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Alkali Metal Doped Picene Layers: Insulating Phase in Multilayer Doped Compounds

Since the discovery of superconductivity in 1911, one of the most interesting issue is to understand the mechanism of Cooper pair formation. New challenges arise also from the discovery of superconductivity  in C60 solid doped with alkali metals. Recently a polycyclic aromatic compound, the picene molecule, was discovered to have a superconducting phase transition when doped with alkali metals, with Tc around 18 K. 

 
Theoretical studies on K3picene showed that the superconductivity could be driven by electron−phonon (intramolecular) coupling, as in the case of K3C60. On the other hand, recent DFT calculations showed that K3picene turns into a Mott−Hubbard insulator when its unit cell experiences an enlargement of 5%, like observed in C60 compounds. Therefore, a fundamental question has to be addressed: do correlation effects and crystal arrangements play a decisive role in doped picene electronic structure and superconductivity as in the case of C60 compounds?
Figure 1 shows the K-doped picene multilayer valence bands at various doping stages, namely K0.5picene and K3picene.
A new state (C) near the Fermi level appears as a consequence of the LUMO − LUMO +1 derived picene bands filled by 4s electrons of K, however this new state is peaked at 0.8 eV of binding energy, and there is no DOS at the Fermi level showing, therefore, an insulating phase for the system.There is a possibility that the metallic state should appear by competing with a phonon-driven (and correlation-assisted) ground state, leading to an insulator−metal transition as a function of temperature.

 
Figure 2 shows the valence band spectra close to the Fermi level of K3picene as a function of temperature, but is evident that the LUMO − LUMO+1 peak remains below the Fermi level, and there is not any DOS appearing at EF.Therefore, the system is insulating also at low temperature. To understand if the observed insulating behaviour is related to correlation effects, we analyze the picene monolayer where possible correlation effects should be   screened by Ag substrate electrons. The evolution of the valence band for the pristine and K (Na) doped monolayers of picene is shown in figure3. The first observation is that in the pristine monolayer there is a Fermi level, and the HOMO-derived band is shifted from 3 to 2.7 eV in the undoped monolayer with respect to the multilayer.

The presence of the Fermi level in the monolayer/substrate system is mainly due to the underlying Ag, but it is evident that the DOS near EF changes as the alkali metals are added to the monolayer, as shown in figure4. Both the shift of the HOMO in the undoped system and the metallic phase for the doped monolayer can be ascribed to the screening effect of the metallic substrate. This behavior could be explained in close analogy to fullerides assuming that the HOMO−LUMO gap and the insulating state, observed in photoemission adding electrons to the LUMO − LUMO+1 bands, may have a strong Coulomb contribution.

However, it is worth noting that the UHV pristine films grown on Ag(111) have a peculiar adsorption geometry, which is different from the expectation based on crystal structure calculations and measurements for picene crystals. Due to strong correlation effects, small variations in the crystal structure (molecular arrangements) may be the reason for the observed insulating state in our doped picene multilayer.



 



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

M. Caputo , G. Di Santo , P. Parisse , L. Petaccia , L. Floreano §, A. Verdini §, M. Panighel , C. Struzzi , B. Taleatu , C. Lal , and A. Goldoni *

ST-INSTM Lab., Sincrotrone Trieste S.C.p.A., s.s. 14 km 163.5 in Area Science Park, 34149 Trieste, Italy
Department of Physics, Trieste University, Via Valerio 2, 34127 Trieste, Italy
§ Lab. TASC, IOM-CNR, s.s. 14 km 163.5 in Area Science Park, 34149 Trieste, Italy
Department of Physics, Obafemi Awolowo University Ile-Ife, Nigeria
Centre for Non-Conventional Energy Resources, University of Rajasthan, Jaipur, India
DOI: 10.1021/jp306640z
Publication Date (Web): September 4, 2012
Copyright © 2012 American Chemical Society

*E-mail: .



 

Last Updated on Monday, 29 September 2014 18:21