Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

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New Fe-based superconductors: properties relevant for applications

We present a phenomenological description of the high-frequency vortex dynamics in and discuss the main parameters related to vortex motion, namely the viscous drag coefficient , the pinning constant (Labusch parameter) and the depinning frequency . We demonstrate experimental results on the angular and temperature dependence of , and in and compare these results with existing models. We show how studies of the vortex viscosity may yield information on the superclean limit. This limit corresponds to the formation of the discrete excitation spectrum in the vortex core due to quantum confinement and small coherence length. From the low-temperature viscosity data we conclude that the superclean limit in is reached for magnetic field perpendicular to the c-axis.

https://iopscience.iop.org/article/10.1088/0953-2048/9/1/001/pdf

High-frequency vortex dynamics in

The penetration depth and surface resistance of a superconductor depend upon the superfluid current density. This dependence gives rise to nonlinear mixing in a superconducting microstrip resonator. Here we discuss the problem of intermodulation in which two signals at ω1 and ω2, laying within the pass band of a microstrip cavity resonance, mix and generate a signal at 2ω1−ω2. An expression relating the power generated at 2ω1−ω2 to the power transmitted at ω1 and ω2 is given. We focus on the high-Tc superconductors where it is believed that the order parameter has dx2−y2 symmetry. We find for a resonator with a large unloaded Q that intermodulation arises dominantly from the reactive nonlinear inductance of the superconducting film.

Theory of intermodulation in a superconducting microstrip resonator

High-T c
 superconductivity has generated a great deal of interest because of the challenges it presents in the fields of material science, condensed matter physics, and electrical engineering, and because of the potential applications which may result from these research efforts. Thin-film passive microwave components may become the first high-temperature superconducting (HTS) devices available for widespread use and commercialization. In this article, we review aspects of material science, physics, and engineering which directly impact high-T

 c
 superconducting microwave devices and discuss issues which determine the performance of these devices. Methods of growing HTS thin films on large-area substrates, techniques for fabricating single-level HTS passive microwave components, and the relevant properties of high-T

 c
 superconducting films are discussed, with a focus on thin films of the HTS material YBa2Cu3O7−δ. Several known mechanisms for microwave loss in both the superconductor and the dielectric substrate are presented. An overview of the general classes of superconducting passive microwave devices is given, and representative microwave devices which have been recently demonstrated are described in detail. Examples of a select few HTS active microwave components are also presented. Potential microwave applications are illustrated with comparisons to current technology.

High-temperature superconducting microwave devices: Fundamental issues in materials, physics, and engineering

We present measurements of the surface impedance as a function of frequency (1--17 GHz), temperature (4.2--91 K), and peak rf magnetic field (0${\mathit{H}}_{\mathrm{rf}}$500 Oe) for high-quality epitaxial ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ thin films using a stripline-resonator technique. The lowest surface resistance at 1.5 GHz was 15 \ensuremath{\mu}\ensuremath{\Omega} at 77 K and 3 \ensuremath{\mu}\ensuremath{\Omega} at 4.3 K. The results for the low- and intermediate-field regions (${\mathit{H}}_{\mathrm{rf}}$50 Oe at 77 K) are explained by a coupled-grain model, which treats the film as a network of superconducting grains connected by grain boundaries acting as resistively shunted Josephson junctions. Quantitative agreement has been obtained between the model and the measurements. The effective junction critical current density, grain size, and shunt resistance are used in the model to characterize the films. The high-field region may be explained by flux penetration into the grains but is not modeled in detail here.

Nonlinear surface impedance for YBa2Cu3O7-x thin films: Measurements and a coupled-grain model.

Microwave hysteretic losses in YBa2Cu3O7-x and NbN thin films.

We present measurements of surface impedanceZ

 S
 as a function of frequency, temperature, and rf magnetic field for high-quality epitaxial YBa2Cu3O7−x
 thin films using a striplineresonator technique that measures theQ of the resonator vs. input power. The results have been modeled using Ginzburg-Landau theory at moderate fields, and the Bean critical state model at high fields. Good agreement has been obtained between the model and the measurements. We also discuss the current distributions in the stripline for both low power whenZ

 S
 is linear and high power whenZ

 S
 is nonlinear.

Measurements and modeling of linear and nonlinear effects in striplines

High-Tc thin films continue to be of interest for passive device applications at microwave frequencies, but nonlinear effects may limit the performance. To understand these effects we have measured the nonlinear surface impedanceZs in a number of YBa2Cu3O7−x thin films as a function of frequency from 1 to 18 GHz, rf surface magnetic fieldHrf to 1500 Oe, and temperature from 4 K toTc. The results at lowHrf are shown to agree quantitatively with a modified coupled-grain model and at highHrf with hysteresis-loss calculations using the Bean critical-state model applied to a thin strip. The loss mechanisms are extrinsic properties resulting from defects in the films. We also report preliminary measurements of the nonlinear impedance of Josephson junctions, and the results are related to the models of nonlinearZS. The implications of nonlinearZS for devices are discussed using the example of a five-pole bandpass filter.

Nonlinear surface impedance of YBCO thin films: Measurements, modeling, and effects in devices

Deoxygenated nonsuperconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.2}$ (YBCO) single crystals have been doped with Br. The resulting crystals become superconducting with ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}92 K, \ensuremath{\Delta}${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}1.0 K as determined by four-point-probe resistivity measurements. The resistivity in our best sample decreases linearly with temperature. The large ratio in the resistivity at 300 K of the brominated to the pristine YBCO single crystals (\ensuremath{\sim}35:1) suggests that bromination greatly increases the scattering rate. The upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) parallel and perpendicular to the ab plane is determined resistively. ${\mathit{H}}_{\mathit{c}2}$(0) and the corresponding coherence lengths ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0) and ${\ensuremath{\xi}}_{\mathit{c}}$(0) are estimated. A comparison with the fully oxygenated YBCO single crystals shows that ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0) remains approximately the same, whereas ${\ensuremath{\xi}}_{\mathit{c}}$(0) decreases by a factor of \ensuremath{\sim}3, suggesting that Br never enters the ${\mathrm{CuO}}_{2}$ planes. The pinning energy for vortex motion in the ab plane decreases after bromination and this decrease can be attributed to the increased anisotropy. For vortex motion along the c axis, however, the pinning energy decreases or increases depending on temperature and field. Compared with the fully oxygenated YBCO single crystals, the critical current density is suppressed by bromination and is strongly dependent on the applied magnetic field. The reduced lower critical field ${\mathit{H}}_{\mathit{c}1}$ in the brominated YBCO single crystals indicates a reduction in the carrier density (by a factor of between 2 and 3), which is consistent with the observed reduction in the plasma frequency.

P. P. Nguyen

Papers

Nonlinear surface impedance for YBa2Cu3O7-x thin films: Measurements and a coupled-grain model.

Microwave hysteretic losses in YBa2Cu3O7-x and NbN thin films.

Measurements and modeling of linear and nonlinear effects in striplines

Nonlinear surface impedance of YBCO thin films: Measurements, modeling, and effects in devices

Transport and magnetic properties of YBa 2 Cu 3 O 7 − δ Br y single crystals