First-principles structural optimization and electronic structure of the superconductor picene for various potassium doping levels

We theoretically explore the crystal structures of Kₓpicene for which a new aromatic superconductivity has recently been discovered for $x=3$, by systematically performing first-principles full structural optimization covering the concentration range $x=1$--4. The crystal symmetry (space group) of the pristine picene is shown to be preserved in all the optimized structures despite significant deformations of each picene molecule and vast rearrangements of herringbone array of molecules. For Kₓpicene ($x=1$--4), optimization indicates that (i) multiple structures exist in some cases and (ii) dopants can enter in the intralayer region as well as in the interlayer region between the stack of herringbone structures. In the electronic structure obtained with the local density approximation for the optimized structures, the rigid-band approximation is invalidated for multiple reasons; the dopants affect the electronic properties not only through the rearrangement and distortion of molecules, but also through hybridizations between the molecules and metal atoms. As a consequence, the resultant Fermi surface exhibits a variety of multiband structures, which take diverse topology for K₁ and K₃picenes.