Microwave Current Research Articles

A self-attenuating superconducting transmission line for use as a microwave power limiter

We have designed, fabricated, and tested microwave power limiter based on high-temperature superconductor thin-film technology. The power limiter takes the form of a 50 /spl Omega/ coplanar waveguide transmission line that is reversibly driven from the low-loss superconducting state to the high-loss normal state when the microwave currents within the device exceed a critical value. When operated at 70 K, the power limiter displays very low insertion loss in the nonlimiting state ( 40 GHz), with constant impedance over the entire microwave range. The maximum power transmitted by the device can be engineered by varying the transmission-line dimensions, and can be further tuned once the device has been fabricated by varying the operating temperature. In the over-power state the device continues to pass a portion of the incident signal, although with reduced linearity. Switching times for the power limiter are estimated to be on the order of microseconds or less, based on both pulsed rf measurements and on measurements using a periodic amplitude-modulated drive signal. We expect this device to be useful for protecting high-performance receiver circuits from over-power conditions without limiting the dynamic range or bandwidth of receiver systems.

MAGNETIC FIELD ORIENTATION DEPENDENCE OF THE MICROWAVE RESPONSE IN YBa2Cu3O7-δ

We present microwave resistivity measurements at 21 GHz in YBa 2 Cu 3 O 7-δ thin film as a function of the temperature and the magnetic field. Measurements are taken in various relative orientations of the microwave current, dc magnetic field and c axis. Attention is focussed on measurements taken with the dc field parallel to the (a, b) planes. In moderate magnetic field, we show that the microwave magnetic response is made up of a vortex motion contribution, which can be described by conventional models, and a noticeable magnetic field induced increase of the quasiparticle density, which exhibits features typical of the existence of lines of nodes in the superconducting gap. We estimate the effective vortex viscosity for vortex motion across the (a, b) planes.

Field dependent microwave resistivity in YBa2Cu3O7-δ

We investigate the effects of directional pinning due to the layered structure and to columnar defects on the microwave response in YBa2Cu3O7−δ films. We present measurements of the field-induced microwave resistivity at 48 GHz and 21 GHz taken in various relative orientations between the microwave currents, dc magnetic field and (a, b) planes. From measurements taken in the Lorentz-force-free configuration we experimentally show the relevance of the magnetic field induced increase of quasiparticle (qp) density. We identify the vortex motion contribution, and extract the vortex parameters. We estimate a pinning frequency of order 30 GHz when the field is aligned to the (a, b) plane. Secondly, we show that the introduction of columnar defects gives rise to a strong pinning along the columns, detectable even at 48 GHz. The pinning frequency appears to be of the same magnitude than for pinning by the layered structure.

Simulation of temperature fields in a superconducting microstrip bridge

The temperature distribution in a microstrip bridge made of a high-temperature superconductor (HTSC) as a result of nonuniform heating by a microwave current is analyzed. Differential equations of heat conduction are solved in the one-and two-dimensional approximations. The size of a localized normal region is estimated. Comparative analysis of the results is performed.

Computer-aided modeling of superconducting striplines with ground planes using critical state models

The effect of ground planes on the current-induced critical states and flux penetration for a thin superconducting strip carrying a microwave current has been calculated numerically. The nonlinear response represented by the dependence of the surface resistance and reactance were also calculated. We find that when the distance d between the ground planes is smaller than the width a of the strip, i.e., d⩽a, the ground planes affect the nonlinear response significantly. In particular we study the effects of ground planes placed at a distance d=a/5. We compare and contrast these effects with the results of similar models without the ground planes. The ground planes reduce the current crowding at the film edges, thus reducing the nonlinearities arising from critical state flux motion. The numerical procedure can be incorporated into computer-aided design of superconducting microwave circuits.

Frequency locking of Josephson junctions in a surface wave resonator

We report on the design, fabrication, and investigation of a series array of Josephson junctions embedded into the surface wave resonator. The resonator is fabricated by using Au-YBa/sub 2/Cu/sub 3/O/sub 7/ bilayer on yttria-stabilized zirconia bicrystal substrate. The arrays were incorporated into the resonator by meandering the bilayer across a grain boundary. The circuit design and the excitation of the proper resonant mode provides series dc biasing and parallel ac biasing of the JJs array. Experimental results show that microwave current is distributed uniformly along the whole 8 mm long array. In the vicinity of the resonant frequency, almost no difference between the microwave currents in the junctions was found.

