The Torque Magnetometry Study of Underdoped YBa2Cu3Oy

Abstract

This thesis describes torque magnetometry study on under-doped YBa2Cu3Oy (YBCO). Torque magnetometry measures the anisotropic magnetization of materials, and is among the most sensitive experimental methods to probe superconducting state. This study's motivation is to apply this method to the open question: how much does the superconducting fluctuations persist into the pseudogap region, where various ordered states are observed?

Experiments at temperatures close to and above Tc was conducted. Diamagnetic component was extracted from the samples' observed magnetization. The result was in agreement with the theory as well as previous study on different under-doped superconducting cuprates. These evidence indicates that the superconducting fluctuations persist deeply into the pseudogap region (quantitative detail see Chapter.3). Furthermore, torque magnetometry was successfully conducted with superconducting samples with Tc ~ 62 K, at temperatures one order of magnitude lower than Tc and DC magnetic field upto 45 Tesla. Before this, torque magnetometry experiment on underdoped cuprates was never conducted so deeply inside the superconducting dome. A "kink'' feature, shown as abnormal behavior in magnetization and more clearly revealed as a peak in differentiated susceptibility dM/dB, was observed at temperatures below 40 K, once the superconducting state with long range coherence was suppressed by strong magnetic field. The "kink'' feature has either a two dimensional or highly anisotropic origin along the ab-plane, and demonstrated very weak temperature dependence. Comparisons with results by thermal conductivity, X-ray scattering, nuclear magnetic resonance (NMR) and ultrasonic methods were made. Apparently, the "kink'' feature shows up on phase diagram in the same region where the charge density wave (CDW) transition was known to exist. Thus, it is suspected that the "kink'' feature is originated from the superconducting fluctuations influenced by a CDW related transition. The observation of the ``kink'' feature, combined with results on diamagnetism and our collaborators' results on thermal conductivity, we are able to rule out a previous estimation of Hc2 that is misleadingly low in magnetic field and/or temperature. Finally, efforts to extend torque magnetometry method to pulsed magnetic field were introduced. Since the highest available DC magnetic field is limited to 45 Tesla, AC magnetic field which could reach 65 Tesla is the only choice for experiments aimed at the deeper parts of the superconducting dome. The situations and their unique challenges, as well as our preliminary results were analyzed in detail.