Hope College Physics Department Research Experiences for Undergraduates Summer 2012 Project Summary

The nonlinear response of High Temperature Superconducting (HTS) microwave resonator samples of Tl₂Ba₂CaCu₂O₈ (TBCCO) and YBa₂Cu₃O7₇(YBCO) on LaAlO₃ (LAO) substrates was analyzed around the transition temperature (T_c). Nonlinearity is an undesirable response in commercially-produced superconductors that can potentially be minimized through the understanding of its effects through our investigation. HTS microstrip lines were examined with a travelling microscope and a scanning electron microscope (SEM) to determine the dimensions, geometry, and edge structures of each sample, and Energy-dispersive X-ray Spectroscopy (EDS) was used to verify the material composition. Multi-tone measurements of even and odd order intermodulation distortion (IMD) currents were performed utilizing a simultaneous and synchronous measurement technique developed at Hope College. Around their respective T_cs, resonators of the two material systems exhibited different even and odd order IMD currents. The degree to which the superconducting current breaks time reversal symmetry (TRSB) is revealed by the ratio of the 2^nd and 3^rd order IMD levels. In YBCO samples, this ratio steadily decreased with increasing temperature, but then rapidly increased through T_c, indicating a considerable amount of TRSB. TBCCO showed a steady decrease in this ratio, as well, with increasing temperature, but did not show any indication of a rise in TRSB whilst approaching T_c. In TBCCO, 3^rd order IMD exhibited a peak around T_c which is consistent with the nonlinear Meissner effect. TBCCO also appears to show nonlinearity past T_c much more than does YBCO, indicating the possibility of quantum mechanical fluctuations: a phenomenon associated with anisotropic superconductivity.

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