Evidence for the formation of a Mott state in potassium-intercalated pentacene

We investigate electronic transport through pentacene thin films intercalated with potassium. From temperature-dependent conductivity measurements we find that potassium-intercalated pentacene shows metallic behavior in a broad range of potassium concentrations. Surprisingly, the conductivity exhibits a re-entrance into an insulating state when the potassium concentration is increased past one atom per molecule. We analyze our observations theoretically by means of electronic structure calculations, and we conclude that the phenomenon originates from a Mott metal-insulator transition, driven by electron-electron interactions.

Figure 1

(Color) (a) Conductivity $\sigma$ and square conductance $G_{◻}$ of three different K-doped PEN films as a function of the ratio $N_{\text{K}}/N_{\text{PEN}}$; under the curves a pentacene molecule. Inset: $N_{\text{K}}/N_{\text{PEN}}$ as a function of doping time. Schematic view (b) of our setup and (c) atomic force microscopy image of a high-quality undoped PEN film showing large crystalline grains.

Figure 2

(Color) Temperature dependence of the conductance of potassium-intercalated pentacene films. (a) The colored dots in the inset of (a) indicate the doping level at which the temperature-dependent conductivity measurements with corresponding color were performed. In black the temperature-dependent conductivity of the Si substrate is shown. The low temperature (5 K) $I\text{−}V$ characteristics of ${\text{K}}_x\text{PEN}$ in the (b) conducting and (c) highly doped insulating states.

Figure 3

(Color online) (a) Conductivity $\sigma$ measured at room temperature as a function of doping time for a 25-nm-thick pentacene film deposited on ${\text{SiO}}_2$. (b) Temperature dependence of the conductivity for a pentacene film grown on ${\text{SiO}}_2$ and doped into the highest conductivity state. The conductivity is rapidly decreasing with lowering the temperature as it is typical for an insulator. [(c) and (d)] AFM images of pentacene films grown on ${\text{SiO}}_2$ and on H-terminated Si. (c) Small and randomly oriented grains with large height fluctuations are observed when the PEN films are deposited on ${\text{SiO}}_2$, (d) whereas PEN films of similar thickness deposited on Si consist of large crystalline grains with a common relative orientation and only relatively small fluctuations in height.

Figure 4

(a) Temperature dependence of the resistivity of a high-quality ${\text{K}}_3{\text{C}}_{60}$ film grown on Si. As expected, the resistivity exhibits a superconducting transition at 18 K. The inset shows the doping dependence of the conductivity. The conductance peak is typical of ${\text{K}}_3{\text{C}}_{60}$. (b) Temperature dependence of the resistivity of a ${\text{K}}_3{\text{C}}_{60}$ film grown on ${\text{SiO}}_2$ showing insulating behavior.

Figure 5

(Color) Crystal structure of potassium-intercalated pentacene ${\text{K}}_1\text{PEN}$ obtained from ab initio computational relaxation with (a) showing the herringbone of the PEN molecules and K atoms in the unit cell and (b) is a side view of the stacked layers of PEN and K, illustrating the potassium intercalation in between the molecular planes. The unit-cell parameters are $a,b,c=6.239,7.636,15.682\text{Å}$ and $\alpha ,\beta ,\gamma =76.98°,88.14°,84.42°$.

Figure 6

(Color) Results of electronic structure calculations for ${\text{K}}_1\text{PEN}$. (a) Single-particle band structure, with the Fermi level $E_F$ (green line) as the zero of energy (Ref. 33). Valence and conduction bands are indicated by the thick blue lines. (b) Carbon (blue) and potassium (orange) projected density of states. (c) Tight-binding fit to the valence and conduction bands (blue thin lines) and the resulting lower and upper Hubbard bands from a mean-field analysis of the corresponding Hubbard Hamiltonian with $U=1.45\text{eV}$ (red thick lines). The arrows indicate the opening of the Hubbard gap. (d) Reciprocal lattice vectors and the first Brillouin zone of ${\text{K}}_1\text{PEN}$.