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IEEE Transactions on Applied Superconductivity

Influence of Material and Geometry of the Dielectric on the Characteristics of the Tank Circuit Resonators for High-Tc RF-SQUID-Based System

Nafise Kalantari, Faezeh Shanehsazzadeh, Farrokh Sarreshtedari, Mehdi Fardmanesh

Published Mar 1, 2016• 5 citations

Abstract

Characteristics of a lumped element L-C tank circuit resonator of high-Tc rf-SQUIDs, which plays an important role in the device operation, have been experimentally investigated, according to the corresponding simulations obtained by COMSOL Multiphysics. We experimentally investigated the characteristics of the resonators by measuring their scattering parameters, as well as evaluating the rf-SQUID signal. Our SQUIDs are made of sputtered YBCO thin films with 3-mm-diameter washer areas. Different configurations of the resonator are investigated, and it is observed that, by increasing the thickness of the dielectric in a specific range, the reflection coefficient (S11) increases. For the experimental verifications, we constructed resonators with different dielectric thicknesses and different configurations. The resonators are made of a dielectric board with a hole in its middle to hold the inductors for close coupling to the SQUID mounted on the backside. An increase in the thickness of the dielectric board confines the RF field distribution close to the SQUID, while also making further distance between the contact pads and the SQUID reducing their parasitic effects on the device signal. This is observed to improve the stability of the operating point of the SQUIDs, while also reducing the noise for measured frequency range of 0.1 to 100 Hz.

DOI: 10.1109/tasc.2016.2537139

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In rf SQUID operation, nearby metal may influence the field distribution and the resonant frequency of the tank circuit, and then cause low frequency noise in the rf SQUID. Using an electromagnetic simulation method, we analysed the influence of metal on the resonant frequencies of three kinds of tank circuit: coplanar resonator, dielectric resonator and lumped-element tank circuit. The simulation results show that metal has the biggest influence on the coplanar resonator and the smallest influence on the dielectric resonator. This is why the high-Tc rf SQUID with a dielectric resonator has less low frequency noise than that with a coplanar resonator.

The operation of YBa2Cu3O7 rf superconducting quantum interference devices (SQUIDs) using a dielectric SrTiO3 resonator as tank circuit was tested. At 77 K, we attained for such resonators unloaded quality factors Q0 up to 7000 at resonant frequencies near 1 GHz, when measuring without electromagnetic shielding. For a washer SQUID with an area of 4×4 mm2 and an inductance of ≂100 pH, which was properly coupled to the resonator, we measured an approximately 60 μV peak-to-peak SQUID signal and a white flux noise spectral density of SΦ1/2=1×10−5Φ0/√Hz, corresponding to a field sensitivity of BN=105 fT/√Hz. With an additional single-layer flux transformer with a diameter of the pickup loop of 22 mm in flip-chip arrangement, the field sensitivity was improved to 30 fT/√Hz.

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Understanding the coupling mechanism between SQUID (superconducting quantum interference device) loop and resonator in high-frequency SQUIDs is essential for the application of these devices in sensitive magnetometers or controllable microwave devices. We have investigated, therefore, two kinds of planar microstrip tank circuit resonators: (1) S-shaped λ/2-resonators with direct or indirect coupling to the SQUID loop with optional flux-focussing pads and (2) conventional rf-washer-SQUID structures for which we have found resonant modes of the washer providing a coupling to the SQUID loop. A lumped element representation of SQUID loop and resonator is presented in order to describe the effective SQUID-resonator coupling in case of direct or indirect coupling. According to this approach, the square of the effective coupling constant k in both cases is equal to the ratio of the magnetic energy stored in the SQUID loop to that stored in the resonator. For the indirect coupled S-shaped λ/2-resonators, k can be described in terms of a geometry-defined mutual inductance. The predictions of the lumped element description are compared with simulations using a standard full-wave analysis computer program in order to quantify the effect of the different layouts on microwave current distribution and on k. A method for the determination of k by simulation is described, leading to useful design rules for the optimization of high-frequency SQUIDs. The simulation results will also be compared with experimental data.

We have developed a matching scheme for the operation of a rf superconducting quantum interference device (SQUID) with a lumped-element tank circuit at pumping frequencies up to 1 GHz. This conventional LC resonant circuit is inductively coupled to a matching coil, which is connected to the 50 Ω transmission line. We characterized the flux noise of high-temperature rf SQUIDs at resonant frequencies between 221 and 950 MHz. At about 900 MHz, we obtained the white flux noise of about 14.6 μφ0/Hz with a SQUID inductance LS=150 pH and 20 μφ0/Hz with LS=260 pH. Several of such tank circuits can be driven in parallel or in series with only one cable.