Nanopatterning and Transport Properties of Cuprate Superconductors

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Nanopatterning and Transport Properties of Cuprate Superconductors

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Title: Nanopatterning and Transport Properties of Cuprate Superconductors
Author: Litombe, Nicholas E.
Citation: Litombe, Nicholas E. 2015. Nanopatterning and Transport Properties of Cuprate Superconductors. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Abstract: Almost 30 years since the discovery of the copper oxide high temperature superconductors, the underlying mechanism describing their behavior continues to elude experimentalists and theorists alike. Understanding the electronic phases and various, possibly competing, orders at the nanoscale continues to be an active and hotly debated research enterprise. Tools available to probe nanoscale electronic behavior such as scanning tunneling microscopy have made tremendous strides in elucidating the nature of this high temperature superconductor ground state. Nevertheless, transport noise measurements on nanostructured cuprates, such as in nanowires, can aid in our physics understanding of nanoscale electronic degrees of freedom particularly for time dependent measurements. To carry out transport measurements on nanodevices, the technique has to be refined to obtain pristine devices where one probes the intrinsic material properties.

In this thesis, motivated by the need to understand nanoscale and fluctuating orders such as stripes and nematics, I will describe the strides made to develop the nanopatterning technology and the recipes for creating superconducting nanowire devices in our material of interest, La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO). LSCO was chosen for study because of: 1. its simple crystal structure containing a single copper oxygen plane and no copper oxygen chains; 2. its ability to be grown at doping concentrations spanning the entire superconducting dome from underdoped to overdoped; 3. and finally it can be synthesized into metallic and insulator bilayer heterostructures sustaining two dimensional superconductivity at their interface. I will show results of small cross-sectional nanowire structures fabricated by correctly identifying the problem, and mitigating them. I will first delineate the key elements in the lithographic process, and identify the technical hurdles that must be overcome to pattern these nanowire structures. I find that our nanowire structures undergo incomplete superconducting transitions and therefore do not reflect intrinsic materials. By varying fabrication process parameters, in a systematic study I will show that the ion beam current during our pattern transfer step is the tunable parameter that determines the preservation of superconductivity in our LSCO nanowire devices. Ours is the first such study in the LSCO cuprate system. I will show results of small cross-sectional nanowire structures created by correctly identifying the problem, and mitigating them.

Next, I explore He$^{+}$ ion beam patterning of LSCO microbridges into nanostructures. I show tunable superconductor-insulator transition by varying the beam doses and energies of the scanning helium writes. I find topographic features resulting from high fluence doses and show dielectric constant contrasts between exposed and unexposed regions of the cuprate microbridge.

Finally, I propose a large scale patterning method for LSCO bilayers sustaining interface superconductivity, grown on vicinal substrates. I characterize critical current anisotropies as a metric of the technique and propose next steps towards obtaining pristine nanostructures that can further elucidate nanoscale electronic phases in the cuprates.
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