The purpose of the MOLCOM Action was to:
-control the factors governing the flow of electrons in crystalline charge-transfer molecular conductors
-prepare new molecular crystals exhibiting 2-D conductivity in order to optimise conditions for superconductivity and, in particular, increase the critical temperature
-elaborate and characterise new materials exhibiting extremely 1-D conducting properties for use as electron paramagnetic resonance (EPR) sensors
-develop a simulation computer programme allowing the prediction of possible obtainable crystal structures starting from a given molecule
-establish guidelines for the synthesis of new materials by modelling their electronic properties.
A computer program was developed to simulate the geometric arrangement of the building blocks in a crystal and the specific properties of individual molecules.
Factors governing the flow of electrons in organic conductors were investigated. A better understanding of this is needed before organic conductors can be used in the fabrication of microelectronics components. Compounds with various degrees of one dimensionality and disorder were prepared and their physical properties studied.
The new donor molecule (TMDTDSF) of the TMTSF type but comprising 2 sulphur and 2 selenium atoms has demonstrated the possibility of tailoring materials of the (TM)2X series. The role of disorder between conducting chains has been investigated by preparing solid solutions. The formation of a solid solution has been established by the x-dependence of the c-lattice parameter. The transport properties at low temperature do not seem to be crucially affected by the weak off chain disorder. However, some effects of the alloying have been detected on the transition temperature towards a spin density wave (SDW) state and the depinning of the SDW.
An oscillating component of the SDW current has been observed with a frequency proportional to the direct current SDW current (TMTSF)2PF6. Theoretical derivation has been achieved of the weak localization correction to the conductivity for a strongly anisotropic 2-dimensional electron gas in the presence of a magnetic field.
Study has been carried out of the alpha-phase of the family (perylene)2[M(mnt)2] involving transport, X-ray and magnetism. Tris-tetrathiafulvalene derivative of triptycene has been synthesized and the growth conditions for the largest superconducting transition in (ET)2Cu(SCN)2 have been optimized.
APPROACH AND METHODS
The materials to be studied can be considered as a loose assembly of units of reduced dimensionality (chains or planes for 1-D and 2-D respectively) in which the molecules are more tightly and strongly packed than between units. Electronic conduction in these materials proceeds via intermolecular electron transfer (along chains or within planes for 2-D materials). Localisation is expected to occur for vanishingly small overlap integrals between molecules. This localisation was to studied through the effects that the temperature and pressure have on:
-structure (averaged and lattice distortions)
-transport properties in the presence of a controlled disorder (obtained by alloying or irradiation)
-intermolecular or interchain disorder and the role of Coulomb forces on the localisation.
-the role of band fitting (commensurability) on the conducting properties
-the nuclear magnetic resonance (NMR) properties (high-resolution NMR techniques in solids are being adapted for molecular conductors).
PROGRESS AND RESULTS
Various conducting systems exhibiting a controlled disorder have been synthesised:
-The new donor molecule (TMDTDSF) of the TMTSF type but comprising two sulphur and two selenium atoms has demonstrated the possibility of tailoring materials of the (TM)2X series which lie between (TMTTF)2X and (TMTSF)2X salts in the generic phase diagra m of these materials.
-The role of disorder between conducting chains has been investigated by preparing solid solutions such as (TMTSF)2AsF6(1-x)SbF6x. The formation of a solid solution has been established by the x-dependence of the c-lattice parameter. The transport proper ties at low temperature (but T > Tsdw) do not seem to be crucially affected by the weak off-chain disorder. However, some effects of the alloying have been detected on the transition temperature towards a SDW state and the depinning of the SDW condensate.-First observation of an oscillating component of the SDW current with a frequency proportional to the dc SDW current in (TMTSF)2PF6.
-Theoretical derivation of the weak localisation correction to the conductivity for a strongly anisotropic 2D electron gas in the presence of a magnetic field.
-Study of the alpha-phase of the family (perylene)2[M(mnt)2] with M=Au, Pt, Pd, Fe, Co, Ni, Cu. Transport, X-ray and magnetism (EPR, susceptibility and NMR) have been studied for M=Au, Cu (diamagnetic acceptors), M=Pt, Pd, Ni, (uniform chain of S=1/2 spi ns), M=Fe (dimerised chain of S=1/2 spins).
-Tris-tetrathiafulvalene derivative of triptycene has been synthesised.
-The growth conditions for the largest superconducting transition in (ET)2Cu(SCN)2 have been optimised.
- A strong and "normal" isotope effect on Tc is observed in H - (ET)2I3 when both central carbons in the ET molecule are replaced by 13C isotopes.
A fairly complete understanding of the generic phase diagram of Bechgaard-like materials has been reached.
Contrary to previous results, it seems that a weak anion disorder does not affect significantly the conductivity properties, although finite effects are seen on the establishment of the SDW ground state and the depinning of the SDW condensate.
A new channel of collective conduction through SDW sliding has been evidence by the generation of an AC current.
The first step towards the fabrication of 2-D organic conductors has been achieved with the synthesis of triptycene molecules.