Transport phenomena in two-phase systems
thesisposted on 27.06.2017, 11:23 by Aidan Wilkinson
The physics of two-phase systems is explored here, particularly magneto-transport and low temperature d.c. conductivity in thin ﬁlms. The extraordinary magnetoresistance (EMR) effect was analysed in the context of previous experimental and theoretical considerations. The magnetoresistance (MR) may be enhanced by up to two orders of magnitude by changing the geometry. This was investigated using ﬁnite element analysis. Thin ﬁlm samples consisting of a layered structure of Germanium-Tin-Germanium (Ge-Sn-Ge) were created in collaboration with Shandong University in China. Ge layers were kept at a constant thickness across all samples, with variable Sn thickness. Regions of Sn form island-like shapes ten times larger than the average ﬁlm thickness, as is seen in scanning electron microscope (SEM) images. Raman spectroscopy was conducted on these samples, from which it is concluded that the Ge layers are amorphous in nature. It was seen that there is a relationship between the electrical resistance and the ﬁlm thickness which is indicative of a metal-insulator transition (MIT). The temperature dependence of resistivity was subsequently investigated. The temperature coefﬁcient of resistivity (TCR) of the samples is seen to become negative as the thickness of the Sn layer is reduced below a certain critical thickness. Depending on their thickness, samples were designated as metallic or insulator, and various models associated with metals and insulators ﬁtted to the data. While it is impossible to be absolutely certain of the validity of each of the models, some are a better ﬁt than others. The same temperature dependence of resistivity was measured with an applied magnetic ﬁeld. This is compared with the previous EMR investigation, however the MR of the samples is only of the order of a few percent which corresponds to ordinary MR, seen in most metals. The magnetic ﬁeld measurements suppress a resistivity down-turn at very low temperatures (T<10K) which suggests the presence of superconductivity. Analysis of dr=dT shows that the onset of superconductivity is lower for samples with a lower Sn thickness. Additionally, the deposition rate of the Sn layer affects the resistivity signiﬁcantly; a higher deposition rate causes a decrease in resistivity. It is supposed that this is due to a change in the microstructure of the ﬁlm. Finally, piezo-resistivity was considered by applying mechanical compression to the samples. The added pressure causes a drop in resistivity.