Abstract:

This thesis describes torque magnetometry studies on unconventional superconductors. Torque magnetometry measures the anisotropic magnetization of samples by recording their torque signals in a tilted magnetic field. Applied to superconductors, this method provides a reliable way to measure the field dependence of magnetization with high resolution under extreme conditions: DC magnetic fields from zero to 45.2 T, and temperature from 300 mK to 300K. The results can be used to determine many important parameters, such as the upper critical field H c2 , the superconducting condensation energy, the onset temperature of diamagnetic signals, and so on. We carried out the torque magnetometry measurements on unconventional superconductors--high Tc superconductors and the p -wave superconductor Sr2 RuO4 --and uncovered new features that do not exist in conventional BCS superconductors.

In high Tc superconductors, our torque magnetometry studies focus on the properties of the vortex liquid state. First, by comparing the observed magnetization curves with the Nernst effect results in Bi 2 Sr2 CaCu2 O8+? , we confirm that the unusually large Nernst effect signals originate from the surviving vortex liquid state above Tc . Second, the M-H curves near the critical temperature Tc suggest that the nature of the transition is the Kosterlitz-Thouless transition. Near Tc , the magnetization response at low field is strongly nonlinear, and the T dependence of the magnetic susceptibility in the low-field limit approaches the predicted curve from the Kosterlitz-Thouless transition. Third, the measurements in intense magnetic field up to 45 T reveal the unusual, weak T -dependence of Hc2 . These observations strongly support the existence of the vortex liquid state above Tc . The superconducting state is destroyed by the phase fluctuation of the pair condensate, while the pair condensate keeps its amplitude above T c .

Further studies in single-layered high Tc superconductors reveal more interesting features. The diamagnetic magnetization is observed in the lightly-doped non-superconducting La2-x Sr x CuO4 , and the inferred Hc 2 values are finite and constant at different T . The observations indicate that the vortex liquid state exists in the lightly doped region. The determination of Hc2 and Tonset at different doping levels x display the dome-like x -dependence of these two quantities. Hc2 is found to be linear with T onset with the g-factor being about 2.1.

In the p -wave superconductor Sr2 RuO 4 , the magnetic hysteresis in the mixed state below Tc displays unusual features: the M-H curves are strongly skewed with a break-in-slope at zero field and reversible M-H curves at low field. These features are different from the hysteretic behaviors in conventional type-II superconductors. These features may be explained by the existence of reversible edge currents at low field, which are intrinsic properties of the p -wave superconducting order parameter.

The torque magnetometry turns to be an effective tool to probe the exotic properties in unconventional superconductors. Our results clarify the phase-disordering nature of the superconducting transition in high Tc superconductors, and reveal a possible edge current in Sr2 RuO 4 . Further exploring other systems with the torque magnetometry may result in more interesting findings.