Surface Resistance Of Superconductors Research Articles

A method based on a dual frequency resonator to estimate physical parameters of superconductors from surface impedance measurements in a magnetic field

High frequency applications of superconductors in a dc magnetic field rely on the estimate of the characteristic crossover frequency νc between low and high losses. Customarily, high sensitivity resonant techniques, intrinsically operating at discrete frequencies, are used to estimate νc. We exploit here a method based on a dual frequency resonator. We show that single-frequency evaluations of νc lead to heavy underestimations of the superconductor surface resistance. We describe a combined analytical and experimental approach that gives more accurate estimates for νc, and additionally allows to test the underlying physical model.

Vortex Dynamics and Dissipation under High-Amplitude Microwave Drive

In this paper, we describe the vortex dynamics under high-amplitude microwave drive and its effect on the surface resistance of superconductors. The vortex surface resistance is calculated with a Montecarlo approach, where the vortex motion equation is solved for a collection of vortex flux lines each oscillating within a random pinning landscape. This approach is capable of providing a detailed description of the microscopic vortex dynamics and in turn important insights into the microwave field amplitude dependence of the vortex surface resistance. The numerical simulations are compared against experimental data of vortex surface resistance at high microwave amplitude measured by means of bulk niobium superconducting-radio frequency cavities operating at 1.3 GHz. The good qualitative agreement of simulations and experiments suggests that the non-linear dependence of the trapped flux surface resistance with the microwave field amplitude is generated by progressive microwave depinning and vortex jumps.

Determination of the magnetic field dependence of the surface resistance of superconductors from cavity tests

We present a general method to derive the magnetic field dependence of the surface resistance of superconductors from the Q-curves obtained during the cryogenic tests of cavities. The results are applied to coaxial half-wave cavities, TM-like "elliptical" accelerating cavities, and cavities of more complicated geometries.

Corrections to "Surface Resistance of Superconductors in the Presence of a DC Magnetic Field: Frequency and Field Intensity Limits" [Aug 17 Art. no. 3500506

We report here a correction to a figure in the paper “Surface Resistance of Superconductors in the Presence of a DC Magnetic Field: Frequency and Field Intensity Limits.”

Surface Resistance of Superconductors in the Presence of a DC Magnetic Field: Frequency and Field Intensity Limits

We revisit the classical rigid-fluxon model which describes the effect of an applied magnetic flux on the RF surface resistance of a superconductor and discuss explicit formulations for a few limit cases. As an application, we discuss the role of magnetic flux on the enhanced surface resistance of RF accelerating cavities based on niobium, depending on its material properties. We also assess the conditions for the use of high-temperature superconductors for carrying the beam image RF currents in future high-energy circular hadron colliders such as the future circular collider being studied at CERN.

Effect of Quasiparticles in the Intragap States on the Superconducting Surface Resistance

The Mattis-Bardeen theory is extended by introducing the complex number of the superconducting gap energy, by which BCS density of states is broadened and the small density of quasiparticle states that appears in the energy gap is well predicted. Using the extended Mattis-Bardeen theory, the surface resistance of superconductors and quality factors of superconducting resonators are calculated. It is shown that simulated temperature dependence of quality factors of superconducting resonators agree very well with the measured ones. It is also demonstrated that the calculated residual quasiparticle number of a superconducting resonator using the density of states consisting of the gap-broadened and localized quasiparticle states agrees quantitatively with the measured one.

Effect of vortex hotspots on the radio-frequency surface resistance of superconductors

We present detailed experimental and theoretical investigations of hotspots produced by trapped vortex bundles and their effect on the radio-frequency (rf) surface resistance ${R}_{s}$ of superconductors at low temperatures. Our measurements of ${R}_{s}$, combined with the temperature mapping and laser scanning of a 2.36-mm-thick Nb plate incorporated into a 3.3-GHz Nb resonator cavity cooled by the superfluid He at 2 K, revealed spatial scales and temperature distributions of hotspots and showed that they can be moved or split by thermal gradients produced by the scanning laser beam. These results, along with the observed hysteretic field dependence of ${R}_{s}$ which can be tuned by the scanning laser beam, show that the hotspots in our Nb sample are due to trapped vortex bundles which contain $\ensuremath{\sim}{10}^{6}$ vortices spread over regions $\ensuremath{\sim}0.1--1$ cm. We calculated the frequency dependence of the rf power dissipated by oscillating vortex segments trapped between nanoscale pinning centers, taking into account all bending modes and the nonlocal line tension of the vortex driven by rf Meissner currents. We also calculated the temperature distributions caused by trapped vortex hotspots, and suggested a method of reconstructing the spatial distribution of vortex dissipation sources from the observed temperature maps. Vortex hotspots can dominate the residual surface resistance at low temperatures and give rise to a significant dependence of ${R}_{s}$ on the rf field amplitude ${H}_{p}$, which can have important implications for the rf resonating cavities used in particle accelerators and for thin-film structures used in quantum computing and photon detectors.

