Tunneling studies of pseudogap in high-Tc superconductor Bi2212

The BCS theory predicts an energy gap in superconductors below their critical temperature T_c. This gap has been observed in conventional as well as high-T_c cuprate superconductors. However, in underdoped cuprates an energy gap has been found to exist even above T_c from several experiments. In this dissertation a study of this normal state gap, called the pseudogap, is presented using ab-plane tunneling spectroscopy of a high-T_c superconductor Bi₂Sr₂CaCu₂O_8+δ (Bi2212) at different dopings δ. Temperature and magnetic field dependent ab-plane tunneling spectra of Bi2212 crystals were studied using a home built low temperature scanning tunneling microscope (STM).; Single crystals of Bi2212 were grown using self flux method, which were than underdoped by annealing in vacuum or in Argon atmosphere at 500–700°C. Resistivity, magnetic susceptibility and X-ray measurements were used for characterization. The T_c of Bi2212 decreases with underdoping while the superconducting energy gap, 2Δ, increases. The energy gap, near the maximum gap direction, at 42K is found to be 30 mV for slightly overdoped (T_c = 85K) and 38 mV for the underdoped (T_c = 70K) crystals giving a 2Δ/ kT_c value of 8.2 and 12.6 for the two compounds, respectively. An energy gap is found to exist above T _c for underdoped Bi2212 while it gets very weak for the overdoped compound. A dip-hump feature at energies ∼2Δ-3Δ is also seen for both the compounds.; The tunneling spectra are analyzed with an interpretation that the pseudogap and the superconducting gaps coexist at low temperatures and that the superconducting gap disappears at T_c. The conservation of states rule is violated but it is recovered if the low temperature spectra are normalized with those above T_c. This normalization also removes the dip-hump and other background features. Such coexistence of two gaps can rule out a scenario that the pre-formed Cooper pairs are responsible for the pseudogap.; A new low temperature STM was designed using a novel coarse approach mechanism, called a piezotube walker. The goal of making this STM was to study the surface images and the tunneling spectra at the same time. The STM was successfully tested down to liquid-He temperatures. Some preliminary images and c-axis spectra of Bi2212 taken with this STM are also presented in this dissertation.