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Superconducting Electronics: High-Transition-Temperature Superconducting Quantum Interference Devices

Date : 10/15/2018

EY Cho, H Li, JC LeFebvre, YW Zhou, RC Dynes, SA Cybart
Applied physics letters 113 (16), 162602

Direct write patterning of high-transition temperature (high-TC) superconducting oxide thin films with a focused helium ion beam is a formidable approach for the scaling of high-TC circuit feature sizes down to the nanoscale. In this letter, we report using this technique to create a sensitive micro superconducting quantum interference device (SQUID) magnetometer with a sensing area of about 100 × 100 μm2. The device is fabricated from a single 35-nm thick YBa2Cu3O7−δ film. A flux concentrating pick-up loop is directly coupled to a 10 nm × 20 μm nano-slit SQUID. Th e SQUID is defined entirely by helium ion irradiation from a gas field ion source. The irradiation converts the superconductor to an insulator, and no material is milled away or etched. In this manner, a very narrow non-superconducting nano-slit is created entirely within the plane of the film.

Date : 2015

Shane Cybart, E.y Cho, B.H Wehlin, Meng Ma, T.J. Wong, R.C. Dynes, Choung Huynh
Microscopy Society of America

In this work, we demonstrate a-b plane superconducting Josephson tunnel junctions for YBa2Cu3O7−δ (YBCO) by utilizing a 500-pm diameter focused helium ion beam to create a very narrow (~nm) tunnel barrier between two superconducting electrodes. The key to this method is that YBCO is very sensitive to point defects in the crystal lattice caused by ion irradiation [3]. Increasing irradiation levels has the effects of increasing resistivity and reducing the superconducting transition temperature. At very high irradiation levels YBCO becomes insulating and no longer conducts or superconducts [3].

Date : 5/12/2015

EY Cho, MK Ma, C Huynh, K Pratt, DN Paulson, VN Glyantsev, RC Dynes SA Cybart
Applied Physics Letters 106 (0), 000000

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 4x1016 ions/cm2 had metallic barrier junctions that exhibited nearly ideal electrical transport characteristics at 50 K and a flux noise of 20 μΦ0 at 10 Hz. At higher irradiation doses, the SQUIDs had insulating barrier Josephson junctions with a quasi particle energy gap edge at 20 meV.

Date : 2/10/2014

SA Cybart, EY Cho, TJ Wong, VN Glyantsev, JU Huh, CS Yung, BH Moeckly, JW Beeman, E Ulin-Avila, SM Wu, RC Dynes
Applied Physics Letters 104 (6), 062601

We have fabricated and tested two-dimensional arrays of YBa2 Cu 3O7− δ superconducting quantum interference devices. The arrays contain over 36 000 nano Josephson junctions fabricated from ion irradiation of YBa2Cu3O7-δ through narrow slits in a resist-mask that was patterned with electron beam lithography and reactive ion etching. Measurements of current-biased arrays in magnetic field exhibit large voltage modulations as high as 30 mV.

Date : 9/14/2009

SA Cybart, SM Anton, SM Wu, J Clarke, RC Dynes
Nano letters 9 (10), 3581-3585

Very large scale integration of Josephson junctions in a two-dimensional series−parallel array has been achieved by ion irradiating a YBa2Cu3O7-δ film through slits in a nanofabricated mask created with electron beam lithography and reactive ion etching. The mask consisted of 15820 high aspect ratio (20:1), 35 nm wide slits that restricted the irradiation in the film below to form Josephson junctions. Characterizing each parallel segment k, containing 28 junctions, with a single critical current Ick we found a standard deviation in Ick of about 16%.

Date : 11/2008

SA Cybart, SM Wu, SM Anton, I Siddiqi, J Clarke, RC Dynes
Applied Physics Letters 93 (18), 182502-182502-3

We have fabricated a series array of 280 superconducting quantum interference devices(SQUIDs) using YBa2Cu3O7−δ thin film ion damage Josephson junctions. The SQUID loop areas were tapered exponentially so that the response of the current-biased array to magnetic field is a single voltage spike at zero field. We fitted the current-voltage characteristics of the array to a model in which we summed the voltages across the SQUIDs assuming a resistively shunted junction model with a normal distribution of SQUID critical currents. At 75 K the standard deviation of these critical currents was 12%.

Superconducting Electronics: Josephson Junctions

Date : 07/09/2018

Ethan Y Cho, Yuchao W Zhou, Jennifer Y Cho, Shane A Cybart
Applied Physics Letters 113 (2), 022604

We report the fabrication of nanoscale wires and Josephson junctions in 25 nm thick YBa2Cu3O7–δ thin films with wire widths as narrow as 50 nm. Our approach utilizes a finely focused gas field ion source from a helium ion microscope to directly modify the material on the nanometer scale to convert irradiated regions of the film into insulators. In this manner, the film remains intact and no material is milled or removed. Transport data show that the electrical properties, critical current and conductance, scale linearly with the lithographically defined width ensuring that the actual and lithographically defined dimensions are commensurate with each other. Unlike in typical ion damage proximity effect Josephson junctions, we observe a low temperature saturation of the critical current and near temperature interdependent resistance which we attribute to a narrower and more resistive barrier.

