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arXiv:1203.3998 · Physical Review B

Unconventional rf photoresponse from a superconducting spiral resonator

Alexander P. Zhuravel, Cihan Kurter, Alexey Vladimirovich Ustinov, Steven M. Anlage

Published Apr 30, 2012• 25 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevb.85.134535

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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.
We present thin-film niobium (Nb\mathrm{Nb}Nb) spiral resonators exhibiting large geometric inductance, high critical magnetic fields, and high single-photon quality factors. These low-loss geometric inductors can be a compelling alternative to kinetic inductors to create high-impedance superconducting devices for applications that require magnetic fields. By varying the spiral pitch, we realize resonators with characteristic impedances ranging from $3.25--7.09\phantom{\rule{0.2em}{0ex}}\mathrm{k}\mathrm{\ensuremath{\Omega}}$. We measure the temperature and magnetic-field-dependent losses and find that the high-impedance resonators maintain an intrinsic quality factor above approximately 105{10}^{5}105 for parallel magnetic fields of up to 1 T. These properties make spiral Nb\mathrm{Nb}Nb resonators a promising candidate for quantum devices that require circuit elements with high impedance and magnetic-field resilience.
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.
Superconducting microcircuits and metamaterials are promising candidates for use in new generation cryogenic electronics. Their functionality is largely justified by the macroscopic distribution of electromagnetic fields in arranged unit cells, rather than by the microscopic properties of composite materials. We present a new method for visualizing the spatial structure of penetrating microwaves with microscopic resolution in planar superconducting macroscopic resonators as the most important circuit-forming elements of modern microelectronics. This method uses a low-temperature laser scanning microscope that examines the phase (i.e., direction) and amplitude of local radio-frequency currents versus the two-dimensional coordinates of the superconducting resonant structure under test. Phase-sensitive contrast is achieved by synchronizing the intensity-modulated laser radiation with the resonant harmonics of the microwave signal passing through the sample. In this case, the laser-beam-induced loss in the illuminated area will strongly depend on the local phase difference between the RF carrier signal and the spatially temporal structure of the focused laser oscillation. This approach eliminates the hardware limitations of the existing technique of radio-frequency microscopy and brings the phase-sensitive demodulation mode to the level necessary for studying the physics of superconducting metamaterials. The advantage of the presented method over the previous method of RF laser scanning micros-copy is demonstrated by the example of the formation of standing waves in a spiral superconducting Archimedean resonator up to the 38th eigenmode resonance.
The issue of intermodulation distortion in superconducting devices for rf and microwave applications is discussed. The classical frequency conversion theory for nonlinear two-port devices in the small signal limit is shown to apply to microstrip resonators regardless of their geometry and material. Two tone, third order intermodulation measurements allow more sensitive detection of nonlinearity compared to surface impedance measurements, provided a low noise, spurious free experimental setup is adopted. Measurements carried out on high quality Nb meanderline microstrip resonators show that at very low power level nonlinearity is dominated by intrinsic device properties. Film defects start to play a predominant role above a critical power level that strongly depends on the defect’s nature.
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The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975K. The lowest-loss resonators that we measure have quality factors of over 106 at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and indicate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.