정합회로 장착 고효율 소형안테나의 해석

This paper deals with the analysis of high efficiency small antenna like superconductor antenna. The superconductor antenna is useful to low frequency range because surface resistance of superconductor increased with the rate of square of frequency. Thus, the used antenna increases in size. For such a reason, the role of the matching circuit is very important in small antenna. In this paper, low loss antenna included the optimal structure of matching circuit is analyzed by using quasi-static approximation. To verify the results of this paper, input impedance and radiation characteristics of this antenna have been evaluated.

The quality factor of a superconducting rf resonator in a magnetic field

The quality factor of a superconducting NbTi resonator at 1.6 MHz in a magnetic field up to 1.2 T as well as its temperature dependence is investigated. A hysteresis effect in the superconducting surface resistance as a function of the magnetic field is observed. An unloaded Q-value of the resonator of 40,500 is achieved at 3.9 K. It is shown that this Q-value is limited by dielectric losses in the FORMVAR insulation of the coils wire. The details of the Q-value optimization are discussed. In the temperature dependence of the Q-value a steep decrease is observed above T approximately = 7.5 K. Finally, the implications of these measurements for real trap experiments are discussed in detail.

Broadband superconducting microstrip patch antenna using additional gap-coupled resonators

We have designed and fabricated a superconducting microstrip patch antenna with a larger bandwidth. The low surface resistance of superconductor compared to normal conductor corresponds to a large quality factor (Q) and improved performance in passive microwave devices. However, the narrow bandwidth that occurs with large Q is a major obstacle to the wider application of superconducting microstrip antennas. To enlarge the bandwidth of superconducting microstrip antennas, two additional gap-coupled resonators (GCRs) are used for the antenna. An electromagnetic simulator based on the moment method was used for design and analysis. The antenna has a center frequency of 11.85GHz and was fabricated using YBa2Cu3O7−δ (YBCO) thin films on CeO2 buffered r-Al2O3 substrates. Return loss and bandwidth of the GCR antenna were measured using a vector network analyzer. The GCR antenna shows an improvement of the bandwidth over a single-element superconducting patch antenna.

Microwave surface resistance of superconductors with grain boundaries

Microwave-field distribution, dissipation, and surface impedance are theoretically investigated for superconductors with laminar grain boundaries (GBs). In the present theory we adopt the two-fluid model for intragrain transport current in the grains, and the Josephson-junction model for intergrain tunneling current across GBs. Results show that the surface resistance ${R}_{s}$ nonmonotonically depends on the critical current density ${J}_{cj}$ at GB junctions, and ${R}_{s}$ for superconductors with GBs can be smaller than the surface resistance ${R}_{s0}$ for ideal homogeneous superconductors without GBs.

Measurement of the absolute penetration depth and surface resistance of superconductors and normal metals with the variable spacing parallel plate resonator

The variable spacing parallel plate resonator (VSPPR) is a microwave transmission line resonator with a continuously variable thickness of the dielectric spacer between the superconducting or metallic plates, filled by cryogenic liquid or vacuum. We measure the dielectric spacer thickness dependencies of the resonator frequency and quality factor, and fit them to theoretical forms, in order to extract the absolute values of penetration depth, λ, and surface resistance, Rs. A cryogenic micropositioning setup is developed to vary the spacer thickness from 0 to 100 μm with a resolution of 8.5 nm, and to maintain parallelism of the resonator plates. Measurement of ac capacitance between the plates is utilized to directly determine the separation between the resonator plates and to reduce the effect of their tilt and nonflatness on the accuracy of the measured Rs and λ. Because the operating temperature is fixed (77 K), the result for a superconductor is independent of an a priori model for the penetration depth versus temperature. This technique can also be employed as a surface impedance standard for characterization of high temperature superconducting films for microwave applications.

Possibility of Nd/sub 1.9/Ba/sub 1.1/Cu/sub 3/O/sub 7+δ/ and Pr/sub 1.14/Ba/sub 1.86/Cu/sub 3/O/sub 7-δ/ single crystals for insulator in high-speed superconducting circuits

Dielectric properties of both Nd/sub 1.9/Ba/sub 1.1/Cu/sub 3/O/sub 7+/spl delta// (NBCO-213) and Pr/sub 1.14/Ba/sub 1.86/Cu/sub 3/O/sub 7-/spl delta// (Pr-rich PBCO) single crystals have been examined at low temperature. These materials have good lattice matching to high-T/sub c/ superconductors (HTS), but they are conductive at room temperature. Below 80 K, they are insulators with low dielectric constants, /spl epsi//sub /spl tau// below 25, and low dielectric loss tan /spl delta/ below 0.1 at 100 kHz. The value of /spl epsi//sub /spl tau// is suitable for insulators in integrated circuits using strip line widths of 10 /spl mu/m order, providing short delay time, no excitation of surface wave, and low radiation loss. The value of tan /spl delta/ is comparable to loss of superconducting surface resistance above 100 GHz. These results indicate the applicability for the insulator layers in multilayer superconducting electronic devices such as Single Flux Quantum (SFQ) circuits operated at high speed.

