Magnetotransport Properties in Underdoped High Tc Superconductors

More than two decades after the first discovery of high-temperature superconductivity (HTS) in copper oxides in 1986, the central issues in the field - the nature of the insulating ground state at low carrier densities and the emergence of HTS from that state with doping, are still unresolved. In this dissertation work, magnetotransport and resistance noise measurements in doped La2CuO4 (LCO), an archetypal HTS family, are performed to address these central questions in several novel aspects. A series of high quality Sr-doped LCO (LSCO) MBE grown thin film samples with doping x=0.01-0.08 have been prepared and measured in high magnetic fields up to 35 T in a wide range of temperatures from room temperature down to 20 mK. These samples span the antiferromagnetic, spin glass, and superconducting phases. For comparison studies, high quality single crystals, such as a superconducting LSCO with doping x=0.06 and a non-superconducting Li-doped LCO (Li-LCO) with doping x=0.01, have also been grown and measured. The main results are as follows. First, detailed magnetotransport measurements suggest the existence of a charge glass state in lightly doped LSCO samples as temperature approaches zero. This charge glass state manifests itself in several ways, such as hysteretic magnetoresistance and the associated history-dependent effects (e.g. memory and zero-field resistance difference after zero-field cooling and field cooling protocols). The doping dependence of such an insulating state has been reported for the first time. Second, in order to investigate the emergence of HTS from the glassy insulating ground state, we have studied two different superconductor-insulator transitions (SITs) by tuning either the charge carrier doping or the external magnetic field. In doping-tuned SIT, measurements have revealed that superconducting fluctuations emerge on the insulating side of the SIT, and more importantly, discovered the coexistence and competition of superconductivity and charge glassiness in HTS materials. The unexpected quenching of the superconducting fluctuations by the competing charge order at low temperatures provides a new perspective on the mechanism for the SIT. Third, the scaling analyses in magnetic-field-tuned SIT and the in-depth studies of superconducting fluctuations in both thin film and single crystal samples suggest the appearance of an intermediate state between the high-temperature insulating state and the low-temperature superconducting state. In addition, the insulating state in the presence of a magnetic field in a wide doping range shows two-dimensional variable-range hopping transport, which indicates a common 2D insulating state in both insulating and superconducting samples. Fourth, the analysis of superconducting fluctuations in a thick superconducting LSCO film indicates the possibility of Berezinskii-Kosterlitz-Thouless (BKT) physics and strong phase fluctuations in lightly doped LSCO. The possibility of a large vortex core energy is also suggested. Fifth, the angular dependence of the magnetoresistance and the possibility of retrieving Fermi surface information from magnetotransport measurements are discussed. Sixth, resistance noise measurements in insulating LSCO thin films are consistent with experiments on single crystal samples. Similar measurements and analysis techniques have been successfully applied in other systems, such as a 2D electron gas in semiconductors. Seventh, in Li-LCO samples, a topologically nontrivial spin arrangement, i.e., a Skyrmion, is also suggested, which could further clarify the relationship between magnetism and superconductivity in HTS. Furthermore, several innovations and improvements in experimental design, instrument control, and capability expansion, etc., have been reported in the main text and appendixes.