Although not an intrinsic superconductor, graphite exhibits superconductivity when intercalated with certain dopants1. Perhaps the most studied of these graphite-based superconductors are the alkali metal–graphite intercalation compounds (GICs) (ref. 2), of which the easiest to fabricate is C8K (ref. 3), with a transition temperature (ref. 2). By increasing the alkali metal concentration (through high-pressure fabrication techniques), the transition temperature can be increased up to 5 K in C2Na (ref. 4). Superconductivity in C6Yb and C6Ca with and 11.5 K, respectively, and at ambient conditions has been observed5. Here we explore the architecture of the states near the Fermi level and identify characteristics of the electronic band structure generic to GICs. In addition to the expected charge transfer from the intercalant atoms to the graphene sheets, resulting from the occupation of the π bands, we find that in all of those—and only those—compounds that superconduct, an interlayer state, which is well-separated from the carbon sheets, also becomes occupied. We show that the energy of the interlayer band is controlled by a combination of its occupancy and the separation between the carbon layers.
To focus our discussion, we begin with an investigation of the newly discovered C6Yb system. Our calculations rely on density functional theory (DFT) techniques6 applied in the local density approximation (LDA)7. As the 4f shell of Yb is found to be filled (in agreement with experiment), the DFT-LDA can be applied with some confidence. Moreover, one may infer that the 4f orbitals play no essential role in superconductivity. The atomic structure of C6Yb, shown in Fig. 1a, involves a stacked arrangement of graphene sheets intercalated with a triangular lattice configuration of Yb atoms. The stacking of the graphene sheets is AAA, whereas the Yb atoms occupy interlayer sites above the centres of the hexagons of the graphene sheets in an ABAB stacking arrangement leading to a P63/MMC crystallographic structure. We use the same geometry for C6Ca.
Figure 1d shows the results of the band structure calculation of C6Yb centred on the Fermi energy. To resolve the qualitative structure of the levels and identify the bands intersecting the Fermi level, it is helpful to compare the dispersion with the corresponding ‘empty’ graphite-like system and the ‘empty’ Yb metal, obtained simply by removing, respectively, the Yb and C atoms from the C6Yb structure whilst holding the lattice constants fixed. In C6Yb, the previously unoccupied graphite π* bands are seen to intersect the new Fermi level (owing to the enlarged interlayer distance, each is almost doubly degenerate). Of these, one remains almost unperturbed by the presence of Yb, whereas the degeneracy of the other is lifted and one of the resulting bands hybridizes strongly with a new band that also crosses the Fermi level. The latter resembles both the free-electron-like state of expanded Yb metal (Fig. 1c) and the interlayer band intrinsic to pure graphite (Fig. 1e). As well as the 5s and 5p orbitals (which lie well outside the range of energies shown in Fig. 1d), the 4f orbitals associated with the two Yb atoms in the unit cell form a set of localized almost non-dispersive bands that appear at about 0.8 eV below the Fermi level. Referring to Fig. 2, and the discussion in ref. 8, we note that a similar phenomenology applies to the band structure of C6Ca.
Although the presence of the interlayer band and its hybridization with the π* band helps to explain the diminished normal state resistance anisotropy observed in C6Yb over that of pure graphite5, could its occupation be significant for superconductivity? To address this question, it is instructive to draw comparison with other superconducting GICs. It is known from low electron-energy-loss spectroscopy measurements of the superconducting compound C8K that, in common with the C6Yb system, the interlayer band lies below the Fermi energy9. However, a more discerning test of the significance of the interlayer state is presented by the Li intercalates. Although intercalation at ambient pressure leads to the C6Li structure, high-pressure techniques can be used to fabricate C3Li and C2Li compounds. Significantly, whereas the C3Li system (like its cousin C6Li) remains non-superconducting, C2Li has a transition at Tc≃1.9 K (ref. 10). Referring to Figs 2 and 3, one may note that, in common with the C6Yb and C6Ca system, the Li GICs also exhibit a free-electron-like interlayer band. The dispersive character of this band is reflected clearly in the Fermi surface (see Fig. 3a). However, significantly, increasing the Li ion concentration (and, with it, the degree of electron doping) leads to a lowering of the interlayer band resulting in its occupancy in the C2Li system. Indeed, combining the available data in Table 1, the coincidence of superconductivity in the GICs with the occupation of the interlayer state is striking.
Studies of the free-electron-like interlayer state in pure graphite and GICs have a long history. Although present in early band-structure calculations in the pure system, the significance of the (unoccupied) band in graphite and C6Li was first emphasized in refs 1112 and, subsequently, its existence was confirmed experimentally in both graphite13 and C6Li (ref. 14). (The same band has been equivalently referred to as a metallic ‘sd band’ in ref. 15.) Referring to the Li-based GICs, it is apparent that the energy and, therefore, also the occupancy of this band can vary considerably with respect to the other bands present in the above systems.
