Micro-bridge type superconducting quantum interference devices

Hanho Lee, Natalia Pankratova, Roman Kuzmin +4 more

Gareth Podd, G. D. Hutchinson, D. A. Williams +1 more

We describe the fabrication and measurement of a micrometer-sized direct-current superconducting quantum interference device (dc-SQUID) in which the critical currents of each of the constriction-type Josephson junctions can be controlled independently and in situ via a process of nonequilibrium (hot-phonon) irradiation from a nanofabricated gated structure. The control mechanism is based on hot phonons which are injected into the superconducting microbridges from close proximity, but electrically isolated, normal-metal constrictions. We have also developed a one-dimensional computer model to analyze the behavior of micro-SQUID devices including situations in which we modify the asymmetry of the device. We show from the model that the experimental results are consistent with a change in effective length of the microbridge junctions with respect to the coherence length of the film. The experimental data, and its interpretation in relation to the micro-SQUID model, confirm that this technique, based on hot-phonon irradiation for controlling the critical current in Dayem bridge Josephson junctions, is compatible with the Josephson effect and a feasible method for post-fabrication parameter control in superconducting circuits using Dayem bridge Josephson junctions.

Gen Uehara, Masanori Higuchi

Hao Wang, Ruoting Yang, Guanqun Li +5 more

Abstract Superconducting quantum interference devices (SQUIDs) with 3D nano-bridge junctions can be miniaturized into nano-SQUIDs that are able to sense a few spins in a large magnetic field. Among all device parameters, the inductance is key to the performance of SQUIDs with 3D nano-bridge junctions. Here, we measured the critical-current magnetic flux modulation curves of 12 devices with three design types using a current strip-line directly coupled to the SQUID loop. A best flux modulation depth of 71% was achieved for our 3D Nb SQUID. From the modulation curves, we extracted the inductance values of the current stripe-line in each design and compared them with the corresponding simulation results of InductEX. In this way, London penetration depths of 110 and 420 nm were determined for our Nb (niobium) and NbN (niobium nitride) films, respectively. Furthermore, we showed that inductances of 11 and 119 pH for Nb and NbN 3D nano-bridge junctions, respectively, dominated the total inductance of our SQUID loops which are 23 pH for Nb and 255 pH for NbN. A screening parameter being equal to one suggests optimal critical currents of 89.6 and 8.1 μ A for Nb and NbN SQUIDs, respectively. Additionally, intrinsic flux noise of 110 ± 40 n Φ 0 /(Hz) 1/2 is calculated for the Nb SQUIDs with 3D nano-bridge junctions by Langevin simulation.

E. Sarnelli, Consiglio Nazionale, Complesso Universitario +1 more

Wenqian Liu, Shuo Xiang, Yicong Huang +11 more

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.