Date : 2017

SA Cybart, A Herr, V Kornev, CP Foley
Supercond. Sci. Technol 30 (090201), 5pp.

Richard Feynman’s Lectures in Physics, Volume 3 [1] final lecture predicted that multiple Josephson junction devices would improve the sensitivity and bandwidth of superconducting devices. Since then there has been the development of both low temperature and high temperature devices and systems for a very wide range of applications, many requiring multiple Josephson junctions and arrays of junctions to operate. Examples are the Josephson voltage standard, superconducting computers, ratchet devices, antennas, THz receivers and detectors, data storage and single flux quantum devices. This focus issue on superconducting devices with multiple Josephson junctions features seven original research papers presenting new concepts, modeling, fabrication and applications. Furthermore this growing research field has regular breakthroughs that then are rapidly superseded. The international development of superconducting electronics in a range of applications combined with the advent of improved miniature cryocooling makes broad application of superconducting electronics a nearer term reality. Figure 1 shows the increasing complexity of different Josephson junction circuits as time progresses.

Date : July/22/2015

E. Y. Cho, Meng K. Ma, Chuong Huynh, R. C. Dynes and Shane A. Cybart
Microscopy and Microanalysis 21.S3 (2015): 1997-1998.

Electrical circuits fabricated from high-transition temperature superconductors (HTS) are very difficult to pattern due to the lack of a reliable etching process. Chemical etching can be used for large features, but undercutting limits the feature size to tens of microns. Dry etching is required for smaller features, but there is no anisotropic reactive ion etch for these materials. Therefore dry etching must be done with isotropic argon ion milling. The ion milling process generates excess heat and unfortunately oxide superconductors are sensitive to high temperature. Overheating the material by ion milling causes it to deoxygenate which turns the superconductor into an insulator. Therefore to prevent overheating the critical dimension for argon ion milling is typically limited to a few microns.

Date : July/22/2016

E Y Cho, K Kouperine, Y Zhou, R Dynes, SA Cybart
Superconductor Science and Technology, Volume 29, Number 9

We have fabricated the two-dimensional arrays of superconducting quantum interference devices (SQUIDs) using YBa2Cu3O7-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.

Date : 2013

S Cybart, P Roediger, K Chen, J Parker, E Cho, T Wong, R Dynes

We investigate the temporal stability of YBa2Cu3O7 Josephson junctions created by ion irradiation through a nano-scale 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 weaklink from diffusion of defects. Shapiro step measurements performed 8 years after fabrication reveal that device uniformity is maintained and is strong evidence that these devices have excellent temporal stability for applications.

Date : 6/2005

SA Cybart, K Chen, RC Dynes
Applied Superconductivity, IEEE Transactions on 15 (2), 241-244

Reproducible low resistance lumped element Josephson junction arrays are desired for many microwave applications. Using our established process of electron beam lithography and ion damage, we have fabricated and demonstrated high quality YBa2Cu3O7-δ superconductor-normal superconductor-superconductor (SS'S) in-plane Josephson junctions. Single junctions and multiple junction arrays with as many as 50 junctions in series have been investigated. These junctions are in close proximity and offer several possible applications. Current-voltage characteristics for single junctions are consistent with the resistively shunted junction model. Junction in pairs have been fabricated which show nearly identical characteristics. Rounding near the critical current occurs for larger number arrays which we attribute to junction nonuniformity. Microwave measurements reveal sharp giant Shapiro steps for junction pairs and 10 junction arrays, rounded steps appear for larger arrays.

Date : 6/2005

K Chen, SA Cybart, RC Dynes
Applied Superconductivity, IEEE Transactions on 15 (2), 149-152

Using electron beam lithography and ion damage, high quality YBa2Cu3O7-δ superconductor-normal metal-superconductor in-plane Josephson junction pairs have been fabricated. These junctions operate at temperatures between 60 and 85 K and have spacing ranging between 150 nm and 800 nm. Central electrodes connecting to the area between the two junctions were made, allowing for simultaneous measurements of the individual junctions as well as the series. Josephson junction pairs with 150 nm spacing were found to have extraordinary properties under microwave radiation with different power. At low power the structure functioned as two independent Josephson junctions in series, while at high power it behaved as one single Josephson junction. A possible origin of this behavior could be that the applied microwave radiation induces weak link behavior in the YBa2Cu3O7-δ between the two junctions.