Measurement of the absolute penetration depth and surface resistance of superconductors using the variable spacing parallel plate resonator

We have developed a modified Parallel Plate Transmission Line Resonator with a smoothly variable thickness of the dielectric spacer filled by liquid nitrogen. A cryogenic linear stage is made to vary the spacer from 200 /spl mu/m down to contact with 0.1 /spl mu/m resolution. Estimates of the absolute penetration depth and the surface resistance are based on the analysis of the spacer thickness dependencies of the resonator frequency and Q-factor. The measurements are performed at fixed temperature (77 K), so the result does not depend on an a priori model for the temperature dependence of the penetration depth. The ability of this technique to be employed as a standard for characterization of HTS films for microwave applications is pointed out.

Improvement of accuracy in measurements of the surface resistance of superconductors using dielectric resonators

Two techniques to minimize the influence of parasitic losses on surface resistance measurements of superconductors employing dielectric resonators have been described. The first method optimizes the sapphire resonator aspect ratio to minimize the parasitic losses for given superconductor sample dimensions and measurement frequency. The second utilizes a reference resonator with a perfect conductor plane to cancel out the influence of parasitic losses by measurements of the resonant frequencies and Q-factors of two resonators. The second technique is recommended when materials having noticeable dielectric losses are used to construct dielectric resonators.

Automation-orientated techniques for quality-factor measurement: of high-Tc superconducting resonators

It is a common technique to use resonators to determine the surface resistance of high-temperature superconductors through the measurements of unloaded Q0 factor and resonant frequency. As the surface resistance may change very significantly between the normal and superconducting states, the quality factor may also have a large variation between the two states. It is thus desirable to develop measurement techniques to deal with the possible large variation in Q0 values. Six methods are proposed in the paper for both one- and two-port coupled resonators. These techniques are suitable for a network-analyser-based automated system used for the determination of surface resistance of superconductors. Example measurements using these techniques are also given.

Precision submillimeter wave laser reflection measurements of high-temperature superconductors

Submillimeter wave laser reflection measurements of surface resistance can provide improved capability in the combination of sensitivity, spatial resolution, and frequency range. We have made reflectivity measurements on metals at 1630 GHz with an uncertainty of less than 0.3%. This sensitivity corresponds to a measurement sensitivity for surface resistance of 0.3 Ω. Assuming anf2 frequency scaling of high-temperature superconductor surface resistance from the microwave to the terahertz frequency range, this sensitivity corresponds to about 1 ×10−5 Ω at 10 GHz. Capability for 10−7 Ω sensitivity could eventually be possible. Preliminary submillimeter wave reflection measurements of a YBCO thin film have been made with a sensitivity of 1%. Submillimeter wave reflectometry can make it possible to determine the spatial dependence of surface resistance in a wide range of material sizes and shapes. The spatial resolution could be on the order of 0.3–0.5 mm.

Bipolaron theory approach to the microwave surface resistance of high-temperature superconductors

Abstract The temperature dependence of the high-temperature superconductor surface resistance is calculated on the basis of the bipolaron Bose condensation model. The correlation between the theoretical results and experimental data is discussed. Several constituents of the residual resistance are described.

55 GHz microstrip bandpass filter made from YBa2Cu3O7−x superconductor thin film

We report the first mm wave passive component made with high temperature superconducting thin films. Microstrip bandpass filters, with bandwidth of 2.2 GHz and a center frequency of 55 GHz, were made using YBa2Cu3O7−x filter patterns on MgO substrates with Au ground planes. The best results were obtained for a film grown with a thin SrTiO3 buffer layer and had Tc=89 K and Jc=106 A/cm2 at 84 K. The filter showed insertion loss of 1.7 db at 77 K, and 1.5 db at 70 K, compared to 7.3 db for Au filters at 77 K. Subtracting the losses in the Au ground plane, we estimated that the superconductor surface resistance is lower than one fourth that of Au at 77 K and 55 GHz.

Confocal resonators for measuring the surface resistance of high-temperature superconducting films

A quasioptical technique of measuring superconductor surface resistance using a confocal resonator has been developed. The method has advantages of nondestructive analysis, high sensitivity, easy extension to higher frequencies, convenient experimental setup, and flexibility in sample size. Tl-Ca-Ba-Cu-O high-temperature superconducting films have been measured with this technique and the measured surface resistances were less than 0.01 Ω at 36.135 GHz and 77 K. The measurements have been performed from 29 to 39 GHz, and all films showed roughly a quadratic dependence of surface resistance with frequency.