Influence of local fullerene orientation on the electronic properties of Na 2 A C 60 ( A = Cs , Rb , K ) compounds

摘要: We have investigated sodium containing fullerene superconductors ${\mathrm{Na}}_{2}A{\mathrm{C}}_{60}$, $A=\mathrm{Cs}$, Rb, and K, by $^{23}\mathrm{Na}$ nuclear magnetic resonance (NMR) spectroscopy at $7.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the temperature range of $10\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Despite structural differences from ${\mathrm{Rb}}_{3}{\mathrm{C}}_{60}$ class superconductors, these compounds NMR line tetrahedrally coordinated alkali nuclei also splits into two lines ($T$ ${T}^{\ensuremath{'}}$) low temperature. In ${\mathrm{Na}}_{2}\mathrm{Cs}{\mathrm{C}}_{60}$ splitting occurs $170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$; quenched cubic phase ${\mathrm{Na}}_{2}\mathrm{Rb}{\mathrm{C}}_{60}$ ${\mathrm{Na}}_{2}\mathrm{K}{\mathrm{C}}_{60}$ we observe split $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. By detailed investigations spectrum, spin-spin, spin-lattice relaxations as well spin-echo double show that different tetrahedral sites are mixed on a microscopic scale. The $T$ ${T}^{\ensuremath{'}}$ differ orientation first-neighbor ${\mathrm{C}}_{60}$ molecules. present evidence orientations neighboring molecules uncorrelated. Thermally activated molecular reorientations cause an exchange between motional narrowing high infer same activation energy $3300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ $125\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. relaxation rate is for down to $125\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ but below. Both Knight shift strongly dependent whole investigated. interpret this variation effect phonon excitations involving rigid librational motion extending understanding structure dynamics results may help clarifying effects superconducting properties.