[Show abstract][Hide abstract] ABSTRACT: This paper evaluates the RF tuning and the microwave nonlinear response of SrTiO<sub>3</sub> thin films by measuring the third harmonics and third-order intermodulation (IMD) products of several coplanar waveguide transmission lines fabricated directly on SrTiO<sub>3 </sub> thin-film samples. From these measurements, we obtained the distributed nonlinear capacitance per unit length as a function of RF bias voltage C(V<sub>rf</sub>) using an accurate equivalent circuit to model the spurious signal generation. A unique value of C(V<sub>rf</sub>) is used to describe the third-harmonic generation at frequencies 3f<sub>1</sub> and 3f<sub>2</sub>, and the IMD products at frequencies 2f<sub>1</sub>+f<sub>2</sub> and 2f<sub>2</sub>+f<sub>1</sub>, where f<sub>1</sub> and f<sub>2</sub> represent the fundamental frequencies of the two-tone excitation signal. We compare C(Vrf) with the distributed nonlinear capacitance measured as a function of the dc-bias voltage C(V<sub>dc</sub>) and obtain excellent agreement at 50 and 76 K. From these results, we conclude that full tunability can be achieved on nanosecond time scales, and that spurious signals generated in SrTiO<sub>3</sub> ferroelectric transmission lines at microwave frequencies can be modeled based on dc-biased measurements
IEEE Transactions on Microwave Theory and Techniques 03/2007; 55(2-55):391 - 396. DOI:10.1109/TMTT.2006.889346 · 2.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We experimentally demonstrate the equivalence of different manifestations of nonlinear response in high temperature superconductor
(HTS) microwave devices. Using a combination of analytical and numerical analysis, we show that the results of intermodulation
distortion measurements, harmonic generation measurements, and power-dependent resonator measurements of different coplanar
waveguide structures patterned onto the same HTS thin-film sample all yield approximately the same values for the nonlinear
penetration depth. The extraction of an underlying nonlinear material parameter that is independent of the specific device
geometry and experimental configuration will allow our results to be quantitatively compared with other nonlinear measurements,
and will therefore help in determining the dominant source(s) of nonlinear response in HTS microwave devices.
Journal of Superconductivity and Novel Magnetism 10/2006; 19(7):531-540. DOI:10.1007/s10948-006-0126-2 · 0.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This work evaluates the microwave nonlinear properties of ferroelectric BaSrTiO thin films by measuring the frequency response of several coplanar transmission lines and interdigital capacitor structures as a function of the applied electric field from 150 Hz to 40 GHz. From these measurements, we obtain the distributed nonlinear capacitance C(V<sub>dc</sub>) as a function of dc bias. We also measure the harmonic generation at microwave frequencies in ferroelectric transmission lines, and use an accurate circuit model to obtain C(V<sub>rf</sub>), the nonlinear capacitance as a function of RF bias. Information about the tuning speed of the film is obtained from a comparison between the two nonlinear capacitances. Characterization of this mechanism is also required to assess the spurious signal generation in ferroelectric-based devices
[Show abstract][Hide abstract] ABSTRACT: In this paper, we characterize microwave nonlinearity in a high temperature superconducting (HTS) thin-film by measuring a geometry-independent current-density scale j<sub>o</sub>. The quantity j<sub>o</sub> specifies the strength of a material-dependent nonlinearity, and can be used to calculate the nonlinear microwave response of planar superconducting transmission-line devices. Our procedure for determining j<sub>o</sub> involves microwave measurements on superconducting coplanar waveguide devices patterned onto the HTS sample. In a YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-x</sub> (YBCO) sample, we obtained a maximum experimental value for j<sub>o</sub> of approximately 3.26×10<sup>8</sup> A/cm<sup>2</sup> at 25 K. From the measured temperature-dependence of j<sub>o</sub> in our YBCO sample, and assuming a theoretical pair-breaking current density j<sub>c</sub> equal to 3×10<sup>8</sup> A/cm<sup>2</sup>, we calculated the temperature dependence of the quasiparticle factor b(T). The curve of the experimentally obtained b(T) matched well with a theoretically-predicted behavior in the scenario of d-wave symmetry in the superconducting order parameter.
[Show abstract][Hide abstract] ABSTRACT: We present new frequency-dependent measurements of both the magnitude and phase of the nonlinear response in superconducting thin films at microwave frequencies obtained using a large-signal network analyzer. Our measurements show that the nonlinear inductance dominates the nonlinear response in thin YBCO films at 76 K, and our analysis yields two current-density scales corresponding to the real and imaginary components of the nonlinear response. The current-density scale associated with the dominant inductive response likely results from intrinsic pair-breaking, while the current-density scale associated with the nonlinear resistive term is smaller than the expected value due to pair-breaking, and could originate from vortex motion or other extrinsic effects.
