Hall effect analysis on Bi-2212 whiskers

In this thesis the transversal (Hall) resistivity in highly-pure whisker-like crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212) is studied. This physical property has been extensively studied among cuprate superconductors, as it shows several anomalous behaviors. In par- ticular, the change of sign of the Hall coe?cient RH around the critical temperature Tc has been measured by numerous groups. However, despite the recent intensive experimental research, this phenomenon has not been yet completely understood. The aim of this thesis is to investigate this phenomenon on whisker- like crystals, which have a number of imperfections considerably lower with respect to other growing techniques. Bi-2212 whiskers have been produced in the Chemistry department of Turin University using the splat cooling method, followed by a thermody- namic cycle in a furnace which made the crystals growing with the desired shape and oxygen content. Subsequently, electrical contacts have been de- posited on the whisker using electron-beam evaporation, photolitography, wet and dry etching and Focused Ion Beam techniques, available at the lab- oratories of the Physics department of Stockholm University. Measurements have been performed in the cryostats of the Experimental Condensed Matter Physics group. Several preliminar analysis have been done in order to verify the suit- ability of the measurement system. A residual magnetization of the magnet has been found, lower than 0.25 mT, as well as a temperature shift of the thermometer, causing a systematic error of 0.3K. The prepared samples have been discovered to be composed by a 2212 phase (Tc ? 76 K), but traces of 2223 phase have also been found (Tc ? 107 K). The anomalous change of sign has been detected in all samples, as well as a dependence of its amplitude on the applied magnetic ?eld B. Concerning the most regular sample (DC010), the intensity of the Hall coe?cient RH has been found comparable with the values in the literature for high magnetic ?elds (? 10 2 cm3 =C). However, an anomalous dependence of RH by the magnetic ?eld value is detected. In particular, very high values of RH at low B have been measured. At 35 mT these values are one order of magnitude higher compared to the ones obtained at high B. The reason of this behavior i