He-FIB Josephson-Kontakt Arrays für einen Hochtemperatursupraleiter Spannungsstandard
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Beschreibung
The definition of the electrical voltage is based on Josephson junction arrays, which exhibit a quantized voltage step under high-frequency excitation. The so called Shapiro step can be used for calibrations. In this work, the use of the high-temperature superconductor YBa2Cu3O7-x instead of the currently employed low-temperature superconductor niobium is investigated. The YBa2Cu3O7-x samples are fabricated at the institute by pulsed laser deposition and structured using photolithography as well as physical and chemical etching steps. The fabrication of the barrier for the Josephson junction is carried out using a helium focused ion beam (He-FIB). Extensive characterization of the fabricated Josephson junctions are performed via automated measurements. The exponential dependence of the Josephson junction parameters on the He-FIB dose, as well as the behavior of these parameters over a wide temperature range, is discussed. Furthermore, the spread of the Josephson junction parameters, which is comparable to other fabrication methods, as a function of different quantities, is presented. The emergence of Shapiro steps under high-frequency excitation in the GHz and THz ranges is demonstrated. Notably, the THz range represents a novelty, as flat Shapiro steps have not previously been demonstrated at these frequencies. Despite the spread in the Josephson junction parameters, superimposed Shapiro steps can be observed with up to three Josephson junctions connected in series. By optimizing the fabrication through the addition of a shunt resistor in parallel to the Josephson junction, this number can be increased to up to ten Josephson junctions, showing a superimposed Shapiro step at 0,8mV when irradiated with 39 GHz. The influence of the shunt resistor on the measurement is discussed in detail. Finally, comparison measurements between a YBa2Cu3O7-x He-FIB Josephson junction and a niobium voltage standard at 4,2K demonstrate the fundamental feasibility of a YBa2Cu3O7-x He-FIB Josephson junction voltage standard.
Autor*in
Max Pröpper
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Hochtemperatursupraleiter Spannungsstandard YBa2Cu3O7-x Josephson-Kontakt Helium-Ionenstrahl Vergleichsmessungen Tieftemperatursupraleiter Niob elektrische Spannung Hochfrequenzanregung He-FIB Josephson-Kontakt