Organic molecular structures and low-dimensional compounds

In quasi one-dimensional organic compounds the Fermi liquid is not a ground state and describes physical properties only approximately, at relatively high temperatures. As temperature decreases, the interparticle interactions come into play and lead to instabilities developing in the charge- or spin-channel. As a result, there emerge novel states such as charge/spin-density wave state, or Luttinger state with spin-charge separation. Similar to the case of 2D system, application of quantizing magnetic field gives birth to novel phases – cascades of the field induced spin density waves with quantized nesting wave vector. For even lower temperatures, quite often, a superconducting state becomes more energetically favorable. The most interesting and less studied are the areas at the phase diagram in the vicinity of boundaries between various states– spin density wave and superconductor, superconductor and normal paramagnet, superconductor and insulator etc. In these areas, due to competition between various phases, there emerge novel phase-inhomogeneous states with rather unusual properties.

The physics of competition between the spin density wave state and superconductivity is studied in the SCES Laboratory using single crystals of the (TMTSF)2PH6, (TMTSF)2ClO4, (TMTSF)2AsF6 compounds.

Organic field effect transistors and transport mechanism in molecular crystals. The quest for high-performance organic field-effect transistors (OFETs) has resulted in a significant increase of the charge carrier mobility. In the best devices based on thin organic films, mobility values of up to 1.5 cm2/V s have been reported. This performance is already comparable with that of amorphous-silicon FETs. However, there are still several important issues to be resolved, most of them being associated with grain boundaries and interfacial disorder in organic thin films. Indeed, currently these structural defects are the major factor which limits the mobility, and results in the broadening of the on/off transition. Grain boundaries can be eliminated in devices fabricated on single crystals of organic semiconductors. Growth in the laboratory of high quality organic single crystals (rubrene, pentacene, etc) and fabrication of the field effect gated structures on their surface is an interesting direction of research aimed at understanding of the charge transport mechanisms in organic molecular crystals, improving carrier mobility and achieving a regime of the diffusive transport. Besides academic interest, it has practical importance for the growing field of plastic electronics.