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arXiv:1709.09960 · IEEE Transactions on Applied Superconductivity

HTS YBCO Resonator Configuration With a Coplanar Optimized Flux Concentrator Strongly Coupled to rf SQUID

Fatemeh Qaderi, Faezeh Shanehsazzadeh, Behnam Mazdouri, Mehdi Fardmanesh

Published Feb 16, 2018• 2 citations

Abstract

We developed a novel magnetic coupling module, formed of a monolayer superconducting flux concentrator that is integrated with a coplanar resonator, strongly coupled to high-temperature superconducting radio frequency superconducting quantum interference device (SQUID). Three types of resonators, including a long stripline resonator between the input loop and the pick-up loop of the flux concentrator, a complementary split ring resonator, and a spiral shape inside the input loop, are explored. The resonance quality factor of different patterns of these three types of the resonators, as well as their coupling to the SQUID, is evaluated using finite-element-method simulations. Several readout methods are tested to couple the electronic system to the resonators by inductive (coil) or capacitive (transmission line) methods, and the optimum readout is reported for each of the resonators. Among the evaluated resonator types, a spiral shape with an optimal behavior at the resonance frequency of 836 MHz, showing the highest quality factor together with the strongest coupling to the SQUID, compared to other types of the resonators, is fabricated using a 200-nm-thick YBCO film on a 1-mm-thick crystalline LaAlO3substrate. The flux concentrator of the module is optimized by the variation of the linewidth of its different parts and also its input loop radius to obtain the maximum flux transformation efficiency.

DOI: 10.1109/tasc.2018.2807180

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We have developed a novel coplanar resonator serving as a tank circuit for radio frequency washer superconducting quantum interference devices (SQUIDs). Two coplanar lines surrounded the circular flux concentrator washer, 13.4 mm in diameter. The SQUID, 3.5 mm in diameter, was coupled to the concentrator in the flip-chip configuration. All parts were fabricated from single-layer epitaxial YBa2Cu3O7 films on LaAlO3 substrates. In this layout, the resonant frequency could be set between 600 and 850 MHz. With SQUID loops of 100×100 μm2 (SQUID) inductance Ls=150 pH or 10×500 μm2 (Ls=260 pH), we determined at 77 K white flux noise levels Sφ1/2 of 1.0×10−5 and 1.5×10−5φ0/Hz, respectively. Both figures corresponded to the field resolution of about 30 fT/Hz.

This article describes three planar layouts of superconducting multiturn flux transformers integrated with a coplanar resonator for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. The best magnetic field noise values of 22 and 11.5 fT/Hz1/2 in the white noise regime were obtained for the layout with two input coils and the layout with the labyrinth resonator, respectively. Excess low-frequency noise (about 200 fT/Hz1/2 at 10 Hz) was present. Computer simulation showed that the loss in this trilayer system was dominated by the high loss tangent of the dielectric film used for the separation of the upper and lower superconducting films. The rf coupling coefficient krf between the resonator and the flip-chip-coupled SQUID was also estimated. The values krf2≈14×10−3 obtained for the layout with two input coils, and krf2≈45×10−3 for the layout with the labyrinth resonator were considerably higher than the typical value of krf2≈7×10−3 for the single-layer coplanar resonator. These high coupling coefficients have compensated the somewhat degraded unloaded quality factor of the resonator, thus securing the optimum operation of the rf SQUID.

Nowadays, the high-critical-temperature radio frequency superconducting quantum interference device (high- T c rf SQUID) is usually coupled to a dielectric resonator that is a standard 10 × 10 × 1 mm 3 SrTiO 3 (STO) substrate with a YBa 2 Cu 3 O 7−δ (YBCO) thin-film flux focuser deposited on it. Recently, we have simulated a dielectric resonator for the high- T c rf SQUID by using the ANSOFT High Frequency Structure Simulator (ANSOFT HFSS). We simulate the resonant frequency and the quality factor of our dielectric resonator when it is unloaded or matches a 50-Ω impedance. The simulation results are quite close to the practical measurements. Our study shows that ANSOFT HFSS is quite suitable for simulating the dielectric resonator used for the high- T c rf SQUID. Therefore, we think the ANSOFT HFSS can be very helpful for investigating the characteristics of dielectric resonators for high- T c rf SQUIDs.

At present, the high-transition-temperature radio frequency superconducting quantum interference device (High- T c RF SQUID) is usually coupled to a dielectric resonator which is a standard 10 mm×10 mm× 1 mm SrTiO3 (STO) substrate with a YBa2Cu3O7-δ (YBCO) thin-film flux focuser deposited on it. The dielectric resonator for the High- Tc RF SQUID has a high quality factor and a resonant frequency in the microwave range. In order to find out the effect of the flux focuser’s geometry on the dielectric resonator’s resonant frequency, we used ANSOFT high frequency structure simulator (ANSOFT HFSS) to simulate the resonance characteristics of some dielectric resonators with different flux focuser geometries. Our simulation results show that when the width of the flux focuser’s slit increases or the radius of the flux focuser’s inner hole decreases, the dielectric resonator’s resonant frequency increases. To estimate the reliability of our simulation results, we selectively prepared a few dielectric resonators and measured their resonance characteristics. The experimental results are virtually consistent with the simulation results. Our study shows that changing the flux focuser geometry is an effective way to adjust the dielectric resonator’s resonant frequency.

We propose a design for planar multi-turn flux transformer for radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. This transformer is integrated with a coplanar resonator, e.g., on a 1 cm2 LaAlO3 substrate. Its pickup loop is connected with two input coils, separate for dc (low-frequency) and rf currents. A double-hole washer SQUID is coupled to these coils in a flip-chip configuration to form a magnetometer. The separation of rf and dc current paths in the transformer made it possible to demonstrate the rf SQUID magnetometer operation.