Crossover in electronic states of fulleride superconductors revealed by electrical transport

Cs$_{3}$C$_{60}$ fullerides with A15 structure found in 2008 [1] and fcc structure in 2010 [2] show superconducting transition at 38K and 35K under pressure, respectively. Both structural phases are Mott insulators in the entire temperature range at ambient pressure. High pressure drives them into metallic and superconducting states and the highest superconducting critical temperature was found at the boundary between unconventional superconducting and conventional BCS superconducting states [1-6]. Recently, we have successfully performed electrical transport measurements on fulleride superconductor families by using a specially designed pressure cell. We will comment on the details of the electronic crossover region of $A_{3}$C$_{60}$ ($A \quad =$ alkali metal) fullerides on a basis of both magnetic [6] and electrical transport measurements. Our new finding by electrical transport can provide a further understanding for highly electron-correlated unconventional molecular superconductors. [1] A. Y. Ganin \textit{et al.}, \textit{Nat. Mater}. \textbf{7}, 367 (2008). [2] A. Y. Ganin \textit{et al.}, \textit{Nature} \textbf{466}, 221 (2010). [3] Y. Takabayashi \textit{et al.}, \textit{Science}, \textbf{323}, 1585 (2009). [4] N. Iwahara, \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{111}, 056401 (2013). [5] N. Iwahara, \textit{et al.}, \textit{Phys. Rev. B} \textbf{91}, 035109 (2015). [6] R. H. Zadik \textit{et al.}, \textit{Sci. Adv.} \textbf{1}, e1500059 (2015).