Person:

Litombe, Nicholas

La2−xSrxCuO4 nanowire devices have been fabricated and characterized using electrical transport measurements. Nanowires with widths down to 80 nm are patterned using high-resolution electron beam lithography. However, the narrowest nanowires show incomplete superconducting transitions with some residual resistance at T = 4 K. Here, we report on the refinement of the fabrication process to achieve narrower nanowire devices with complete superconducting transitions, opening the path to the study of novel physics arising from dimension-limited superconductivity on the nanoscale.

Helium-ion beams (HIB) focused to sub-nanometer scales have emerged as powerful tools for high-resolution imaging as well as nano-scale lithography, ion milling or deposition. Quantifying irradiation effects is essential for reliable device fabrication but most of the depth profiling information is provided by computer simulations rather than experiment. Here, we use atomic force microscopy (AFM) combined with scanning near-field optical microscopy (SNOM) to provide three-dimensional (3D) dielectric characterization of high-temperature superconductor devices fabricated by HIB. By imaging the infrared dielectric response we find that amorphization caused by the nominally 0.5 nm HIB extends throughout the entire 26.5 nm thickness of the cuprate film and by about 500 nm laterally. This unexpectedly widespread structural and electronic damage can be attributed to a Helium depth distribution substantially modified by internal device interfaces. Our study introduces AFM-SNOM as a quantitative nano-scale tomographic technique for non-invasive 3D characterization of irradiation damage in a wide variety of devices.

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.