Influence of La-doping of YBa/sub 2/Cu/sub 3/O/sub 7/ on transport properties of interface-engineered ramp-edge junctions

We have investigated the influence of La-doping of the YBa/sub 2-x/La/sub x/Cu/sub 3/O/sub 7/ (YBCO) thin film electrodes on dc and ac Josephson properties of ramp-edge junctions with interface-engineered barriers. Non-doped optimized junctions exhibit critical current densities, j/sub c/, of 5/spl middot/10/sup 4/ A/cm/sup 2/ and normal sheet resistances, R/sub N//spl middot/A, of 3/spl middot/10/sup -9/ Ohm/spl middot/cm/sup 2/ at 77 K. The transport properties of junctions with x=0.03 La-doping are quite similar to those of non-doped junctions. La-doping of x=0.05 and x=0.07 leads to an increase of R/sub N//spl middot/A and decrease of j/sub c/ by about one order of magnitude. Devices with j/sub c/<10/sup 4/ A/cm/sup 2/, which are operating in the short junction limit exhibit current-voltage characteristics without any excess current and 100% modulation of critical current in external magnetic field. The current voltage characteristics exhibit well defined Shapiro steps. The dependence of current-step height on microwave current can be described in the resistively-shunted junction model. The strong changes in the temperature dependencies of j/sub c/ and R/sub N//spl middot/A suggest different electrical transport properties for junctions fabricated with non-doped and La-doped YBCO. The temperature dependence of the critical current and the normal resistance allows us to draw conclusions to the transport properties of our junctions.

Design optimization for microwave coupling structures of a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions

A new coupling scheme of the external microwave signal for a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions has been proposed. It is suggested to couple the meander-like array inductively to a parallel microwave transmission line. Within the resistively shunted junction model different types of microwave coupling structures were analyzed with the goal to reduce the mutual phase locking and to achieve high output power and phase-locking stability. A harmonic balance technique was used to derive an analytic expression connecting the amplitudes of microwave voltages and currents in the array and the frequency of the external signal. Numerical simulations were performed to determine the boundaries of stable locking as a function of circuit parameters.

Nonlinear impedance of a microwave-driven Josephson junction with noise

The nonlinear impedance of a point Josephson junction is calculated under various conditions for the resistively shunted junction model in the presence of noise. The calculation proceeds by solving the Langevin equation for the mechanical problem of a Brownian particle in a tilted cosine potential in the presence of a strong ac force ignoring inertial effects. The exact solution of the infinite hierarchy of equations for the moments (expectation values of the Fourier components of the phase angle), which describe the dynamics of the junction, is expressed in terms of a matrix continued fraction. This solution allows one to evaluate the nonlinear response of the junction (nonlinear microwave impedance, for example) to an ac microwave current of arbitrary amplitude. Strong nonlinear effects in the resistance and the reactance are observed for large ac currents as is demonstrated by plotting the nonlinear response characteristics as a function of the model parameters. For weak ac currents and low noise strengths, our results agree closely with previously available linear response and nonlinear response noiseless solutions, respectively. Applications of the model to the interpretation of recent experimental data found in the literature for the nonlinear behavior of microwave impedance of superconducting weak links are discussed.

Microwave properties ofYBa2Cu3O7−δthin films in linear and nonlinear regime in a dc magnetic field

We report an experimental and theoretical study of the linear and nonlinear microwave impedance of thin ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films in a dc magnetic field of different orientations. The measurements were done using the parallel-plate resonator technique at 5.7 GHz, $21\mathrm{K}&lt;T&lt;80\mathrm{K},$ and for $H&lt;0.8\mathrm{T}.$ We demonstrate that the contributions to the microwave response from nonlinearity and from the dc magnetic field are almost additive for our films (excluding very low microwave power). The increase of microwave current simply leads to a parallel shift of the dc magnetic field dependences of the surface impedance. This result shows that widely discussed nonlinear mechanisms related to vortices produced by the dc field are not relevant for our experimental conditions. The theoretical analysis of other nonlinear mechanisms points out that the most probable source of nonlinearity is vortices produced by microwave currents in weak links. A qualitative description of the experimentally observed nonlinear microwave response is provided by the critical-state Bean model, which was modified for thin films.

Modeling the nonlinear surface impedance of high-Tc thin films

Abstract To explore the idea that weak links are responsible for power dependence, we have measured and modeled the microwave impedance of fabricated Josephson-junctions, in which we have observed evidence for Josephson-vortex creation and annihilation by the microwave currents. We believe that we have observed long-junction effects in fabricated high- T c Josephson-junctions and that these are the first observation of microwave-frequency vortex creation. To explain the nonlinear surface impedance of thin films of the high- T c materials, an extended-coupled-grain model is introduced and discussed. The model incorporates a series array of Josephson-junction defects with a distribution of I c R n products. Comparison with experiments shows good qualitative agreement.

