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Superconductor Science and Technology

The effects of annealing a 2-dimensional array of ion-irradiated Josephson junctions

Ethan Y. Cho, K Kouperine, Y Zhuo, R. C. Dynes, Shane A. Cybart

Published Jul 22, 2016• 11 citations
Publisher

Abstract

Abstract We have fabricated the two-dimensional arrays of superconducting quantum interference devices (SQUIDs) using YBa 2 Cu 3 O 7- δ ion-irradiated Josephson junctions, and we have studied the effects of post-annealing the arrays at 100 ◦ C in oxygen. The maximum voltage modulation, V B , in a magnetic field for DC biased arrays at 50 K is initially 1.2 mV, but increases to 3 mV after annealing. Furthermore, the temperature where the largest V B occurs increases from 45 K to 48.5 K after annealing. We present and simulate a model where annealing causes diffusion and recombination of the low-energy oxygen defects that narrows the barrier, resulting in an increase in the Josephson binding energy. We show that this process stabilizes after 40 minutes of annealing and leads to a significant improvement in the properties of the array.

DOI
10.1088/0953-2048/29/9/094004

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Abstract We have fabricated the two-dimensional arrays of superconducting quantum interference devices (SQUIDs) using YBa 2 Cu 3 O 7- δ ion-irradiated Josephson junctions, and we have studied the effects of post-annealing the arrays at 100 ◦ C in oxygen. The maximum voltage modulation, V B , in a magnetic field for DC biased arrays at 50 K is initially 1.2 mV, but increases to 3 mV after annealing. Furthermore, the temperature where the largest V B occurs increases from 45 K to 48.5 K after annealing. We present and simulate a model where annealing causes diffusion and recombination of the low-energy oxygen defects that narrows the barrier, resulting in an increase in the Josephson binding energy. We show that this process stabilizes after 40 minutes of annealing and leads to a significant improvement in the properties of the array.
Ion-irradiated YBa2_22​Cu3_33​O7_77​ Josephson arrays
We designed, fabricated and tested short one dimensional arrays of masked ion-irradiated YBa2_22​Cu3_33​O7_77​ Josephson junctions (JJ) embedded into log-periodic spiral antennas. Our arrays consist of 4 or 8 junctions separated either by 960~nm or 80~nm long areas of undamaged YBCO. Samples with distanced junctions and with closely spaced junctions showed qualitatively different behaviors. Well separated arrays demonstrated giant Shapiro steps in the hundreds-GHz band at 66K and were tested as Josephson mixers with improved impedance matching. All closely spaced arrays behaved as one junction with a lower superconducting transition temperature, hence forming a single weak link on distances up to 880~nm. Such design opens a new way to increase the Ic_{c}c​RN_{N}N​ product of ion-irradiated junctions and we speculate that the phenomena and physics behind it might be similar to the so-called "giant" Josephson coupling observed in cuprates.
We investigate the temporal stability of YBa2Cu3O7-δJosephson junctions created by ion irradiation through a nanoscale implant mask fabricated using electron beam lithography and reactive ion etching. A comparison of current-voltage characteristics measured for junctions after fabrication and eight years of storage at room temperature show a slight decrease in critical current and increase in normal state resistance consistent with broadening of the weak link from diffusion of defects. Shapiro step measurements performed eight years after fabrication reveal that device uniformity is maintained and is strong evidence that these devices have excellent temporal stability for applications.
Abstract We report on the noise properties of ion-irradiated YBa 2 Cu 3 O 7 Josephson junctions. This work aims at investigating the linewidth of the Josephson oscillation with a detector response experiment at ≃132 GHz. Experimental results are compared with a simple analytical model based on the Likharev–Semenov equation and the de Gennes dirty limit approximation. We show that the main source of low-frequency fluctuations in these junctions is the broadband Johnson noise and that the excess 1 f noise contribution does not prevail in the temperature range of interest, as reported in some other types of high- T c superconducting Josephson junctions. Finally, we discuss the interest of ion-irradiated junctions to implement frequency-tunable oscillators consisting of synchronized arrays of Josephson junctions.
We present a theoretical model for two-dimensional (2D) superconducting quantum interference device (SQUID) and superconducting quantum interference filter (SQIF) arrays with overdamped Josephson junctions for uniform bias-current injection at 77 K. Our simulations demonstrate the importance of including Johnson thermal noise and reveal that only the SQUID-loops self-inductance contributions are of importance. Our numerical results establish the validity of a scaling behavior between the voltages of one-dimensional (1D) and 2D SQUID arrays and show that the same scaling behavior applies to the maximum transfer functions. The maximum transfer function of a 2D SQUID array can be further optimized by applying the optimal bias current, which depends on the SQUID-loop self-inductance and the junction critical current. Our investigation further reveals that a scaling behavior exists between the maximum transfer function of a 2D SQUID array and that of a single dc SQUID. Finally, we investigate the voltage response of 1D and 2D SQIF arrays and illustrate the effects of adding spreads in the heights and widths of SQUID loops.
YBa2_22​Cu3_33​O7_77​ nano-constriction Josephson junctions and SQUIDs fabricated by focused helium-ion-beam irradiation
By focused 30 keV30\,\mathrm{keV}30keV He ion beam irradiation, epitaxially grown YBa2_22​Cu3_33​O7_77​ (YBCO) thin films can be driven from the superconducting to the insulating state with increasing irradiation dose. A properly chosen dose suppresses superconductivity down to 4 K4\,\mathrm{K}4K, while crystallinity is still preserved. With this approach we create areas of normal-conducting YBCO that can be used to define resistively shunted constriction-type Josephson junctions (cJJs) on the nanometer scale. We also demonstrate that the fabricated cJJs can be incorporated in direct current superconducting quantum interference devices and can be used as detector junctions in THz antennas.
In this work, we demonstrate the ability to fabricate superconducting quantum interference devices (SQUIDs) by directly writing Josephson junctions into the plane of YBa2Cu3O7−δ thin films with a focused helium ion beam. This technique allows for the control of the Josephson barrier transport properties through the single parameter, ion dose. SQUIDs written with a dose of 4 × 1016 ions/cm2 had metallic barrier junctions that exhibited nearly ideal electrical transport characteristics at 50 K and a flux noise of 20 μΦ0/Hz at 10 Hz. At higher irradiation doses, the SQUIDs had insulating barrier Josephson junctions with a quasi particle energy gap edge at 20 meV.