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arXiv:1504.01937 · Applied Physics Letters

Dual cut-off direct current-tunable microwave low-pass filter on superconducting Nb microstrips with asymmetric nanogrooves

Oleksandr V. Dobrovolskiy, Michael Huth

Published Apr 6, 2015• 42 citations
arXivHTMLPDF

Abstract

We present a dual cut-off, dc-tunable low-pass microwave filter on a superconducting Nb microstrip with uniaxial asymmetric nanogrooves. The frequency response of the device was measured in the range 300 KHz–14 GHz at different temperatures, magnetic fields, and dc values. The microwave loss is most effectively reduced when the Abrikosov vortex lattice spatially matches the underlying washboard pinning landscape. The forward transmission coefficient S21(f) of the microstrip has a dc-tunable cut-off frequency fd which notably changes under dc bias reversal, due to the two different slope steepnesses of the pinning landscape. The device's operation principle relies upon a crossover from the weakly dissipative response of vortices at low frequencies when they are driven over the grooves, to the strongly dissipative response at high frequencies when the vortices are oscillating within one groove. The filter's cut-off frequency is the vortex depinning frequency tunable by the dc bias as it diminishes the pinning effect induced by the nanopattern. The reported results unveil an advanced microwave functionality of superconducting films with asymmetric (ratchet) pinning landscapes and are relevant for tuning the microwave loss in superconducting planar transmission lines.

