Charge imbalance in $λ$-(BETS)$_2$GaCl$_4$ and their interplay with superconductivity

Abstract

The two-dimensional organic superconductor λ-(BETS)2GaCl4 exhibits pronounced charge fluctuations below T ≈ 150 K, in contrast to the sibling compound κ-(BETS)2GaCl4 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 κ-salts. When the organic molecules are arranged in the λ-pattern, however, a strong vibrational ν27(b1u) mode is present, that forms a narrow doublet. Most important, when cooling λ-(BETS)2GaCl4 below 150 K, two weak side modes appear due to charge disproportionation that amounts to 2δ = 0.14e. In analogy to the β ′′ -type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in λ-(BETS)2GaCl4 at Tc = 4.7 K. We discuss the possibility of a charge-density-wave that coexists with the proposed spin-density-wave state.