Density functional calculations of electronic structure and magnetic properties of the hydrocarbon ${\mathrm{K}}_3$picene superconductor near the metal-insulator transition

We have investigated the electronic structures and magnetic properties of a newly discovered hydrocarbon superconductor, ${\mathrm{K}}_3$picene, having $T_c$ $=$ 18 K. We have shown that the metal-insulator transition is driven in ${\mathrm{K}}_3$picene by 5% volume enhancement with the formation of the local magnetic moment. Active bands for the superconductivity near the Fermi level ($E_F$) have hybridized character of LUMO and LUMO$+$1 of the picene molecule. Fermi surfaces of ${\mathrm{K}}_3$picene manifest neither prominent nesting feature nor marked two-dimensional behavior. By estimating the ratio of the Coulomb interaction, $U$, and the bandwidth, $W$, of the active bands near $E_F$, we have demonstrated that ${\mathrm{K}}_3$picene is located in the vicinity of the Mott transition. Our findings suggest that ${\mathrm{K}}_3$picene is a strongly correlated electron system.

Figure 1

(a) Crystal structure of ${\mathrm{K}}_3$picene. There are two stacked picene molecules in a unit cell. K atoms are intercalated between the stacked picene molecules. (b) Molecular structure of a picene.

Figure 2

DOS for NM (blue solid lines) and AFM (red dotted lines) phases of ${\mathrm{K}}_3$picene. Inset shows the amplified DOS near $E_F$. (a) ${\mathrm{K}}_3$picene with the original volume (K${}_3$P[OV]), and (b) ${\mathrm{K}}_3$picene with 5% volume expansion (K${}_3$P[EV]).

Figure 3

(a) Band structure for NM phase of ${\mathrm{K}}_3$picene with original volume (K${}_3$P[OV]). (b) Band structure for AFM phase of ${\mathrm{K}}_3$picene with 5% volume expansion (K${}_3$P[EV]). Notice that two bands near $E_F$ are split into U and L which are represented by blue (dark) color.

Figure 4

Charge densities (CDs) of (a) U and (b) L bands in Fig. 3. CDs of (c) LUMO $+$ 1 and (d) LUMO orbitals of a picene molecule. CD of the U band shows mostly LUMO $+$ 1 character with small hybridization with LUMO, while CD of the L band shows hybridized character of LUMO and LUMO$+$1.

Figure 5

Fermi surfaces of ${\mathrm{K}}_3$picene. (a) NM phase of ${\mathrm{K}}_3$P[OV], (b) AFM phase of ${\mathrm{K}}_3$P[OV], and (c) NM phase of ${\mathrm{K}}_3$P[EV].