DOI
10.1063/1.4917229

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We present a dual cut-off, dc-tunable low-pass microwave filter on a superconducting Nb microstrip with uniaxial asymmetric nanogrooves. The frequency response of the device was measured in the range 300 KHz–14 GHz at different temperatures, magnetic fields, and dc values. The microwave loss is most effectively reduced when the Abrikosov vortex lattice spatially matches the underlying washboard pinning landscape. The forward transmission coefficient S21(f) of the microstrip has a dc-tunable cut-off frequency fd which notably changes under dc bias reversal, due to the two different slope steepnesses of the pinning landscape. The device's operation principle relies upon a crossover from the weakly dissipative response of vortices at low frequencies when they are driven over the grooves, to the strongly dissipative response at high frequencies when the vortices are oscillating within one groove. The filter's cut-off frequency is the vortex depinning frequency tunable by the dc bias as it diminishes the pinning effect induced by the nanopattern. The reported results unveil an advanced microwave functionality of superconducting films with asymmetric (ratchet) pinning landscapes and are relevant for tuning the microwave loss in superconducting planar transmission lines.
We introduce a fluxonic metamaterial on the basis of nanopatterned superconducting Nb microstrips and employ it for modulation and synthesis of quantized loss levels in the lower GHz range by a sine-wave quasistatic ac drive. The nanopatterns are uniaxial nanogrooves with identical and different slope steepness, which induce a pinning potential of the washboard type for Abrikosov vortices. For the fundamental matching field, when the location of vortex rows geometrically matches the nanogrooves, the following effects are observed: The forward transmission coefficient S21(f) of the microstrips can be controllably modulated within a range of about 3 dB by the ac. For the sample with symmetric grooves, depending on the choice of the operation point in the current-voltage curve, the shape and the duty cycle of the output signal can be tuned. For the sample with asymmetric grooves, depending on the ac amplitude, a sine-to-triangular or a sine-to-rectangular pulse shape conversion is observed. The possibility of synthesizing quantized loss levels by a serial connection of the two samples with different nanopatterns is exemplified and can be used for the development of multilevel excess-loss-based fluxonic devices.
A lattice of Abrikosov vortices in type II superconductors is characterized by a periodic modulation of the magnetic induction perpendicular to the applied magnetic field. For a coherent vortex motion under the action of a transport current, the magnetic induction at a given point of the sample varies in time with a washboard frequency fWB = v/d, where v is the vortex velocity and d is the distance between the vortices in the direction of motion. Here, by using a spectrum analyzer connected to a 50 nm-wide Au nanowire meander near the surface of a superconducting Nb film, we detect an ac voltage induced by coherently moving fluxons. The voltage is peaked at the washboard frequency, fWB, and its subharmonics, fTOF = fWB/5, determined by the antenna width. By sweeping the dc current value, we reveal that fWB can be tuned from 100 MHz to 1.5 GHz, thereby demonstrating that patterned normal metal/superconductor nanostructures can be used as dc-tunable generators operating in the radiofrequency range.
A superconductor bandpass filter with tunable central frequency in the range of 2-3.5 GHz has been implemented using superconducting quantum interference devices (SQUIDs). The filter is designed using two pi-network resonators connected by a transmission line. Both resonators have a SQUID inductor with a tuning range of 65-200 pH, controlled by current magnetically coupled to the SQUIDs. Over a frequency tunability of 1.5 GHz, the filter has a mid-band insertion loss of 0.5-3.0 dB and corresponding maximum unloaded quality factor of 40. Due to the presence of active elements, tunability of the filter depends on the power of the microwave signal. A maximum power of -52 dBm corresponds to a frequency tuning range of 15%. Spectral measurements by controlling the central frequency of the filter with variable pulsewidth shows that the filter can be tuned at a rate of 120 GHz/mus.
We develop a microwave frequency counter based on the complex conductivity modulation of a DC-biased superconducting transmission line while maintaining the bias below the critical current. A resonator-free platform is proposed for microwave frequency estimation. It utilizes a bias-controlled phase delay in a superconducting coplanar waveguide structure, which imprints frequency-dependent signatures on the transmitted microwave tone. Instead of relying on intentionally designed resonators, our method leverages the complex transmission spectra arising from numerous weak resonance structures due to subtle fabrication imperfections. However, even in the absence of such features, the method remains effective because the DC bias induces frequency-dependent phase shifts that modulate the microwave signal’s propagation velocity. Mixed signals are externally generated via in-phase-quadrature mixing with a reference signal, which manifests as interference-like patterns enabling high-resolution spectral identification. We measure the frequency-dependent changes in the interference-like patterns under this bias to catalog characteristic transmission loss profiles across a wide frequency range (2–8 GHz). These profiles serve as supervised training data for a machine learning algorithm that identifies unknown input microwave frequencies with high-resolution and accuracy. Our method achieves a frequency resolution (F/ΔF) of over 200 000 and a relative error in identification of below 0.01%. This scalable approach to distributed microwave sensing may be useful for quantum measurement and signal classification and is particularly well-suited for dark matter searches, such as hidden-photon detection, where broadband frequency identification without prior tuning is essential. The measured frequency range is primarily limited by the bandwidth of external microwave components, such as low-noise amplifiers and bias-tees. The superconducting device itself may be used up to its superconducting gap frequency (∼280 GHz for TiN at 3.8 K).
Microwave resonators fabricated from superconducting thin films are playing a critical role with recent advances in superconducting quantum computing technology and photon detectors. There has been an intensive worldwide effort to study the sources of loss that limit the quality factors of these resonators. In this thesis, I have focused on the measurements of the microwave response of vortices in Al and Re superconducting thin film resonators cooled in magnetic fields comparable to or less than that of the Earth. Previous work on vortex dynamics at microwave frequencies has involved large magnetic fields, orders of magnitude larger than the Earth's fields. Al and Re are common materials used in superconducting resonant circuits for qubits and detectors. Despite the similarities of Al and Re superconductors, the microwave vortex response is strikingly different in the two materials from my resonator measurements. I present a quantitative model for the dissipation and reactance contributed by the vortices in terms of the elastic pinning forces and the viscous damping from the vortex cores. The differences in the vortex response in Al and Re were due to the vortex pinning strength in the two films The critical role played by pinning in determining the microwave response motivated us to try to modify the pinning in our Al films by nanostructuring the film surface. A single narrow slot along the center line of a resonator was used to increase the pinning in the resonator traces and resulted in a reduction of the loss from vortices by over an order of magnitude. Such patterned pinning techniques could be used on resonators in systems with insufficient shielding or pulsed control fields to reduce the loss from unwanted trapped vortices. Finally, we explored the possibility for using measurements of the power dependence of the resonators to determine if trapped vortices are present.
We present the design, fabrication and testing of a low-temperature superconducting (LTS) analog microwave variable attenuator. The two-port circuit includes two 90ohybrid couplers and a phase shifter to realize a variable attenuation at the output port. The wideband analog phase shifter is implemented using LTS microstrip transmission line (TL) coupled to an array of radio frequency superconducting quantum interference devices (rf-SQUIDs). The phase shift is achieved by passing a dc current through a TL adjacent to the SQUID-loaded microstrip line which controls the flux through the rf-SQUIDs. The proposed variable attenuator shows more than 30 dB of precise analog flat attenuation tuning range over 8 GHz to 12 GHz with a dc control current ranging from 0 mA to 4.5 mA. The device exhibits a wideband operation with variable attenuation seen not only in theX-band but also inKu-band. The device is fabricated using a multilayer niobium-based superconducting process SFQ5ee by MIT Lincoln Laboratory, and has a compact footprint of 1.3 mm2. To the best of our knowledge, this is the first implementation of a variable attenuator using distributed rf-SQUIDs. This device has implications in a variety of applications such as cryogenic receivers and qubit circuit measurements.