Charge imbalance in $\lambda$-(BETS)${}_2$GaCl${}_4$ and their interplay with superconductivity

Abstract

The two-dimensional organic superconductor $\lambda$-(BETS)${}_2$GaCl${}_4$ exhibits pronounced charge fluctuations below $T\approx 150$~K, in contrast to the sibling compound $\kappa$-(BETS)${}_2$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 $\kappa$-salts. When the organic molecules are arranged in the $\lambda$-pattern, however, a strong vibrational ${\nu }_{27}(b_{1u})$ mode is present, that forms a narrow doublet. %indicating static charge imbalance of about 2\%. Most important, when cooling $\lambda$-(BETS)${}_2$GaCl${}_4$ below 150~K, two weak side modes appear due to charge disproportionation that amounts to $2\delta =0.14e$. In analogy to the ${\beta }^{\prime \prime }$-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in ł at $T_c=4.7$~K. We discuss the possibility of a charge-density-wave that coexists with the proposed spin-density-wave state.

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Cited by in corpus (1)

• Kink in Stoner Factor as a Signature of Changing Magnetic Fluctuations in the Organic Conductor $\lambda$-(BETS)${}_2$GaCl${}_4$