Charge-and Magneto-transport In Multiphase Composite Systems

In recent years, the multiphase composite systems have attracted much attention in material science and condensed physics. The multiphase composites are characteristic of the heterogeneous structure and all the phases, which may have different elements or physical properties from each other, are distributed with certain statistical law. The changes of the concentration, structure and distribution of the single phases will all lead to the changes of the macroscopic properties of the system accordingly. The most important parameter to characterize such multiphase composite systems is the microscopic morphology, which consists of materials' geometry, mainly referring to the sizes and shapes of the phases, and materials' topology, describing the how the phases connect to one another, and materials' surface structure, reflecting the roughness features. Of course all them are closely related to the length scale we choose to observe and measure the materials. There has been a long history since we studied such systems and with the rapid progress of the material science and nano-technology, people are now focusing on the design, preparation, and assembly of various mesco-composites to achieve particular functions. The man-made composites have been utilized in all aspects of our daily life, ranging from semiconductors, metals, superconductors, magnets and oxide-ceramics to polymers. They may exist as micro- or nano- polycrystal, grains, thin film, fiber and other heterogeneous structures. The studies of the multiphase composites have broken some of the old concepts and rules, making people to re-investigate the physical process governing such systems.Modern technologies like tunneling electron microscope (TEM), scanning tunneling microscope (STM) have shown up the composites' heterogeneous structure at mesoscopic or even microscopic length scale. The microscopic morphology and its relation to the macroscopic physical features, however, have not been understood thoroughly and need our further discussion. Development of the theory and calculation method may help to better understand and predict the effective physical properties, like the effective conductivity, effective dielectric constant and effective module of such systems. The most successful approaches ever used are the continuum models and the discrete models. Based on solving the classical continuum equations of transport, the continuum models can predict the effective features and transport properties of the composite materials. Rarely, however, can such continuum models provide accurate predictions of the macroscopic properties of strongly disordered multiphase materials. In particular, when the contract between phases is large or the phases form large clusters, most continuum models break down. At the same time, due to their very nature, the discrete models, which are based on a lattice representation of a material's morphology, have the ability for providing accurate predictions for the effective properties of such heterogeneous systems. In this sense, the discrete models are good complementary to the continuum models. Though theories have been established about such discrete models, they have not been well applied to the studies of the transport characters in multiphase composite systems.Half-metallic oxides granular systems like polycrystalline samples, cold pressed powers and other composites are one of the most widely studied multiphase composite systems. The nano-size half metallic grain phase always coexists with the grain boundarie (GB) regions. The "grain boundaries" here include not only natural but also some artificial GB junctions, grain surfaces in pressed powders and various other composites. Grain-boundary magnetoresistance effect is found due to the existence of lots of GBs, which play a key role in the magneto-transport process in such systems, because the magneteresistance is closely related to the spin-polarized process in the boundaries. Actually, the properties of GBs, such as the density of local spins inside, the electronic and magnetic features of GBs will result in different transport mechanisms like direct tunneling, resonant tunneling and etc.. When applied magnetic or electrical field or under the press, the half-metallic and GBs phases will form some kinds of topology structures, i.e. different transport network, and result in complex grain-boundary magnetoresistance behaviors accordingly. Therefore, half-metallic multiphase composites have fundamental meanings and extensive technological applications. Conducting polymers system is another widely studied multiphase composites, and is a new kind of functional materials as well. Microstructure studies point out that there are nanosized crystalline regions where the polymer chains are regularly and densely packed and therefore can be treat as metallic grains. Outside these regions the order in chain arrangement is poor and the chains form amorphous media. The whole system has the feature of heterogeneous disorder, and the two phases couple into a chain-linked granular network. The coupling between the metallic grains is provided by single chains, which may extend into several regions, therefore the electrical connection is totally different from that of dot connection in conventional granular systems. Some novel transport phenomenon are find such as it has no well defined critical point for its disorder induced metal-insulator transition, which has been observed over a relatively wide temperature range and can be tuned by applying external pressure and/or magnetic fields, the same to its effective conductivity and dielectric coefficient. But to date no models can give a comprehensive description about the conducting polymer multiphase composite systems.Theoretical and experimental researches have proved that high performance of semi-conducting polymer thin film transistors with high mobility resulted from the increase of the improvement of polymer structures. The most studied poly(3-alkylthiophene)s can achieve great increase in mobility by self-organize to polycrystalline lamella structures after some