It is notable that the dispersive 5d band of Yb in the expanded metal (Fig. 1c) and the interlayer state in pure graphite (Fig. 1e) both bear strong resemblance to the dispersion of the band of free electrons in the absence of external potential (Fig. 1f). Indeed, we can understand the important features of this nearly free-electron-like state by studying the ‘empty’ graphite system with the intercalant ions removed. In Fig. 4 we show the effect of charging and layer separation on the energy of the resulting interlayer band. An increase in the c-axis lattice constant and in the accumulated charge both lead to a lowering of the band energy and, thus, an increase in its occupancy. It is interesting to note that, for a wide range of experimentally known GICs, the interlayer band energy follows closely that of the pure graphite model.
Motivated by preliminary electronic structure calculations, theoretical studies16,17 have emphasized the significance of partially occupied two-dimensional graphite π* bands and three-dimensional interlayer bands for superconductivity. Indeed, these studies have shown that aspects of superconductivity in some GICs can be described well by a two-band phenomenology. Yet the underlying microscopic pairing mechanism remains in question. Significantly, unlike the fullerene-based superconducting compounds, the π* bands are decoupled from the out-of-plane lattice vibrations of the graphene sheets and couple only weakly to the in-plane modes. By contrast, one may expect the interlayer band to engage with lattice vibrations of the metal ions and, through their hybridization with the π* bands, the graphene sheets. Indeed, numerical calculations have shown the presence of strong electron–phonon coupling in C6Ca, facilitated by the occupation of the free-electron-like band8. Whether an electron–phonon mechanism alone can explain the broad distribution of Tc observed across the range of GICs, as well as the staging dependence18, remains a question for future investigation. In this context, it is interesting to note that, in those compounds with a low transition temperature, the interlayer band remains nearly orthogonal to the π* bands, as it is in pure graphite, whereas in C6Yb and C6Ca the hybridization is strong.
Intriguingly, through its weak coupling with the graphene layer, one may expect occupancy of the intrinsic interlayer band of graphite to provide an ideal environment for soft charge fluctuations that could promote (s-wave) superconductivity by means of an excitonic pairing mechanism19. Indeed, the same weak coupling diminishes the potential for charge density wave instability, which might otherwise compete with the superconducting pair correlations. In this context, it is interesting to note that a parallel pairing mechanism involving the exchange of acoustic plasmons has been suggested20 and was much discussed after the discovery of superconductivity in the cuprates. The model has been re-evaluated for layered metals21. Bearing in mind that low-frequency plasma oscillations will, of course, couple to the ionic positions—conventional acoustic phonons can be viewed as simply coupled ionic and electronic plasma oscillations—such processes present an effective mechanism to enhance electron–phonon coupling.
Recognizing the auspicious role played by the free-electron-like state in the superconducting GICs, it is tempting to look for a common phenomenology in the isoelectronic family of boron intercalates and, in particular, the well-studied superconducting compound MgB2, which exhibits a Tc=39 K (ref. 22). Although the material indeed possesses interlayer states, the band structure (Fig. 2) shows that it remains unoccupied. However, in contrast to C6Yb and the other superconducting GICs, in this case charge carriers are transferred from the σ to the π* band. The resulting band of hole-like σ states couples strongly to the host lattice providing a natural environment to realize a high-temperature phonon-mediated superconductivity.
Finally, it is interesting to note that the interlayer states find their analogue as nearly free-electron-like states in the carbon nanotube system: early theoretical23 and experimental24 investigations of the alkali-metal nanotube intercalates indicated a phenomenology in doping dependence similar to the GICs. On this background, an investigation of possible superconductivity in electron-doped carbon nanotubes would seem to be a timely and worthwhile enterprise.
The results reported here were obtained from an implementation of the LDA in the framework of the CASTEP package25. The methodology, which includes the use of ultrasoft pseudopotentials, has been validated in many environments including the rare-earth compounds26. Here, we take advantage of the fact that, in using ultrasoft pseudopotentials, one can exercise a considerable degree of freedom in deciding which electron shells to treat as fixed core and which to treat as valence, free to participate in bonding.
The numerical results presented for C6Yb were computed by treating all of the electrons below the 5s level for Yb and below the 2s level for C as core. Plane waves up to 480 eV were used to expand the electronic states and the Brillouin zone was sampled with a 12×12×6 mesh. (A test calculation using a 600 eV cutoff did not yield appreciably different results.) To check for possible correlation effects, we used the LDA+U method as implemented in the PWSCF package27 with the parameter U varying between 3 and 7 eV.
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We are grateful to S. S. Saxena and M. Ellerby for bringing the C6Yb and C6Ca compounds to our attention and to G. G. Lonzarich for contributing to our understanding.
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Csányi, G., Littlewood, P., Nevidomskyy, A. et al. The role of the interlayer state in the electronic structure of superconducting graphite intercalated compounds. Nature Phys 1, 42–45 (2005). https://doi.org/10.1038/nphys119
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