Novel Orientational Ordering and Reentrant Metallicity in ${\mathrm{K}}_x{\mathrm{C}}_{60}$ Monolayers for $3\le x\le 5$

STM studies on ${\mathrm{K}}_x{\mathrm{C}}_{60}$ monolayers reveal new behavior over a wide range of the phase diagram. As $x$ increases from 3 to 5 ${\mathrm{K}}_x{\mathrm{C}}_{60}$ monolayers undergo metal-insulator-metal reentrant phase transitions and exhibit a variety of novel orientational orderings, including a complex 7-molecule, pinwheel-like structure. The proposed driving mechanism for the orientational ordering is the lowering of electron kinetic energy by maximizing the overlap of neighboring molecular orbitals. In insulating (metallic) ${\mathrm{K}}_x{\mathrm{C}}_{60}$ this gives rise to orbital versions of the superexchange (double-exchange) interaction.

Figure 1

(a) STM topograph ($V=-0.2\mathrm{V}$, $I=5\mathrm{pA}$) of a ${\mathrm{K}}_{4+\delta }{\mathrm{C}}_{60}$ ML ($\delta \sim 0.5$) displaying new 7-sublattice pinwheel structures (highlighted in blue). Inset: Topograph of the stoichiometric ${\mathrm{K}}_4{\mathrm{C}}_{60}$ ML shows the cross phase. (b) $dI/dV$ spectroscopy for a single pinwheel molecule (red curve) is similar to spectroscopy measured for molecules in the ${\mathrm{K}}_4{\mathrm{C}}_{60}$ cross phase (black curve).

Figure 2

(a) STM image ($V=-0.2\mathrm{V}$, $I=5\mathrm{pA}$) of a domain of pinwheels where the 7-sublattice pinwheels form a close-packed $\sqrt{7}\times \sqrt{7}$ superstructure. (b) A close-up image and (c) a schematic structure of a single 7-sublattice pinwheel. The observed line node in STM topography is represented here by a red line.

Figure 3

(a) STM topograph ($V=-0.2\mathrm{V}$, $I=5\mathrm{pA}$) of ${\mathrm{K}}_5{\mathrm{C}}_{60}$ flower phase exhibiting $2\times 2$ reconstruction. Inset: Topograph of the ${\mathrm{K}}_3{\mathrm{C}}_{60}$ ML. (b) $dI/dV$ spectra of molecules in ${\mathrm{K}}_5{\mathrm{C}}_{60}$ flower phase show finite density of state at $E_F$. Inset: Schematic ${\mathrm{K}}_5{\mathrm{C}}_{60}$ electronic structure. (c),(d) Model structures for the ${\mathrm{K}}_5{\mathrm{C}}_{60}$ and ${\mathrm{K}}_3{\mathrm{C}}_{60}$ phases. Blue dots represent pentagons on the ${\mathrm{C}}_{60}$ equatorial plane.

Figure 4

(a) Local electronic density of states for the HOMO and LUMO of a JT-distorted $\mathrm{C}_{60}{}^{4-}$ molecule calculated by tight binding. (b) Schematic diagram of the proposed superexchange mechanism between two neighboring $\mathrm{C}_{60}{}^{4-}$ molecules. (c) The orientational dependence of $-t^2/U$ obtained from tight-binding calculations for adjacent molecules. The angle $(0,0)$ corresponds to the experimentally observed cross phase.