Intercalation and High Temperature Superconductivity of Fullerides

Intercalation of polyatomic molecules into a superconductor can drastically affect the properties of the compound. A mechanism responsible for a large increase in Tc for such systems is proposed. It explains the recent remarkable observation of high Tc superconductivity in the hole-doped C60/CHX3 (X≡Cl,Br) compounds and the large shift in their Tc upon Cl→Br substitution. The increase in Tc is due to contribution to the pairing arising from the interaction of electrons with the vibrational manifold of the molecule. The proposed mechanism opens up the possibility to observe a site-selective isotope effect. We also suggest that intercalating CHI3 would further increase the critical temperature to Tc ≃ 140K. PACS. 74.70.Wz Superconductivity; Fullerenes and related materials – 74.72.-h High-Tc compounds This paper is concerned with the impact of intercalation by polyatomic molecules on the properties of superconductors. The study has been motivated by the recent remarkable observation of high Tc caused by intercalation of CHBr3 (bromoform) and CHCl3 (chloroform) molecules into hole-doped fullerides [1] (see also description of [1] in Ref. [2]). The discovery of high temperature superconductivity [1] in these systems (Tc = 117K for C60/CHBr3; Tc = 80K for C60/CHCl3) has attracted a lot of attention and raises the fundamental question about the nature of this phenomenon. The usual superconducting fullerides (see ,e.g., the reviews [3, 4]) are chemically electron-doped compounds (e.g., Rb3C60, K3C60). It is believed that their superconductivity is due to the coupling of electrons to the intramolecular vibrational modes. The vibrational spectrum spreads over a broad region (ΩL ≃ 270cm−1 → ΩH ≃ 1500cm−1). Strictly speaking, all modes contribute to the pairing but the question of which region of the vibrational spectrum plays a major role is still controversial (see, e.g. [4]). A new exciting development was described in [5]; with the use of gate-induced doping, a hole-doped fulleride compound was created without chemical dopant. As a result a major increase in Tc to 52K(!) was observed. This is caused by the different properties of the valence band relative to the conduction band such as the density of states. Recently, this technique was complemented by intercalating the molecules CHBr3 and CHCl3 into the crystal [1]. These molecules are placed in regions near the C60 units. This has resulted in a dramatically higher Tc again: Tc ≃ 117K(!). It was indicated in [1] that the change in lattice spacing may be responsible for the increase in Tc. This is an interesting scenario which needs to be studied in detail. Here we propose another additional mechanism. It is of interest in its own right; furthermore, it can provide for a quantitative explanation of the drastic increase in Tc observed in [1]. It is very essential to note that the experimentalists [1] observed not only a remarkable high Tc for the C60/CHX3 compound (X≡ Br,Cl), but also a large difference in the value of Tc for X≡ Br vs. X≡ Cl. Whereas Tc ≡ T Br C ≃ 117K was observed for X≡Br, the value of Tc for X≡ Cl turned out to be much smaller (≃ 80K), although still very high. The structures of the bromoform and chloroform molecules are similar and therefore a microscopic explanation of the strong change in the critical temperature is also of definite interest. The picture we propose is that the intercalated molecules themselves actively, and importantly, participate in the pairing. The internal vibrational modes of the molecules provide for an additional attraction which leads to higher Tc. Such a mechanism was considered by one of the authors in [6]. Therefore, our major focus is on the vibrational spectra of these molecules. Let us start with the parent hole-doped fulleride (Tc = 52K). The order parameter for this superconductor is described by the equations: