Studies On The Theory Of Abnormal Magnetoresistance Of Non-stoichiometric Silver Chalcogenides

An abnormal magnetoresistance (MR) of non-stoichiometric silver selenide and telluride (called by a joint name silver chalcogenide) is found firstly in 1997. It was shown that the MR of silver chalcogenide samples exhibits a linear dependence on magnetic field from 1mT to 5.5 T in the range of temperature from 4 to 300K. Silver chalcogenide material is expected to be measuring instrument for the high magnetic field, pulse laser technology, magnetic technology and information read technology. U.S. Department of Energy plans it as one of the most attentive materials in the next decade. However, the preparation technology for the materials need be improved further more and the investigation of materials' nature is inadequate. The reports about materials are even contradiction. Driven by the experimental results, theory researchers proposed models to interpret the unusual linear MR effect. Although the theory study has achieved some progress but no model has been compared with the experimental data quantitatively. A theory based on the first principle for the MR of non-stoichiometric silver chalcogenides is not built.Taking this into consideration, we observed the experimental fact, especially the structure of materials, and investigated the existing theory, then explored the mechanism of the MR effect of materials thoroughly and systemically. Based on two levels, we proposed four models for the MR of materials which results can be compared with the experimental results quantitatively.A first model proposed by us is a phenomenological model on the first level. The basic physical thought of the model is that the silver chalcogenide is regard as two-component (silver metal component and semiconductor component) composition. The key of the model is that the dependence of the mobility of carriers in the materials on the temperature and magnetic field is found. Based on the effective media approximation (EMA), the model for the MR of the three-dimension bulks is proposed and the results by the model agree well with the experimental data for the first time. The model takes the MR as a function of magnetic field, temperature and the conductivity of two components without magnetic field, and works when T > 112.5K . A critical volume fraction of silver metal is found, at which the MR reaches the maximum value in the same magnetic field and temperature. We suggest that the critical volume fraction is the result of the percolation between silver particles in silver selenide and telluride bulks.The second model on the first level is a duality transformation model of two-component composition. Since boundary effect of the thin films is remarkable, the EMA model is unsuitable for the thin films. So a new model need be proposed for the MR of thin films. We hold the supposition that the silver chalcogenide is two-component composition which can be depicted by the "random droplet" model without magnetic field. Now we must calculate the effective conductivity tensor for the films in a magnetic field. A general duality transformation is found which maps the conductivity-tensor into a new, isomorphic, well-defined conductivity tensor. It is indicated that the model predictions agree well with the available experimental data.For discussing the physical mechanism of MR effect intuitively and thoroughly, a quasi-random network model was proposed to simulate the real material on the second level (the half-phenomenological level). The main thought of the model is that the current in the real material can be described by the current in the resistor network.The thin film is dispersed into a two-dimension resistor network which is constructed from four-terminal resistors and the bulk is dispersed into a three-dimension resistor network which is constructed from six-terminal resistors. The mobility of carriesμwithin the network has a quasi-random distribution, i.e. a Gaussian distribution with two constraint conditions. The two constraint conditions are as follows: 1) The boundary values ofμare themobility of silver metalμ₁ and semiconductorμ₂ respectively; 2) The average value ofμis described by the parallel connection model of composite. The model predicts the dependence of the MR with the magnetic field and temperature. A good agreement is found between the theoretical MR and the available experimental data. The 2D and 3D visualizations of current flow within the network are indicated: the inhomogeneous of mobility in the network induce the intense curving flowing of electric current and MR effect of materials in magnetic field. This work is helpful for the mechanism of the MR of materials based on the first principle. The longitudinal MR of the non-stoichiometric silver chalcogenide was reported by Hu et al. It is indicated that the p-type and n-type doped materials has complicated longitudinal MR, including positive MR and negative MR. The only literature proposed a model which shows the negative MR. In this work, a random network model was proposed to investigate the complex phenomenon of longitudinal MR of materials. The resistivity tensor of material and the tensor of resistor unit in the network are corrected because of the inhomogenous nature of materials and the nonzero transverse-longitudinal coupling component of resistivity tensor. The model predictions are compared with the experimental result Semi-quantitatively. The visualizations of current flow within the network indicates the intense flowing of electric current along sample's thickness direction which analogous to the effect known as current jetting. The findings indicate that the inhomogenous nature of materials induces the complicated longitudinal MR.