Date : 10/2004

K Chen, SA Cybart, RC Dynes
Applied physics letters 85 (14), 2863-2865

We have fabricated in-plane high-Tc Josephson junction pairs and series arrays using our established nanolithography and ion damage process. Junctions in a pair showed nearly identical electrical properties. The ten-junction array exhibited current–voltage characteristics that can be described by the resistively shunted junction model at 78K. Flat giant Shapiro steps were observed in both cases. We believe that the ion-damaged high-TcsuperconductingJosephson junction is a good candidate to form large numbers of junctions in series arrays that can function above 77K for quantum voltage standards and other applications.

Superconducting Electronics: Magnesium Diboride

Date : 5/5/2014

SA Cybart, TJ Wong, EY Cho, JW Beeman, CS Yung, BH Moeckly, RC Dynes
Applied Physics Letters 104 (18), 182604

Magnetic field sensors based on two-dimensional arrays of superconducting quantum interference devices were constructed from magnesium diboride thin films. Each array contained over 30 000 Josephson junctions fabricated by ion damage of 30 nm weak links through an implant mask defined by nano-lithography. Current-biased devices exhibited very large voltage modulation as a function of magnetic field, with amplitudes as high as 8 mV.

Date : 1/2/2006

SA Cybart, K Chen, Y Cui, Q Li, XX Xi, RC Dynes
Applied physics letters 88, 012509

We have fabricated planar thin-film MgB2Josephson junctions and 20-junction series arrays using 200-keV ion implantation and electron-beam lithography. Resistively shunted junctionI-V characteristics were observed in the temperature range of 34–38K. The ac Josephson effect was observed and flat giant Shapiro steps in arrays suggest good junction uniformity with a small spread in junction parameters. The temperature dependence of the critical current suggests that the nature of the interface between the superconductor and normal region can be described using a soft boundary proximity effect coupling model. We believe that the higher operating temperature and close spacing of these junctions make them promising candidates for quantum voltage standards and other devices.

Superconducting Electronics: Simulations

Date : 6/1/2014

TN Dalichaouch, SA Cybart, RC Dynes
Superconductor Science and Technology 27 (6), 065006

A model is developed to investigate the effects of mutual inductance on the voltage– field (V− B) characteristics of two-dimensional arrays of Josephson junctions. The V− B characteristics were numerically simulated for arrays with and without mutual inductance contributions. We find mutual inductances can have a strong impact on the voltage response of SQUID arrays and mutual inductance contributions from nearest neighboring SQUIDs only do not represent a good approximation.

Date : 2013

S Wu, S Cybart, S Anton, R Dynes

The voltage as a function of applied magnetic field (V-B) was calculated for arrays of superconducting quantum interference devices (SQUIDs) connected in series. Comparisons were made between arrays of equal area SQUIDs and superconducting quantum interference filters (SQIFs). The areas for the SQIFs were varied exponentially, so that the V-B had a sharp minimum at zero field. We used equations for the dc SQUID based on resistively shunted junctions, with typical parameters for YBa2Cu3O7 - δ ion damage Josephson junctions. The maximum transfer coefficient of the central minimum VB = (∂V/∂B)max of the SQIF decreases as the area range increases. We find that the equal area array is more robust to the effects of non-uniform junction critical currents than the SQIF, for the junction parameters and SQUID area distributions chosen. Furthermore, we find that slight variations (~5%) to the area due to fabrication irregularities have little effect on the central minimum of V-B for either device.

Date : 9/2012

SA Cybart, TN Dalichaouch, SM Wu, SM Anton, JA Drisko, JM Parker, BD Harteneck, RC Dynes
Journal of Applied Physics 112 (6), 063911-063911-6

We have fabricated series-parallel (two-dimensional) arrays of incommensurate superconducting quantum interference devices(SQUIDs) using YBa2Cu3O7−δ thin film ion damage Josephson junctions. The arrays initially consisted of a grid of Josephson junctions with 28 junctions in parallel and 565 junctions in series, for a total of 15 255 SQUIDs. The 28 junctions in the parallel direction were sequentially decreased by removing them with photolithography and ion milling to allow comparisons of voltage–magnetic field (V–B) characteristics for different parallel dimensions and area distributions. Comparisons of measurements for these different configurations reveal that the maximum voltage modulation with magnetic field is significantly reduced by both the self inductances of the SQUIDs and the mutual inductances between them. Based on these results, we develop a computer simulation model from first principles which simultaneously solves the differential equations of the junctions in the array while considering the effects of self inductance, mutual inductance, and non-uniformity of junctioncritical currents. We find that our model can accurately predict V–B for all of the array geometries studied. A second experiment is performed where we use photolithography and ion milling to split another 28 × 565 junction array into 6 decoupled arrays to further investigate mutual interactions between adjacent SQUIDs. This work conclusively shows that the magnetic fields generated by self currents in an incommensurate array severely reduce its performance by reducing the maximum obtainable modulation voltage.