Strong Correlations in Electron Doped Phthalocyanine Conductors Near Half Filling

Abstract

We propose that electron doped nontransition metal-phthalocyanines (MPc) like ZnPc and MgPc, similar to those very recently reported, should constitute novel strongly correlated metals. Due to orbital degeneracy, Jahn-Teller coupling and Hund's rule exchange, and with a large on-site Coulomb repulsion, these molecular conductors should display, particularly near half filling at two electrons/molecule, very unconventional properties, including Mott insulators, strongly correlated superconductivity, and other intriguing phases. PACS numbers:Building novel metals by doping molecular crystals such as polyacetylene, fullerenes, TTF-TCNQ, (TMTSF) 2 X, (TMTTF) 2 X and (BEDT-TTF) 2 X salts, etc., is a well trodden route [1,2], but remains an exciting and ever moving front. Very recently the Delft group showed that thin films of transition metal phthalocyanines (MPcs) FePc, CoPc, NiPc, CuPc, initially insulating, can be turned genuinely metallic through potassium doping [3]. Electron doping appears to take place largely in the twofold degenerate lowest unoccupied e g molecular orbital (LUMO) [4,5] of the molecules. The simplest rigid band model naturally explains, according to [3], why the MPcs, initially insulating when pristine (n = 0) [6], become metallic upon increasing doping (0 < n < 4), ending up again as insulators at full doping (n = 4). It is not yet clear whether stoichiometric compound phases may exist here as they do in alkali doped fullerides, but there is an otherwise striking overall analogy, to the point that even the conductance values reported at optimal metallic doping are close in magnitude and temperature (in)dependence to those of K x C 60 films.While the pursuit of metallicity-cum-magnetism in the transition metal doped MPcs [5] will in the future constitute an interesting goal in itself, the scope of this note is to point out newer directions and possibilities that can make slightly different metallic doped MPcs, yet to be realized, potentially even more exciting. Doped MPcs are not, we claim, regular metals, but constitute strongly correlated electron systems, akin to doped two-band Mott insulators. Characterized by a strong on-site electron repulsion, by the e g orbital degeneracy, and by intra-site Jahn-Teller and electron-electron multiplet couplings, the doped MPc molecular system should approach near n = 2, as foreshadowed by recent studies [7,8,9] a novel unstable fixed point heralding a wealth of possible low-temperature phenomena and phases. We conduct in this letter a preliminary exploration of this scenario by addressing theoretically -and thus proposing the experimental realization of -new metals obtained through electron doping of ZnPc, MgPc and other such non-transition metal MPcs. The extra electrons should flow into the MPc 2e g lowest unoccupied molecular orbital (LUMO) [4], to form a two band metal. The LUMO states are ligand shell orbitals loosely surrounding the central metal ion, with a large intramolecular Coulomb repulsion U in comparison with the narrow electron bandwidth...

Potassium-intercalated H2Pc films: Alkali-induced electronic and geometrical modifications

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18] The possibility to modify the electronic structure of the molecular film allows the tuning of specific properties and this is clearly highly desired to further improve the performance of Pc-based devices. For this reason, the interesting results reported by Craciun et al 19 and already predicted by a theoretical study by Tosatti et al 20 about the observation of an insulator-to-metal transition of Pc films upon alkali intercalation (between 0 and 4 K atoms per molecule) have inspired a number of studies. A majority of these investigations has dealt with the intercalation of CuPc and other metal Pc (MePc) films by K, [16][17][18][19] Rb, 21 and Cs 22,23 with a particular focus on the investigation of the modifications of the electronic structure induced by the alkali.…”

Low-energy spin dynamics in the[YPc2]0

“…2 In fact, the employment of these materials in different areas, ranging from the fabrication of organic light emitting diodes, to contrast agents or spintronics materials, has been envisaged. Pc-based compounds have attracted significant interest in the last decade after it has been suggested that high temperature superconductivity could be induced in these materials by alkali doping 3 and, more recently, when it has been recognized that neutral [LnPc 2 ] 0 molecules, with Ln a lanthanide ion, are molecular nanomagnets with extremely long coherence times at liquid nitrogen temperature. [4][5][6][7][8] Owing to the flat shape of Pc molecules, the structure of Pc-based materials is typically characterized by chains along which Pc molecules tend to stack.…”

Soft x-ray spectroscopy study of the element and orbital contributions to the electronic structure of copper hexadecafluoro-phthalocyanine

“…The fundamental properties 1 and technological applications 2 of metal phthalocyanines ͑M-Pc͒ have been the subject of much research, and indeed M-Pcs are one of the foremost planar organic molecular semiconductors used in many organic optoelectronic devices. 3 Of this family, there has been increased attention in recent years paid to CuPc and its derivatives F 16 CuPc and K-doped CuPc.…”