Quarter-filled systems close to charge order

Quarter-filled systems close to charge order

Fig 7: Phase diagram proposed [2] for 1/4-filled organic conductors. Arrows show positions we suggest for the recently studied in our projects compounds: α-(BEDT-TTF)2MHg(SCN)4 (M=Tl, K, NH4) and α-(BEDT-TTF)2I3.

These materials are of a great interest both to experimental and theoretical physicists not only for their superconducting properties, but because they are ideal models of two-dimensional conductors. It was proposed recently that the exotic ground states observed for BEDT-TTF-based conductors, superconductivity, charge ordered insulating state, and deviations from the Drude behavior in the metallic state, are all driven by the same effect - strong electron-electron interactions (see e.g. [2]). As a result, a tiny change of the ratio of electronic correlations to bandwidth leads to a change of a ground state of a system. An example is a phase diagram in Fig.7, where a dependence of a ground state on a ratio between electronic repulsion and bandwidth is displayed; by arrows we suggest positions of the materials we recently studied in our project.

The characteristic features of each ground state are nicely observed in the optical conductivity spectra (Fig. 8) received through polarized reflectance measurements . In the spectra of α-(BEDT-TTF)2I3 an insulating gap is present at temperatures below TMI =135 K. A superconducting gap of 25 cm-1 was observed below Tc=8 K for αt-(BEDT-TTF)2I3. In the spectra of this material and of a superconductor (Tc~1 K) α-(BEDT-TTF)2NH4Hg(SCN)4 a robust Drude peak is present in low-frequency spectra of the normal state. Of superior interest are the systems α-(BEDT-TTF)2MHg(SCN)4 (M=K, Tl): d.c. conductivity shows that they are metals down to 4 K, while in optical spectra a pseudogap at about 300 cm-1 appears below 100 K. We interpret it as an evidence of charge order fluctuations close to the phase transition; part of the electronic system becomes ordered, while a narrow Drude peak is responsible for metallic conductivity. Indeed, these experimental results support the calculated phase diagram (Fig. 7).

Fig 8: Optical conductivity of ½ and ¼ filled compounds. k-(BEDT-TTF)2Cu(NCS)2 is a superconductor close to a Mott insulator and magnetic order. (BEDT_TTF)4[Ni(dto)2] is also half filled but exhibits very peculiar optical properties due to correlation effects. The Superconductor α-(BEDT-TTF)2NH4Hg(SCN)4: large U and small V: metallic at any temperature; metals α-(BEDT-TTF)2MHg(SCN)4 (M=K, Tl): large U and moderate V: pseudogap below 200 K due to presence of V, charge-order fluctuations; complete charge order α-(BEDT-TTF)2I3 large U and large V: metal-to-insulator transition at 135 K due to charge ordering.

The dependence on the band filling was also probed by optical measurements. In agreement with theoretically predicted behavior, in the strongly-correlated 1/5-filled system β"-(BEDO-TTF)5[CsHg(SCN)4]2, the pseudogap also exists in the spectra, but the Drude peak has a much higher intensity, since commensurate ordering is not possible for this filling [4]. These resent results gave us an idea of a general phase diagram showing a dependence of a ground state on the size of electronic correlations and on band filling. The aim of our present project is to prove this picture.

Theoretical predictions of a charge-fluctuation driven superconductivity could be verified by experimental studies of a quarter filled compounds close to the charge-ordered insulating state of b²-(BEDT-TTF)2SF5CH2CF2SO3. Vibrational spectroscopy of molecular vibrations can locally probe the fluctuating charge order, in addition a strong fluctuation band appears in infrared reflectance spectroscopy. The decrease of the effective electronic interaction in an isostructural metal suppresses both charge-order fluctuations and superconductivity, pointing to their interplay. The results can be described by calculations on the extended Hubbard model.


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    Proximity of the Layered Organic Conductors alpha-(BEDT-TTF)2MHg(SCN)4 (M = K,NH4) to a Charge-Ordering Transition
    Phys. Rev. Lett. 90, 167002 (2003).
  2. M. Dressel und N. Drichko
    Optical Properties of Two-Dimensional Organic Conductors: Signatures of Charge Ordering and Correlation Effects
    Chemical Review 104, 5689 (2004).
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    Indications of electronic correlations in the 1/5-filled two-dimensional conductor &beta"-(BEDO-TTF)5[CsHg(SCN)4]2
    Phys. Rev. B 72, 024524 (2005).
  4. M. Dressel, N. Drichko und J. Merino
    Evidence of charge ordering in the electronic spectra of two-dimensional organic conductors
    Physica B 359–361, 454 - 456 (2005).
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    Electronic properties of correlated metals in the vicinity of a charge-order transition: Optical spectroscopy of α-(BEDT-TTF)2MHg(SCN)4 (M=NH4, Rb, Tl)
    Phys. Rev. B 74, 235121 (2006).
  6. J. Merino, A. Greco, N. Drichko und M. Dressel
    Non-Fermi Liquid Behavior in Nearly Charge Ordered Layered Metals
    Phys. Rev. Lett. 96, 216402 (2006).
  7. N. Drichko, S. Kaiser, Y. Sun, C. Clauss, M. Dressel, H. Mori, J. Schlueter, E. Zhilyaeva, S.A. Torunova und R. Lyubovskaya
    Evidence for charge order in organic superconductors obtained by vibrational spectroscopy
    Physica B 404, 490 (2009).
  8. S. Kaiser, M. Dressel, Y. Sun, A. Greco, J. Schlueter, G.L. Gard und N. Drichko
    Bandwidth Tuning Triggers Interplay of Charge Order and Superconductivity in Two-Dimensional Organic Materials
    Phys. Rev. Lett. 105, 206402 (2010).
  9. M. Dressel
    Quantum criticality in organic conductors? Fermi-liquid versus non-Fermi-liquid behavior
    J. Phys.: Condens. Matter 23, 293201 (2011).