Hund-Enhanced Electronic Compressibility in FeSe and its Correlation with $T_c$

We compute the compressibility of the conduction electrons in both bulk orthorhombic FeSe and monolayer FeSe on a ${\mathrm{SrTiO}}_3$ substrate, including dynamical electronic correlations within slave-spin mean-field $+$ density-functional theory. Results show a zone of enhancement of the electronic compressibility crossing the interaction-doping phase diagram of these compounds in accord with previous simulations on iron pnictides and in general with the phenomenology of Hund’s metals. Interestingly, at ambient pressure FeSe is found slightly away from the zone with enhanced compressibility but moved right into it with hydrostatic pressure, while in monolayer FeSe the stronger enhancement region is realized on the electron-doped side. These findings correlate positively with the enhancement of superconductivity seen in experiments, and support the possibility that Hund’s induced many-body correlations boost superconductive pairing when the system is at the frontier of the normal- to Hund’s metal crossover.

Figure 1

Color map of the electronic compressibility ${\kappa }_{\text{el}}=dn/d\mu$ (in color scale) of FeSe. White color denotes a divergence in the electronic compressibility. From left to right: electronic compressibility for 0.0, 6.6, and 9.0 GPa cases and for a monolayer of FeSe over a substrate of STO. Vertical yellow dashed lines represent the stoichiometric filling of bulk FeSe $n=6.0$ (while for $\mathrm{FeSe}/\mathrm{STO}$—rightmost panel—it is set to $n=6.1$ instead to illustrate its electron-doped character) and horizontal lines our estimated value of $U=4.2\mathrm{eV}$ for this system. The crossing point locates the stoichiometric compound in this $U$-filling plane.

Figure 2

Upper panels: electronic compressibility as a function of the on-site Coulomb interaction $U$. Middle-upper panels: quasiparticle weights of the different orbitals as a function of $U$. Middle-lower panels: mass enhancement of the different orbitals ($1/Z_m$) as a function of $U$. Lower panels: total local spin-spin correlation function as a function of $U$. All calculations are performed for $J/U=0.2$. The electron filling is fixed to 6.0 for FeSe bulk and to 6.1 for $\mathrm{FeSe}/\mathrm{STO}$.