The nonlinear surface impedance of YBa2Cu3O7−δ thin films in zero and large applied fields

We present coplanar resonator measurements of the nonlinear microwave surface impedance of laser ablated and electron-beam coevaporated YBa2Cu3O7−δ thin films at 8 GHz in zero and applied magnetic fields, including the effects of irradiation with heavy ions. Correlations between the changes in surface resistance and reactance as a function of microwave current suggest that there are different contributions to the nonlinear behavior at low and high microwave currents in both zero and applied fields. In zero field, we suggest that microwave-induced flux lines are responsible for the observed nonlinear behavior, resulting in thermal runaway at large, sample dependent, microwave currents. The observed nonlinearity completely masks any possible contribution from the intrinsic pair breaking expected for a d-wave superconductor. In a magnetic field at relatively low currents, the nonlinearity can be described by the nonlinear dynamics of flux lines pinned by defects with a continuous range of pinning strengths. At higher currents, microwave field nucleation of flux lines is again considered important. On irradiation with 3.6 MeV/nucleon Xe-129 ions, the microwave losses and nonlinearity are significantly decreased in both zero and applied fields, which could have important implications for microwave device applications.

Coupled-grain/RSJ series array for modeling of nonlinear microwave surface impedance of YBCO thin films

The nonlinear microwave impedance of YBCO thin films may be due to Josephson-junction-like defects. In this work, high-T/sub c/ thin films are modeled as a series of ideal superconducting grains coupled by resistively shunted Josephson junctions that simulate the defects. The model calculates the surface resistance and reactance as a function of microwave current. Comparisons with experimental results from YBCO thin-film stripline resonators show good quantitative agreement for resistance at frequencies in the range 1-6 GHz and at several temperatures. Long junction effects are proposed to explain deviations between measured and computed results above 6 GHz. We have also modeled a phase-shifting device consisting of 300 high-T/sub c/ Josephson junctions in series along a microstrip transmission line. Computed results are in good quantitative agreement with the measured device phase shift versus microwave input power. Similarities in the power dependencies of the series array of junctions and the patterned film resonators are discussed.

Removal of a choanal polyp with the wire loop of an aural snare after microwave coagulation

A 59-year-old man with chronic sinusitis of many years' duration complained of obstruction of the right nostril, particularly on expiration. Rhinoscopic examination revealed a globular tumor, measuring about 1 cm in diameter, behind the right middle meatus. Because the polyp could not be encircled by the loop of a nasal snare, a double-needle electrode (width, 1 mm) was inserted into the polyp three times. Each time a microwave current of 40 watts was applied for 3 seconds to cause coagulation. The polyp became smaller and could be removed with the wire loop (thickness, 0.3 mm) of an aural snare. We consider microwave coagulation therapy useful in the management of selected cases of choanal polyps.

Intrinsic mechanism of phase locking in two-dimensional Josephson junction networks in presence of an external magnetic field

We present numerical simulations of the dynamics of two-dimensional Josephson junction arrays to study the mechanism of mutual phase locking. We show that in the presence of an external magnetic field two mechanisms are playing a role in phase locking: feedback through the external load and internal coupling between rows due to microwave currents induced by the field. We have found the parameter values (junction capacitance, cell loop inductance, impedance of the external load) for which the interplay of both these mechanisms leads to the in-phase solution. The case of unloaded arrays is discussed as well.

RF power dependence study of large area YBCO thin films

In an effort to develop HTS superconducting filters with sufficient power handling capability for PCS (Personal Communication Services) base station transmit applications, we have undertaken a study of the power dependence of large area YBCO thin films on LaAlO/sub 3/ substrates. We employed a coplanar-wave-guide (CPW) resonator technique to obtain the changes of loss and inductance versus circulating microwave currents in the films. Data have been collected on uniform large area (2 diameter) films grown by coevaporation and off-axis sputtering techniques under varying deposition conditions. We found correlations between the RF power dependence and other film properties such as penetration depth and crystal structure. The most intrinsic sample, from the coevaporation technique, characterized by the smallest penetration depth, good orthorhombicity and absence of tetragonal phase, shows the least amount of nonlinearity. Such correlations can be used to prescreen films for fabrication and monitor the film production line. However, films from coevaporation and off-axis sputtering show very distinct power dependent behaviors.

