LUMO photoemission lineshape in quasi-one-dimensional ${\text{C}}_{60}$ chains

The low-energy single-particle excitations of highly ordered ${\text{C}}_{60}$ chains adsorbed on a vicinal copper substrate are investigated by angle-resolved photoemission spectroscopy. The interface state previously identified on ${\text{C}}_{60}/\text{Cu}\left(111\right)$ shows a one-dimensional dispersion on Cu(553). In contrast, no significant momentum dependence is detected for emission from the lowest unoccupied molecular orbital (LUMO). The LUMO displays similar phonon features as in ${\text{C}}_{60}/\text{Cu}\left(111\right)$ but it does not peak toward the Fermi level for all considered potassium dopings and its photoemission lineshape is broader than in any other monolayer system investigated so far. This behavior is not easily reconciled with existing theory and indicates that the one-dimensional character of the chains affects the electronic structure of the monolayer in an intricate way.

Figure 1

(a) and (b) Low-energy electron diffraction (LEED) patterns of 1 ML ${\text{C}}_{60}/\text{Cu}\left(111\right)$ and ${\text{C}}_{60}/\text{Cu}\left(553\right)$ measured with 40 and 37 eV electron energy, respectively. The ${\text{C}}_{60}$ chains on Cu(553) run along the $\left[\overline{1}10\right]$ direction of the copper substrate. (c) Low-temperature $\left(T=10\text{K}\right)$ $\text{He}I\alpha$ angle-integrated photoemission spectra of one monolayer of ${\text{C}}_{60}$ in a 1D and in a 2D molecular arrangement on Cu(553) and Cu(111), respectively. The arrows mark the positions of the satellites due to electron-phonon coupling. The inset shows angle-integrated spectra of 1 ML ${\text{C}}_{60}/\text{Cu}\left(553\right)$ and clean Cu(553) over a wider energy range.

Figure 2

(Color online) Angle-resolved photoemission data of ${\text{C}}_{60}$ chains on Cu(553) measured at low temperature $\left(T=10\text{K}\right)$. (a) Set of high-resolution EDCs measured along the chains. (b) Dispersion plot illustrating the electronic structure of ${\text{C}}_{60}/\text{Cu}\left(553\right)$ in the direction along the chains. The dashed lines highlight the satellites from the nondispersing intramolecular phonons; the black circles follow the dispersion of the interface state in the HOMO-LUMO gap. (c) Constant energy maps at (a) $-0.95\text{eV}$, (b) $-0.6\text{eV}$, and (c) $E_F$. The ${\text{C}}_{60}$ Brillouin zones are indicated by a dashed grid. Maximum intensity corresponds to black. $k_y$ is oriented along $\left[\overline{1}10\right]$, the direction parallel to the chains. $k_x$ is oriented along the perpendicular direction $\left[\overline{3}\overline{3}10\right]$.

Figure 3

${\text{K}}_x{\text{C}}_{60}/\text{Cu}\left(553\right)$ angle-integrated photoemission spectra measured for different doping levels of the LUMO $\left(T=10\text{K}\right)$. The LUMO occupancy $x$ was calibrated with the procedure described in Ref. 30. We found that in the undoped monolayer $1.2e/{\text{C}}_{60}$ are transferred from the substrate to the molecule.

Figure 4

(Color online) Spectral function calculations for a coupled electron-phonon system in a square lattice with different hopping anisotropies. (a)–(c) Electron self-energy calculated for a half-filled band coupled to two $H_g$ phonons (${\omega }_{ph}=0.195$ and 0.096 eV, respectively). The hopping anisotropy are as follows: (a) $t_{\parallel }/t_{\perp }=1$, isotropic dispersion, (b) $t_{\parallel }/t_{\perp }=2$, and (c) $t_{\parallel }/t_{\perp }=10$, highly anisotropic dispersion similar to the ${\text{C}}_{60}$ chains. The insets show the corresponding Fermi surfaces [gray (red) line]. In the calculations we included the fivefold degeneracy of these ${\text{C}}_{60}$ vibrational modes. (d)–(f) Occupied part of the interacting DOS convoluted with a Gaussian $\left(\sigma =10\text{meV}\right)$ for the three different hopping anisotropies. The insets show calculated dispersion plots. A constant impurity scattering of 100 meV has been added for the plots of $A\left(k,\omega \right)$. The solid lines correspond to the noninteracting bands.