Absence of metallicity in K-doped picene: Importance of electronic   correlations
  Potassium-doped picene (K$_x$picene) has recently been reported to be a superconductor at $x=3$ with critical temperatures up to 18 K. Here we study the electronic structure of K-doped picene films by photoelectron spectroscopy and {\it ab initio} density functional theory combined with dynamical mean-field theory (DFT+DMFT). Experimentally we observe that, except for spurious spectral weight due to the lack of a homogeneous chemical potential at low K-concentrations ($x \approx 1$), the spectra always display a finite energy gap. This result is supported by our DFT+DMFT calculations which provide clear evidence that K$_x$picene is a Mott insulator for integer doping concentrations $x=1$, 2, and 3. We discuss various scenarios to understand the discrepancies with previous reports of superconductivity and metallic behavior. 
Andreas Ruff, Michael Sing, Ralph Claessen, Hunpyo Lee, Milan Tomi\'c,   Harald O. Jeschke, and Roser Valent\'i
cond-mat.str-el cond-mat.supr-con