. Our scenario reconciles contrasting evidences on the electronic correlation strength, implies a strong asymmetry between hole and electron doping, and establishes a deep connection with the cuprates.

Figure 1

Top: experimental mass enhancements in doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ assessed by different techniques (see the Supplemental Material [30]) [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]. The experimental phase diagram, including the antiferromagnetic metallic (AF) and the superconducting (SC) phases is reproduced as a background. The solid line is a fit of the specific heat data. ARPES and quantum oscillation data corresponding to a given doping represent the values estimated for the different sheets of the Fermi surface. Correlations increase monotonically as the filling is reduced, and the estimates from different techniques spread more and more (see text). Bottom: orbitally resolved mass enhancement calculated within local-density approximation (LDA)$+$slave-spin mean-field theory for doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (lines) and for ${\mathrm{KFe}}_2{\mathrm{As}}_2$ (squares).

Figure 2

Calculated orbitally resolved quasiparticle weight $Z_{\alpha }$ (upper panel) and electron population (middle panel) for doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and stoichiometric ${\mathrm{KFe}}_2{\mathrm{As}}_2$ (squares) within slave-spins mean field. Note that for each orbital $\alpha$, $Z_{\alpha }$ tends to vanish when $n_{\alpha }$ approaches half filling. Indeed, when plotting $Z_{\alpha }$ as a function of $n_{\alpha }$ for each orbital (lower panel), a striking linear behavior (typical of a doped single-band Mott insulator) appears showing that each orbital is only sensitive to its doping away from the half-filled Mott insulator found at a total population $n=5$, independently from the others. The dots signal the values for the stoichiometric compound, having a total population $n=6$. The same analysis is performed for dynamical cluster approximation (DCA) calculations on the bidimensional Hubbard model of Gull et al. [32], representative of the pseudogap phase of underdoped cuprates, pointing to a common mechanism.

Figure 3

Possible unified phase diagram for pnictides and cuprates. We plot here the experimental phase diagram for doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (from [45]) as a function of the average orbital doping. In the hypothesis that the magnetically ordered and orthorhombically distorted phase (light gray area) is accidentally favored by the total commensurate filling $n=6$ around the stoichiometric compound thus suppressing the superconductivity of the tetragonal phase, we artificially eliminate it and complete the superconducting dome (blue area). We then see that this dome is centered around 20% doping (per orbital) away from the half-filled Mott insulator (here at $n=5$), as in cuprates. In between, we find a strongly correlated Mott selective (and bad metallic [34]) phase, while at higher orbital dopings (corresponding, respectively, to the electron doped region in pnictides and the overdoped region in cuprates), we recover a moderately correlated Fermi liquid.