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IEEE Transactions on Applied Superconductivity

Broad-Band Coupling With Improved Efficiency Between the Nano-SQUID and Coplanar Waveguide

Long Wu, Lei Chen, Linxian Ma, Xiaoyu Liu, Xiaohan Chen, Yue Wang, Zhen Wang

Published Mar 8, 2019• 0 citations
Publisher

Abstract

Besides its well-known magnetic sensitivity, the nano superconducting quantum interference device (SQUID) has an almost instantaneous response time. However, transferring an ultrafast magnetic signal to a nano-SQUID is a technical challenge. We describe a very efficient direct-coupling structure between the SQUID and the coplanar waveguide (CPW). The center conductor of the CPW and its grounding planes are connected by a short line. We fabricated devices having direct and indirect coupling between the short line and nano-SQUIDs. The coupling efficiencies of both schemes are compared quantitatively. By injecting microwaves of frequencies 0.1-18 GHz through the CPW, we found that microwave power of 100 nW and 158 μW is required to suppress the critical current of nano-SQUIDs for the direct and indirect-coupling devices, respectively. By applying a dc current to the short line, we were able to measure the flux-modulation curves of SQUIDs. To achieve a flux quanta in SQUIDs requires currents of 0.46 and 11 mA for the direct- and indirect-coupling devices, respectively. Therefore, the direct-coupling scheme improves the efficiency by a factor of 39 from 0.1 to 18 GHz and 24 at dc, respectively. Furthermore, while sending a pulse-modulated signal of 1 μW power, a SQUID incorporating the direct-coupling scheme produced a working flux modulation.

DOI
10.1109/tasc.2019.2898694

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We study the microwave properties of a superconducting tunable coplanar waveguide (CPW). Pairs of Josephson junctions are forming superconducting quantum interference devices (SQUIDs), which shunt the central conductor of the CPW. The Josephson inductance of the SQUIDs is varied in the range of 0.08–0.5 nH by applying a dc magnetic field. The central conductor of the CPW contains Josephson junctions connected in series that provide extra inductances; the magnetic field controlling the SQUIDs is weak enough not to influence the inductance of the chain of the single junctions. The circuit is designed to have left- and right-handed transmission properties separated by a variable rejection band; the band edges can be tuned by the magnetic field. We present transmission measurements on CPWs based on up to 120 Nb–AlO x –Nb Josephson junctions. At zero magnetic field, we observed no rejection band in the frequency range of 8–11 GHz. When applying the magnetic field, a rejection band between 7 GHz and 9 GHz appears. The experimental data are compared with numerical simulations.
DC SQUIDs with outer loop size ∼10 μmx 10 μm have been fabricated with nanobridge Josephson junctions in thin Niobium films (25, 100 and 200 nm) using either electron beam or focused ion beam techniques. Such SQUIDs are ideally suited to incorporate into the centre strip of a superconducting coplanar waveguide. The current-voltage characteristics and magnetic field modulation of the SQUIDs are compared for the two methods. The temperature dependence of the critical current is quite linear for the e-beam devices whilst the focused ion beam devices fit an exponential. Here we also investigate trimming methods to reduce the critical currents of the nanojunctions fabricated in the thicker films, in order to improve the SQUID parameters at lower temperatures.
Abstract We propose the fabrication of micro-superconducting quantum interference devices via nano laser direct writing (NLDW). A 20 nm thick Nb film with a critical transition temperature of 9.2 K was deposited on a silicon substrate by sputtering, and nanopatterning was performed with NLDW. After optimizing parameters such as the laser pulse width and power, we investigated the electrical performance of SQUIDs by varying the laser power between 245 mW and 270 mW in 5 mW steps. Experimentally, the voltage–flux (V–Φ) characteristics of micro SQUIDs exhibited typical periodic modulation, confirming the Josephson effect and magnetic flux quantization effect based on the NLDW process. The noise spectral density measurements of SQUID fabricated with laser power of 265 mW achieved a white noise level of 3 μ Φ 0 /Hz 1 / 2 , demonstrating comparable flux noise sensitivity compared to electron beam and ion beam fabrication processes. Furthermore, the devices retained stable superconducting and electrical performance after 101 days, illustrating excellent long-term stability. To some extent, the NLDW shows the potential to perform micro-nanopatterning of superconducting devices efficiently and cost-effectively, as well as an avenue for conducting nanoscale thermal engineering on superconducting devices for basic science research.