Charge Imbalance In Lambda-(Bets)(2)Gacl4 And Their Interplay With Superconductivity

The two-dimensional organic superconductor lambda-(BETS)(2)GaCl4 exhibits pronounced charge fluctuations below T approximate to 150 K, in contrast to the sibling compound kappa-(BETS)(2)GaCl4 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 kappa-type salts. When the organic molecules are arranged in the A-type pattern, however, a strong vibrational nu(27)(b(1u)) mode is present that forms a narrow doublet. Most important, when cooling lambda-(BETS)(2)GaCl4 below 150 K, two weak side modes appear due to charge disproportionation that amounts to 2 delta = 0.14e. In analogy to the beta ''-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in lambda-(BETS)(2)GaCl4 at T-c = 4.7 K. We discuss the possibility of a charge-density wave that coexists with the proposed spin-density-wave state.