Arpaia, Riccardo (2014) YBCO nanowires for ultrasensitive magnetic flux detectors and optical applications. [Tesi di dottorato]

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Abstract

In this thesis we have realized YBCO nanowires, with cross sections down to 40x50 nm^2, and employed them in nanoSQUIDs and optical sensors. We have overcome the limiting issues in establishing reliable nanofabrication routines for YBCO, due to the chemical instability (related to oxygen out-diffusion), and the extreme sensitivity to defects and disorder due to very short coherence length of this material. Through an improved nanopatterning procedure, based on e-beam lithography in combination with an amorphous carbon mask and a very gentle Ar^+ ion etching, by keeping a capping layer of Au on the top of the nanostructures, we have achieved nanowires preserving ``pristine'' superconducting properties, very close to the as-grown films. The benchmark of their quality is given by the very high critical current densities they carry (up to 10^8 A/cm^2), which are very close to the theoretical depairing limit. Therefore, these structures represent model system to study HTS, since the properties they show are strictly related to the superconducting material, scaled to the nanoscale, and they are not ruled by damages and defects associated to the nanopatterning procedure. Different experiments have been done, employing these nanowires in more complex devices. The results we got pave the way for new exciting experiments, which will be object of future works, exploring HTS superconductivity at the nanoscale both for fundamental studies and applications: We have fabricated and characterized YBCO nanoSQUIDs, employing very short nanowires in the so-called Dayem bridge configuration. These devices, working in the whole temperature range from 300 mK up to their critical temperature (~ 82 K), have revealed an ultra white low-noise below 1 μΦ_0 Hz^(-1/2) above 10 kHz, corresponding to a predicted spin sensitivity of only 50 μ_B Hz^(-1/2). These properties make our devices very attractive for many applications, as for the investigation of the magnetic moment in small ensembles of spins in a wide range of temperatures and magnetic fields. We have realized YBCO nanorings, measuring the magnetoresistance close to T_C. Large oscillations have been measured, which can be associated to the vortex dynamics triggering our nanowires to the resistive state. The FFT spectra have shown, for the nanorings with narrower linewidth, a single sharp peak: this peak, associated to h/2e periodicity, as predicted for optimally doped YBCO, represents a clear evidence of a uniform vorticity of the order parameter inside the rings, confirming the high degree of homogeneity of our structures. The homogeneity of our nanowires, together with the fast relaxation times of the YBCO, gave the boost for the realization of devices, aimed at the detection of single photons. This goal required the use of very long wires (up to 10 μm), covering large areas (~ 300 μm^2). We have done preliminary photoresponse measurements on these devices, detecting signals whose main origin appears to be mostly bolometric. The use of a ferromagnetic LSMO layer, employed as capping for preliminary photoresponse studies appears to be the natural route for future investigation.

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