Intrinsic tunneling spectroscopy of low-Tc cuprate superconductors : Pseudogap, fluctuations and electrical doping

Sammanfattning: In this thesis, intrinsic tunneling and magnetic transport measurements of the cuprate superconductor Bi2+xSr2-yCuO6+δ (Bi-2201) are presented. The compound is characterized by a single-CuO2 plane, has a very low critical temperature Tc ~ 4K and a relatively low upper critical field Hc2 ~ 10T. Therefore, superconductivity can be suppressed even at low temperatures, which opens the possibility to study normal-state properties with a relatively low influence of thermal fluctuations. Understanding the mechanism of Tc suppression in this compound might help understanding the mechanism of high Tc in other cuprates. The measurements where performed on small mesa structures with areas less than a square micrometer, which allow for measurements with nearly no artifacts caused by heating or crystal defects.This series of experiments demonstrate that all superconducting characteristics are reduced inproportion to Tc , but the corresponding c-axis pseudogap characteristics remains similar to that in high-Tc Bi-2212 and Bi-2223 compounds with 20-30 times larger Tc. This large disparity reveals that the pseudogap is not connected to superconductivity, instead it possibly represents a different spin-singlet order that is universal to all cuprates. Furthermore, the experiments show that the thermal fluctuation region is not extraordinary large in this compound and is therefore not a cause of the low Tc, which leads to the assumption that this is caused by a weaker coupling, resulting in a smaller Cooper pair energy.In addition, the change of mesa properties by electrical doping is studied on this compound, as well as on Bi-2212. The superconducting and normal state properties can be altered in a similar way than by conventional doping, without the use of different samples. This makes doping dependent studies much simpler and helps to understand the puzzling behavior of existing states within these compounds.