Spectroscopic studies of high-temperature superconductors on planar tunnel junction devices and nanofabricated point contact junctions

Planar tunnel junction;Point contact spectroscopy;Superconductor;Nanofabrication

Zhao, Han

关键词: Planar tunnel junction;    Point contact spectroscopy;    Superconductor;    Nanofabrication;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/95494/ZHAO-DISSERTATION-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Since the discovery of high-temperature superconductors (HTS), there has been lots of effort to study their physical properties. Two powerful methods for this investigation are tunneling spectroscopy and point contact spectroscopy. The tunneling spectroscopy directly probes the density of states whereas the point contact spectroscopy detects the sample properties via quasiparticle scattering. Two new approaches to fabricate devices for spectroscopic studies are presented in the thesis. The first one is atomic layer deposition (ALD) to grow ultra-thin and low-defect Al2O3 tunnel junction barriers, developed on sputter-deposited Nb thin films. The conductance characteristics at low temperature show the clear density of states signature of superconducting Nb. The junction resistance times area product increases exponentially with barrier thickness, further supporting the high quality of the junctions, in which single-step elastic tunneling predominates. The second new approach is the focused ion beam (FIB) nanofabrication technique which can achieve precise control of the geometry of point contact junctions. Two different types of PCS devices (vertical and in-plane) have been studied, and the application of vertical junctions on niobium thin films shows consistent data insensitive to thermal cycling, which opens the possibility to perform PCS on materials while varying external variables, such as temperature, directional-dependent magnetic field, and stress. We also use this new method of FIB-fabricated PCS junctions to probe the strongly correlated election system FeTe0.55Se0.45, whose normal state spectra show a conductance enhancement around zero bias, which, through comparison with previous experiments, we associate with electronic nematicity.Finally, preliminary results of planar tunnel junction devices on FeSeTe thin films and crystals to test the superconducting gap order parameter symmetry using the proximity effect are also presented.