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arXiv:cond-mat/0209614 · Applied Physics Letters

Influence of LaAlO3 surface topography on rf current distribution in superconducting microwave devices

Alexander P. Zhuravel, Alexey Vladimirovich Ustinov, K. Solomon Harshavardhan, Steven M. Anlage

Published Dec 19, 2002• 33 citations
arXivHTMLPDF

Abstract

A laser scanning microscope with a thermal spot size of about 4 μm is used to measure a quantity proportional to the rf current density in an operating superconducting coplanar waveguide microwave resonator. The twinning of the LaAlO3 substrate produces a meandering of the current at the edges due to irregularities in the wet etching of the YBa2Cu3O7−δ film associated with substrate twin domain blocks, and an (∼20%) enhancement of the rf photoresponse at these locations. These irregularities are candidates for enhanced nonlinear response from the device.

DOI
10.1063/1.1530753

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A laser scanning microscope with a thermal spot size of about 4 μm is used to measure a quantity proportional to the rf current density in an operating superconducting coplanar waveguide microwave resonator. The twinning of the LaAlO3 substrate produces a meandering of the current at the edges due to irregularities in the wet etching of the YBa2Cu3O7−δ film associated with substrate twin domain blocks, and an (∼20%) enhancement of the rf photoresponse at these locations. These irregularities are candidates for enhanced nonlinear response from the device.
Micro-channels of nanosized columnar tracks were planted by heavy-ion irradiation into superconducting microwave microstrip resonators that were patterned from YBa2Cu3O7-x thin films on LaAlO3 substrates. Three different ion fluences were used, producing different column densities, with each fluence having a successively greater impact on the nonlinearity of the device, as compared to a control sample. Photoresponse images made with a 638 nm rastered laser beam revealed that the channel is a location of enhanced photoresponse and a hot spot for the generation of intermodulation distortion. The microwave photoresponse technique was also advanced in this work by investigating the role of coupling strength on the distribution of photoresponse between inductive and resistive components.
This work presents new experimental results on low-temperature (LT) characterization of local RF properties of passive superconducting (SC) microwave devices using a novel laser scanning microscope (LSM). In this technique, a modulated laser beam is focused onto and scanned over the surface of a resonant SC device to probe the spatial distribution of RF current. The highly localized photo-induced change of the kinetic inductance of the SC device produces both a shift of the resonant frequency f/sub 0/ and change of the quality factor Q. An image of these changes is recorded as the laser spot is scanned over the device. We present the first measurements of spatially resolved intermodulation response in a high temperature superconducting (HTS) co-planar waveguide resonator, opening up a new window into the local origins of nonlinearity in the HTS materials.
Abstract We report a technique for measuring the spatial profile of microwave‐frequency currents in resonant thin‐film superconducting microstrip devices operated near resonance. We probe the patterned superconducting device as a function of position by scanning a modulated light beam across the region to be probed. With the light focused to a spot size much smaller than the microstrip width, we have measured the current density distribution as a function of position across the width of the strip. Using an elementary model for the superconducting current density distribution, we fit an effective penetration depth at the edge of the microstrip. © 1993 John Wiley & Sons, Inc.
Superconducting thin film resonators employing strip geometries show great promise in rf/microwave applications due to their low loss and compact nature. However, their functionality is limited by nonlinear effects at elevated rf/microwave powers. Here, we show that by using a planar spiral geometry carrying parallel currents in adjacent turns, this limitation can be minimized. We investigate the rf current distributions in spiral resonators implemented with Nb thin films via laser scanning microscopy. The rf current density profile along the width of the individual turns of the resonators reveals an unconventional trend: maximum current in the middle of the structure and decaying toward its edges. This unusual behavior is associated with the circular nature of the geometry and the cancellation of magnetic field between the turns, which is favorable for handling high powers since it allows the linear characteristics to persist at high rf current densities.
This paper reports the design, fabrication, and measurements of optically tunable superconducting microwave resonators and delay lines. The devices are comprised of superconducting coplanar waveguide (CPW) transmission lines fabricated from a 100 nm-thick YBCO film, interrupted by 3- and 5-m YBCO meander structures of the same thickness on top of an LAO substrate. The CPW is tested at 77 K up to a frequency of 50 GHz, where it exhibits a transmission loss of about 0.12 dB/mm. The measurements are done in a cryogenic microwave probe station, where all the calibrations have been performed at 77 K. It has been observed that the resonance frequencies of the superconducting resonators can be changed by more than 2 GHz under the optical illumination at a wavelength of 1550 nm. The delay lines also exhibit a phase retardation of up to 90 at the same operating conditions.
We measured the spatial distribution of microwave fields and dissipative surface currents in various passive and active superconducting thin-film devices during their operation at a temperature of about 4 K. We used low-temperature scanning electron microscopy which offers a spatial resolution of about 1 μm and we present sample response models for interpreting the imaging results. Such investigations may have important consequences for the improvement of the design and preparation of superconducting thin-film microwave devices.