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Tunneling Studies of Superconducting Cuprate with Helium Disordered Barriers

  • Author(s): Wang, Yan-Ting
  • Advisor(s): Cybart, Shane
  • et al.

Quantum computation has aroused many attentions toward the superconducting devices. One of the essential elements is the Josephson junction. This dissertation presents a novel technique for fabricating Josephson junction applying to several superconducting materials using a focused helium ion beam. The fabrication procedure and transport properties of the devices made with different superconducting materials are reported individually.

One of the most well-know high-T$_\text{C}$ superconducting cuprate material, YBa$_2$Cu$_3$O$_{7-\delta}$, is first implemented with the helium ion technique. Numerous Josephson junctions have been made with YBa$_2$Cu$_3$O$_{7-\delta}$ thin films. A deeper analysis for junction characteristics is provided to point out the critical parameters for describing junction behaviors.

The other famous high-T$_\text{C}$ cuprate superconductor investigated in this dissertation is Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals. I fabricated the first planar Josephson junction on the \textit{a-b} plane of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ with helium ion irradiation. The junction orientations are confirmed by the magnetic field measurements, and the nearly perfect Fraunhofer patterns is revealed. This is a breakthrough for development of multidimensional superconducting circuits.

The mechanisms for cuprate high-T$_\text{C}$ superconductivity hasn't been fully understood yet. This work compares the tunneling spectra of the cuprate thin films (YBa$_2$Cu$_3$O$_{7-\delta}$) and the cuprate single crystals (Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$). I further study the direct tunneling from a high-T$_\text{C}$ cuprate superconductor to another homogeneous high-T$_\text{C}$ cuprate superconductor. The spectra reveals tunneling from superconducting carriers and quasi-particles for both materialss. The temperature dependence of the gap-like structures in the spectrum can be depicted with the Bardeen-Cooper-Schrieffer theory, which implies a potential \textit{s}-wave behavior. In contrast, the gapless spectra demonstrate the possible existence of \textit{d}-wave component.

Last, the same technique of high-energy helium ions was employed to irradiate a conventional superconductor, niobium nitride, and a iron-based superconductor. The effects of ion irradiation to these superconducting materials are also presented to assist the potential extension of the technique.

In conclusion, this work provides another perspective to the fundamental mechanism for cuprate high-T$_\text{C}$ superconductivity through helium ion disorder barriers with YBa$_2$Cu$_3$O$_{7-\delta}$ thin films and Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystlas. I also established the fabrication procedure for ion disordered \textit{a-b} plane Josephson junctions on the fresh cleaved Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ crystals at any desired angles, which can facilitate the advancement for superconducting circuits. The technique of using helium ion irradiation is further extended to other non-cuprate superconductors and set the footing stone for future applications.

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