Organic metals are unique materials: the crystal is built by organic molecules and conduct electricity at room temperature as well as common metals; on cooling down some of these crystals become even superconducting below 13 K, some stay metallic and some undergo a metal-insulator transition . To answer the question Why does this happen? is the aim of our project.

Fig1.
A schematic crystal structure of a BEDT-TTF based organic conductor. A single BEDT-TTF molecule is shown in red

The crystals are built by layers of BEDT-TTF (bis(ethylenedithio)-tetrathiafulvalene) molecules (Fig.1) sandwiched between the sheets of counter anions. The π-electron orbitals of the BEDT-TTF aromatic rings overlap and form a conductance band. The anion layer donates electrons to the BEDT-TTF molecules, charging them up to approximately +0.5e per molecule; consequently, the conductance band is partly filled and the material is metallic. The metallic properties are observed only within the layers; in the perpendicular direction the insulating anion layer blocks charge transfer. For that reason these materials are called two-dimensional conductors. The reduced dimensionality makes them attractive objects for theoretical studies and for experimental test of the predictions by theory.

The history of this field is an exciting example, how a physical theory can trigger a new field of solid state physics and lead to the creation of new materials. In 1964 a paper was published by J. Little [1], predicting a room-temperature superconductivity in structures, that can be synthesized using organic molecules (polymers at that time) rich with ?-electrons; ironically, the paper was basically wrong, as it was dealing with one-dimensional structures, which can be metallic at high temperatures, but will always have an insulating ground state, as was shown by Peierls [2]. However, material scientists started to work, and in 1979 superconductivity under pressure was observed in quasi one-dimensional crystals now called Berchgaard salt, and in 1984 a first normal-pressure organic conductor β-(BEDT- TTF)2I3 appeared. By changing chemical and thus physical parameters of the crystals the temperature of superconducting transition was increased up to 12 K for κ-(BEDT- TTF)2Cu[N(CN)2]Br.

Fig2.
Phase diagram proposed [2] for -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. [3]). 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. 2, 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. 3) 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. 2).

Fig2.
Optical conductivity of filled compounds. 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 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 a size of electronic correlations and on band filling. The aim of our present project is to prove this picture.

Further reading: M. Dressel, N. Drichko. Optical properties of Organic Conductors. Review article. Chemical Reviews (2004 in press).

References

Contact: N. Drichko, M. Dressel