Influence of the ${\mathrm{C}}_{60}$ filling on the nature of the metallic ground state in intercalated peapods

We present a study of the electronic properties of potassium doped ${\mathrm{C}}_{60}$ filled inside single-walled carbon nanotubes (SWCNT), so-called peapods using high-resolution photoemission. In the pristine state the overall spectral response is derived from ${\mathrm{C}}_{60}$ and the SWCNT. The bundles consist of one third of metallic tubes which are correlated electronic systems exhibiting a power law renormalization in the density of states. Compared to SWCNT bundles the Tomonaga-Luttinger liquid (TLL) renormalization parameter is hardly increased. Upon doping, the absence of distinct line phases or intercalation stages in this hybrid system leads to a broadening of the ${\mathrm{C}}_{60}$-derived states. This yields a complex doping behavior and charge distribution between the electronic states of the SWCNT and the encaged ${\mathrm{C}}_{60}$ chains. Close to the Fermi level, this has important consequences on the doping induced crossover from a TLL to a conventional Fermi liquid which is found at much higher charge transfers as compared to intercalated SWCNT bundles.

Figure 1

Valence band spectrum of ${\mathrm{C}}_{60}$ peapods (black line) and single-walled carbon nanotubes (grey line).

Figure 2

Valence band spectrum of ${\mathrm{C}}_{60}$ peapods near the chemical potential. Inset: Double-logarithmic representation. The value for $\alpha$ is obtained from the linear fit (black line).

Figure 3

Doping dependence of the ${\mathrm{C}}_{60}$ peapod valence band (right panel) compared to solid ${\mathrm{C}}_{60}$ taken from Ref. 22 for the ${\mathrm{K}}_x{\mathrm{C}}_{60}$ ($x=0$, $\alpha$, 3, 4, and 6) and from Ref. 28 in the case of the ${\mathrm{Rb}}_1{\mathrm{C}}_{60}$ spectrum (left panel). The numbers in the figures correspond to the $x$ in ${\mathrm{A}}_x{\mathrm{C}}_{60}$ $(\mathrm{A}=\mathrm{K},\mathrm{Rb})$ in the case of solid ${\mathrm{C}}_{60}$ and to the $\mathrm{C}∕\mathrm{K}$ ratio in the case of ${\mathrm{C}}_{60}$ peapods.

Figure 4

Doping dependence of ${\mathrm{C}}_{60}$ peapods near the chemical potential. The shift of the vHs derived from the semiconducting [$S_1$, $S_2$, and metallic tubes $\left(M_1\right)$ is indicated by the arrow]. The numbers next to the graph indicate the $\mathrm{C}∕\mathrm{K}$ value.

Figure 5

Change of the power law scaling factor $\alpha$ depending on the doping level for ${\mathrm{C}}_{60}$ peapods (black squares) in comparison to previous results on single-walled carbon nanotubes (open circles). The numbers serve to identify the respective doping step in Table and Fig. 7.

Figure 6

Doping dependence of the valence band spectra of ${\mathrm{C}}_{60}$ peapods for higher doping levels. The numbers next to the graph indicate the $\mathrm{C}∕\mathrm{K}$ ratio of the respective doping level. Inset: Comparison of the valence band spectra of pristine (black line, upscaled for clarity) and highly doped (grey line, $\mathrm{C}∕\mathrm{K}=14$) ${\mathrm{C}}_{60}$ peapods.

Figure 7

Schematic representation of the electronic band structure of ${\mathrm{C}}_{60}$ encapsulated in SWCNT. In both figures the grey lines represent the metallic bands whereas the semiconducting bands are drawn in black. The horizontal lines in the left figure indicate the Fermi level shift of the different doping levels and are accordingly marked with the numbers which correspond to the values in Fig. 5. The grey area on the right figure indicates filled states in the pristine peapods while the striped rectangle represents the ${\mathrm{C}}_{60}$ ${\mathrm{t}}_{1u}$-derived band which is not filled in pristine peapods. The latter is marked on the left figure with a horizontal line.