Charge-order fluctuations and superconductivity in two-dimensional organic metals

Abstract

We report comprehensive Raman and infrared investigations of charge-order (CO) fluctuations in the organic metal ${\beta }^{\prime \prime }$-(BEDT-TTF)${}_2$SF${}_5$CHFSO${}_3$ and superconductor ${\beta }^{\prime \prime }$-(BEDT-TTF)${}_2$SF${}_5$CH${}_2$CF${}_2$SO${}_3$. The charge-sensitive vibrational bands have been analyzed through an extension of the well-known Kubo model for the spectral signatures of an equilibrium between two states. At room temperature, both salts exhibit charge fluctuations between two differently charged molecular states with an exchange frequency of about $6\times 10^{11}{\mathrm{s}}^{-1}$. The exchange rate of the metallic salt remains roughly constant down to 10 K, while in the superconductor the exchange velocity starts to decrease below 200 K, and a "frozen" charge-ordered state emerges, and coexists with the charge-order fluctuation state down to the superconducting temperature. These findings are confronted with other existing spectroscopic experiments, and a tentative phase diagram is proposed for the ${\beta }^{\prime \prime }$ BEDT-TTF quarter-filled salts.

10 pages, 10 figures, submitted to Phys. Rev. B

References in corpus (5)

• Quantum frustration in organic Mott insulators: from spin liquids to unconventional superconductors
• Theoretical Aspects of Charge Ordering in Molecular Conductors
• Absence of charge order in the dimerized κ-phase BEDT-TTF salts
• Charge ordering and coexistence of charge fluctuations in quasi-two-dimensional organic conductors $\theta$-(BEDT-TTF)${}_{2}X$
• DC and high-frequency conductivity of the organic metals beta"-(BEDT-TTF)2SF5RSO3 (R = CH2CF2 and CHF)

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• Effects of Electron Correlations and Chemical Pressures on Superconductivity of ${\beta }^{\prime \prime }$-type Organic Compounds
• The metal-insulator transition in the organic conductor ${\beta }^{\prime \prime }$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl
• Charge imbalance in $\lambda$-(BETS)${}_2$GaCl${}_4$ and their interplay with superconductivity
• Raman signatures of the strong intra- and inter-molecular charge oscillations in bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) \k{appa}-phase salts