454 research outputs found
Optical-phonon-mediated charge transport in substituted morpholinium TCNQ2 salts
Substituted morphollnium TCNQ2 compounds exhibit some universal features with respect to the field, frequency and temperature dependence of the conductivity. These characteristics are different from the closely related morphollnlum TCNQI compounds. We show that a natural way to account for these phenomena comes about by careful examination of the order parameter space, i.e. the space created by the two different order parameters which together determine the gap at the Fermi surface: (i) the regular Pelerls dimerization of the acceptor-acceptor overlap and (li) a coherent shift of the donor ~olecules with respect to the accep$ors. By describing small revolutions through parameter space (realized by phase coupling of two different phonons producing the above mentioned gap) the ground state electron gas is shifted with respect to the lattice, thereby producing a current. The model allows for a qualitative description of conductivityrelated phenomena in these systems
Dopant-induced crossover from 1D to 3D charge transport in conjugated polymers
The interplay between inter- and intrachain charge transport in bulk polythiophene in the hopping regime has been clarified by studying the conductivity s as a function of frequency ¿/2p (up to 3 THz), temperature T, and doping level c. We present a model which quantitatively explains the observed crossover from quasi-one-dimensional transport to three-dimensional hopping conduction with increasing doping level. At high frequencies the conductivity is dominated by charge transport on one-dimensional conducting chains
Spin dynamics in the Cu(2)-O planes of tetragonal and orthorhombic YBa2Cu3O7-delta as probed by 89Y NMR
The 89Y-nuclear relaxation rate is found to be almost similar in the orthorhombic and tetragonal modifications of YBa2Cu3O7-d. This result is seen as evidence for the unchanged spin dynamics in the Cu(2)-O planes in both compounds and as support for those theories that decouple charge and spin carriers
The conductivity and band structure of some synthetic semiconductors based on M(dmit)2 (M = Ni, Pd, Pt)
The conductivity of the following six new low-dimensional compounds is reported: (Me4N)(Ni(dmit)2), (Et4N)(Ni(dmit)2), (Pr4N)(Ni(dmit)2), (Bu4N)(Ni(dmit)2), (Bu4N)(Pt(dmit)2) and (Et4N)0.5(Pd(dmit)2). Semiconducting behaviour is found for all the compounds investigated, with activation energies between 0.1 and 0.5 eV. The crystal structures have been established by X-ray diffraction. Using these, the band structure has been determined. It is shown that the compounds investigated are semiconductors each having a correlation gap resulting from an effective electron-electron repulsion of 1.0 eV
Measurement of the complex dielectric constant down to helium temperatures. I. Reflection method from 1 MHz to 20 GHz using an open ended coaxial line
Metals in Catalysis, Biomimetics & Inorganic Material
Understanding the doping dependence of the conductivity of conjugated polymers: dominant role of the increasing density of states and growing delocalization
In variable-range-hopping theories for the dc conductivity, the extension of sites where the charges are located and the energy dependence of the density of states (DOS) are usually neglected. We show that these dependences are the dominant factors for understanding the strong doping dependence, and present an analytical theory for arbitrary DOS. We verify the theory with systematic data over a broad range of temperature and doping for FeCl3-doped poly(p-phenylene vinylene). By combining theory and data, we reconstruct the energy-dependent DOS and the extension of sites
Quantum tunnelling of magnetization in Mn12-ac studied by 55Mn NMR
We present an ultra-low temperature study (down to T = 20 mK) of the nuclear
spin-lattice relaxation (SLR) in the 55Mn nuclei of the molecular magnet
Mn12-ac. The nuclear spins act as local probes for the electronic spin
fluctuations, due to thermal excitations and to tunnelling events. In the
quantum regime (below T = 0.75 K), the nuclear SLR becomes
temperature-independent and is driven by fluctuations of the cluster's
electronic spin due to the quantum tunnelling of magnetization in the ground
doublet. The quantitative analysis of the nuclear SLR shows that the presence
of fast-tunnelling molecules, combined with nuclear intercluster spin
diffusion, plays an important role in the relaxation process.Comment: RevTex, 5 pages, 3 eps figures; presented at the Internation
Conference on Molecular Magnets (Valencia, 5 - 10 Oct. 2002); to be published
in Polyhedro
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