LAUSR

The two-dimensional organic superconductor $\ensuremath{\lambda}\ensuremath{-}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ exhibits pronounced charge fluctuations below $T\ensuremath{\approx}150$ K, in contrast to the sibling compound $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ that remains metallic down to milli-Kelvin. Infrared spectroscopy reveals only minor splitting in the vibrational features of the latter compound, common to other strongly dimerized $\ensuremath{\kappa}$-type salts. When the organic molecules are arranged in the $\ensuremath{\lambda}$-type pattern, however, a strong vibrational ${\ensuremath{\nu}}_{27}({b}_{1u})$ mode is present that forms a narrow doublet. Most important, when cooling $\ensuremath{\lambda}\ensuremath{-}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ below 150 K, two weak side modes appear due to charge disproportionation that amounts to $2\ensuremath{\delta}=0.14e$. In analogy to the ${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in $\ensuremath{\lambda}\ensuremath{-}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ at ${T}_{c}=4.7$ K. We discuss the possibility of a charge-density wave that coexists with the proposed spin-density-wave state.