[Show abstract][Hide abstract] ABSTRACT: We propose a modified two-tone method that could be used for sensitive measurements of the intrinsic microwave surface impedance (Z<sub>S</sub>) of thin superconductor films and the tanδ of a low-loss dielectric. An open-gap resonator scheme is used to measure the penetration depth (λ) of thin superconductor films and extract the intrinsic Z<sub>S</sub> of the superconductor films from its measured R<sub>S</sub><sup>eff</sup> and λ. We use a very small gap of 10 μm between the top plate and the rest parts of the resonator. The tanδ of rutile in the low 10<sup>-7</sup> range and the dielectric constant as high as ∼110 are observed at temperatures below 10 K at ∼15.2 GHz, which enable to measure the R<sub>S</sub> of the 10 mm-in-diameter YBCO films as low as ∼100 μΩ at the same frequency (f). The discrepancy between the R<sub>S</sub><sup>eff</sup> at ∼15.2 GHz and that at ∼8.5 GHz scaled to ∼15.2 GHz appears less than 2% when the relations of R<sub>S</sub> ∝ f<sup>2</sup> and tanδ ∝ f are used. We describe usefulness of our measurement method for measuring the intrinsic microwave properties of various superconductor samples.
[Show abstract][Hide abstract] ABSTRACT: We report on the development of a microwave power limiter based on high-temperature superconductor technology. The power limiter takes the form of a 50 Ω coplanar waveguide transmission line that can be reversibly driven from the low-loss superconducting state into the high-loss normal state as microwave currents within the device exceed a critical value. This device has demonstrated very low insertion loss (<0.5dB/cm at 70K, 40GHz) and extremely wide bandwidth (constant impedance up to 40 GHz) in the signal-pass state, with variable attenuation in the signal-block state. Switching times for transitions from the signal-pass state to the signal-block state are estimated to be on the order of a nanosecond or less. Reversible operation has been demonstrated for continuous-wave (CW) signals up to 10 W at 3GHz, and for 100 μs transient signals up to 100W. This device should be valuable for protecting high-performance receiver components from high-power transients encountered in real-world applications.
[Show abstract][Hide abstract] ABSTRACT: We describe the design, fabrication, and testing of a superconducting passive nonlinear reference device that has a calculable phase relationship between the fundamental RF drive signal and resulting higher order harmonic components. We show this passive device to be a useful standard for verifying the phase calibration of nonlinear vector network analyzers, as the magnitude and phase of the third-order product are found directly from independent measurements of the field-dependent surface impedance. Microwave power-dependent measurements of coplanar waveguide (CPW) resonators fabricated from thin film high-T<sub>c</sub> superconductor materials yield the transmission line resistance and inductance per unit length as a function of rf current. With these values and the line geometry we computed both the nonlinear surface impedance of the material and the phase and magnitude of the third-order product of a traveling wave in a quasi-linear transmission line. To demonstrate our CPW reference device, we measured both the magnitude and the phase of third-harmonic components generated in a number of 133 mm long meander transmission lines using a commercial nonlinear vector network analyzer. We demonstrate agreement to within 10 degrees between the measured and the predicted phase for third-harmonic signals relative to the fundamental.
ARFTG Microwave Measurements Conference, 2003. Fall 2003. 62nd; 01/2004
[Show abstract][Hide abstract] ABSTRACT: The origin of the detrimental nonlinear response in high T<sub>c</sub> superconductor (HTS) microwave devices is currently not well understood. In order to help elucidate the origin of these nonlinear effects, we have developed a description of the nonlinear response in superconductors in terms of a current-dependent complex conductivity. We demonstrate that such a treatment can consistently describe the results of power-dependent surface impedance measurements in resonator geometries as well as harmonic generation and intermodulation distortion effects in transmission line geometries. This approach yields a device-independent quantity that describes the nonlinear response of the superconducting material itself, which is suitable for comparisons of different materials and for material optimization. A further benefit of this description of nonlinear effects is that the relative importance of the nonlinear resistive and inductive components of a superconductor can be examined. We use this approach to predict the phase of the nonlinear response in HTS planar transmission lines, and compare our predictions with new phase-sensitive measurements made using a nonlinear vector network analyzer.