Experimental evaluation of the nonlinear surface resistance of YBa2Cu3O7?x step-edge grain boundary Josephson junctions

The temperature, microwave current, and dc magnetic field dependencies of the effective surface resistanceRs of YBa2Cu3O7−x grain boundary Josephson junctions have been investigated at 6 GHz. The junctions were prepared on stepped LaAlO3 substrates and incorporated into tapered linear half-wavelength microstrip resonators. The characteristic parameters of the heterogeneous resonator were analyzed theoretically considering the junction as a lumped element of complex impedance. The microwave properties of the junction could then be extracted and related to their characteristic dc transport properties. The critical currentIc of the junction was found to limit the linear power dissipation in the superconducting strip. At microwave currents aboveIc, highly nonlinear microwave losses occurred, which displayed a characteristic magnetic field modulation. At even higher currents,Rs saturated at a level corresponding to the normal resistanceRn of the junction.

Microwave power dependence of YBa2Cu3O7 thin-film Josephson edge junctions

Using a stripline resonator technique, we measured the dependence on microwave current of the impedance of fabricated superconductor/normal-metal/superconductor Josephson junctions in YBa2Cu3O7 thin films. The results are compared with predictions of a resistively shunted junction model that yields good agreement at low and intermediate values of the microwave current. At high currents deviations from the model predictions are observed, which we propose are due to flux penetration into the junction. The results are consistent with previously proposed explanations of the rf power dependence of the high-Tc materials.

Catastrophic quenching of superconductivity in melt-textured YBa2Cu3O7−δ with second-phase additions

Using rf (microwave) currents, the quenching of superconductivity in melt-textured YBa2Cu3O7−δ with second-phase additions at temperatures near the superconducting onset temperature has been observed. The current density was varied from roughly 104 to 105 A/cm2. Quenching is indicated by dramatic changes in the surface resistance, induced by a combination of the rf power level (current density), and an applied magnetic field. The catastrophic response is interpreted as the failure of a percolative network of junctions. The observed ‘‘failure’’ is attributed to variations in the critical current of the junctions. The junctions which fail are thought to be intragranular, not ‘‘weak links’’ associated with grain boundaries.

Nonlinear electrodynamics of superconducting NbN and Nb thin films at microwave frequencies.

The surface resistance ${\mathit{R}}_{\mathit{s}}$ of NbN and Nb thin films with particular reference to nonlinear effects is reported at high values of the microwave magnetic field ${\mathit{H}}_{\mathrm{rf}}$\ensuremath{\le}500 Oe using a stripline resonator in the frequency range 0.6\ensuremath{\le}f\ensuremath{\le}12 GHz. In the stripline geometry, the microwave current is concentrated on the narrow center conductor. Thus a high microwave current density and, therefore, a high ${\mathit{H}}_{\mathrm{rf}}$ can be achieved with moderate power. For Nb thin films, ${\mathit{R}}_{\mathit{s}}$ does not increase with ${\mathit{H}}_{\mathrm{rf}}$ for low ${\mathit{H}}_{\mathrm{rf}}$ values, as expected from weak-link theory. However, for the NbN thin films, ${\mathit{R}}_{\mathit{s}}$ at temperature T follows a ${\mathit{R}}_{\mathit{s}}$(T,f)=${\mathit{R}}_{\mathit{s}0}$(T,f)+S(T,f)${\mathit{H}}_{\mathrm{rf}}$ dependence, for f6 GHz where ${\mathit{R}}_{\mathit{s}0}$(T,f) is the surface resistance at zero ${\mathit{H}}_{\mathrm{rf}}$, and the slope S(T,f) is proportional to ${\mathit{f}}^{2,3}$. This nonlinear effect is consistent with Halbritter's weak-link theory. For f&gt;6 GHz, ${\mathit{R}}_{\mathit{s}}$ shows a plateau in the dependence on ${\mathit{H}}_{\mathrm{rf}}$ the magnitude of which varies as ${\mathit{f}}^{3,5}$, which is not predicted theoretically. When ${\mathit{H}}_{\mathrm{rf}}$ increases above a critical value ${\mathit{H}}_{\mathrm{rf}}^{\mathit{c}}$, the resonance curves for the strip-line resonator become asymmetrical and the intermodulation products saturate, indicating strong nonlinearities. The temperature dependence of ${\mathit{H}}_{\mathrm{rf}}^{\mathit{c}}$(T) for Nb, in contrast to NbN, thin films follows that of the dc ${\mathit{H}}_{\mathit{c}1}$. Information on the granularity of NbN, derived from the present study, provides insights into the surface impedance of the granular high-${\mathit{T}}_{\mathit{c}}$ copper oxide thin films.