Metallic behavior in the potassium-doped fullerene-cubane copolymer

${\text{C}}_{60}$ and ${\text{C}}_{70}$ form with cubane $\left({\text{C}}_8{\text{H}}_8\right)$ molecules rotor-stator phases in which the fullerenes are rotating and the cubanes are static. Heating the rotor-stator phase up to 470 K a fulleride copolymer, poly $\left({\text{C}}_{60}{\text{C}}_8{\text{H}}_8\right)$, is obtained. Here, we study using microwave conductivity measurements, electron-spin resonance, and superconducting quantum interference device magnetometry the effect of potassium intercalation on the electric and magnetic properties of poly $\left({\text{C}}_{60}{\text{C}}_8{\text{H}}_8\right)$. The resulting ${\text{K}}_2{\text{C}}_{60}{\text{C}}_8{\text{H}}_8$ copolymer is metallic for $T>200\text{K}$ in contrast to the insulating pristine poly $\left({\text{C}}_{60}{\text{C}}_8{\text{H}}_8\right)$ copolymer. At lower temperatures, a disorder-driven Anderson localization of the electrons takes place and the ground state is a magnetic insulator.

Figure 1

Temperature dependence of the 9 GHz ESR linewidth of pristine poly $\left({\text{C}}_{60}{\text{C}}_8{\text{H}}_8\right)$ (circles) and ${\text{K}}_2{\text{C}}_{60}{\text{C}}_8{\text{H}}_8$ (squares) polymers.

Figure 2

Temperature dependence of the microwave conductivity, $\sigma \left(T\right)$, of ${\text{K}}_2{\text{C}}_{60}{\text{C}}_8{\text{H}}_8$ normalized to the RT value. At 400 K the conductivity changes reflect a transition from a metal to a semiconducting behavior.

Figure 3

Temperature dependence of the static susceptibility of ${\text{K}}_2{\text{C}}_{60}{\text{C}}_8{\text{H}}_8$ measured by SQUID (squares) and 9 GHz ESR (circles). Inset: temperature dependence of the inverse of the susceptibility. Note the non-Curie-Weiss behavior.

Figure 4

225 GHz ESR spectrum of ${\text{K}}_2{\text{C}}_{60}{\text{C}}_8{\text{H}}_8$ measured at 120 K. The fit (gray line) assumes a three-axis $g$-factor anisotropy with $g_1$, $g_2$, and $g_3$ given in the text. The difference between the fitted and the measured spectrum is also shown.

Figure 5

Temperature dependence of the 225 GHz ESR linewidth. Note the line broadening at temperatures below about 40 K.