Absence of metallicity in K-doped picene: Importance of electronic correlations

Potassium-doped picene (K_xpicene) 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 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≈1), the spectra always display a finite energy gap. This result is supported by our DFT+DMFT calculations which provide clear evidence that K_xpicene 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.