Experimental Study of Pristine and Alkali Metal Doped Picene Layers: Confirmation of the Insulating Phase in Multilayer Doped Compounds

Abstract

The electronic structure and the geometric arrangement of picene molecules adsorbed on Ag(111) were studied by means of photoemission and near-edge X-ray absorption fine-structure spectroscopies (NEXAFS). While the valence band of a picene monolayer shows a clear metallic state that evolves with alkali metal doping, in the case of a picene multilayer there is no evidence of metallic states evolving with doping. Our data suggest that the bulk ultrahigh-vacuum films of K x picene are in an insulating phase, and we attribute this behavior to a strong electron–electron Coulomb interaction that is instead screened in the monolayer. The NEXAFS profiles of different picene layers show a coverage-dependent orientation, from flat molecules (monolayer) to an orientation of ∼40° of the molecular long axis with respect to the substrate surface (multilayer). The observed molecular orientations are in disagreement with the expected crystal structure of the bulk material and may explain the presence of insulating states in strongly correlated doped picene multilayers.

Absence of Metallicity in K-doped Picene: Importance of Electronic Correlations

“…This representation nicely shows the evolution of in-gap spectral weight upon doping, with none of the spectra displaying any metallic Fermi edge. The observation that even the x = 3 phase, for which superconductivity has been claimed, remains gapped is most surprising, but in agreement with two previous reports [24,25]. As one possible explanation polaronic effects have been suggested [24], which may cause a pseudogap at the Fermi level as has been observed, e.g., in oxides [37].…”

“…In fact, the importance of electron-electron correlations in K 3 picene was pointed out by Giovannetti et al [21] and Kim et al [22] who found an insulating state with antiferromagnetic order from ab initio calculations. Experimentally, the picture of the electronic structure has remained controversial: while one photoemission study reported the appearance of metallic spectral weight at the Fermi level for x = 1 [23], other authors [24,25] failed to observe a metallic Fermi edge for a wide concentration range from x = 0.22 to 3.6 and speculated about the possible role of correlations and polaronic effects [24].…”

Superconductivity in aromatic hydrocarbons

“…The Photoemission spectroscopy (PES) reported by Okazaki et al showed metallic behavior in thin films of K-doped picene [21,22], i.e., the PES peak was observed at the Fermi energy with K-doping of picene thin films and the peak disappeared with excess K-doping. However, the PES reported by other groups showed no metallic behavior [23][24][25]. Okazaki et al suggested that the observation of metallic behavior originated in the different crystal structures of picene thin-films, which change depending on the substrates used.…”

Electronic excitation spectrum of calcium-doped picene: Electron energy-loss spectroscopy study

“…[38][39][40][41][42] Most importantly, various photoelectron spectroscopy studies revealed that thin films of potassium-intercalated picene show a negligible density of states at the Fermi level which has been explained by such correlations effects. [40][41][42] It therefore seems to be quite likely that also for Cadoped picene the electronic correlations are strong enough to localize the charge carriers and to lead to a nonmetallic ground state. Moreover, the fact that there is hybridization between Ca and picene derived states in Ca-doped picene (see previous discussion) demonstrates that a rigid bandlike behavior upon intercalation and associated charge transfer certainly is not appropriate to describe this compound.…”