deposit surface treatment process, which will lead to good polymer chain packing. And the boundaries between the crystalline parts determine the charge transport and the high mobility achieved so far all related to the properties of these boundaries. But theoretical explanations are still lack.The purpose of our work is focused on the charge- and magneto-transport in the two multiphase composite systems. The main results are listed as follows:1. The magneto-transport in half-metallic granular systemsWe study the magnet-transport process in half-metallic granular systems, including manganite polycrystalline system and CrO₂ cold pressed powders. We found that the existing resistor series connection model and parallel connection model can't well describe the experimental results. Investigating the detailed micro-structure in such dispersed systems, we find that most of the half-metallic are packed and separated by the grain boundaries. The transport across the grain boundaries dominates. Considering the mesoscopic heterogeneous characters, we establish a random resistor network (RRN) model with the resistors are distributed, which reflect the disorder of the system and may overcome the disadvantages of the previous series and parallel connection model. We can conveniently use RRN model to discuss the effect of the network dimensionality and the resistors random distribution on the magneto-transport process since it is based on the discrete model.For granular half-metallic oxides like some polycrystalline samples and pressed powders, we only consider the direct tunneling mechanism since it dominates in such systems. We build the random resistor network (RRN) model and compare it with the series connection model and parallel connection model. The latter two are based on the continuum model therefore are not sufficient to describe the transport process. Our calculations prove that for the half-metallic granular systems with high spin polarization, the transport network effects can't be ignored. With the increase of the dimension of the network, the magnetoresistance value decreases and such effect is more prominent for higher spin polarization materials. The random distribution of the tunneling resistance will influence the magnetotransport in such a way that the more disordered system has the larger magnetoresistance value, and all these well agree with the related experimental data.We conclude that in half-metallic multiphase composites, the RRN model is a much more better candidate to describe the micro-structure and its relation with the magnetotransport properties.2. The charge—transport of highly conducting polymer systemsWe study the carrier dynamics in conducting polymers such as polypyrrole and polyaniline. We observe that the systems are composite medium with metallic regions randomly distributed in the amorphous parts. Within the metallic regions, the polymer chains are regularly and densely packed, outside which the poorly arranged chains form amorphous host. The grains have quantum size their shapes can't be described by a single geometric form. The energy scale for interchain charge transfer is about two orders of magnitude smaller than that for intergrain charge transfer. The intergrain transport is essentially an intrachain process that proceeds along the covalently bound chains. We further the previous work, and based on the random resistor network (RRN) model we propose for the first time a chain linked granular system. Including the coherent and incoherent transport mechanism in our RRN model, we reproduce the dielectric response through an metal-insulator transition in conducting polymers.In our RRN model, we place metallic and insulating dots on the lattice sites. The resistors inserted between them represent different couplings, which may change with exploring frequency and temperature. The effective conductivity and dielectric can therefore be calculated within the framework of RRN model at different external conditions. Our calculation show that the highly conducting polymers are a particular composite near metal to insulator transition and the low energy carrier dynamics is determined by the competition between the coherent and incoherent channels, which result in the unusual frequency dependent conductivity and multiple zero crossing of dielectric function of highly conducting polymers. Our calculations are in good agreement with the experimental results.3. The charge—transport of semi-conducting polymer systemsIn this section we study the charge transport in semi-conducting polymers of poly-(3hexylthiophene), (P3HT) with polycrystalline lamella structures, where in the crystalline parts, the chain packing densely and orderly, while the parts between them form the disordered regions. The mobility in the disordered boundaries is several orders less than that of the crystalline parts, therefore determines the effective mobility of the composite system. The improvement in the structures of these boundaries is most dominant factor.We here build several boundary structures resulting from the crystallites orientations and study their electronic structures using the first principle method. Our calculation results show that the charge transport in such system is mesco-anisotropic, and the conductance for different boundary structures ranges for sever orders in magnitude. Our calculations give a theoretical proof that it is necessary to consider heterogeneous structures and especially the electronic process in boundaries.In summary, the multiphase composite systems, for both the half-metallic granular materials and conducting polymers, are characterized with the heterogeneous structures. There are always two or more phases, which have remarkable difference in physical features. Both the materials' geometry, mainly including the sizes and shapes of the phases, and materials' topology, describing the how the phases connect to one another, determine the transport process and therefore the macro-properties of such systems. Our RRN model, since based on the discrete model, can successfully use to describe the microstructures of the composites and reproduce many experimental results.