Jonathan R. Dizon

Combining Near-Field Scanning Microwave Microscopy With Transport Measurement for Imaging Current-Obstructing Defects in HTS Films

Identification of defects that obstruct electrical current in high-temperature superconductors (HTS) is of great importance for applications. A technique that combines near-field scanning microwave microscopy (NSMM) with transport measurement was developed to obtain multiple sets of complementary maps on the same sample area. This technique takes advantage of the NSMMs unique capability to function both as an EM wave emitter that can locally heat a spot on a current-biased sample and also as a detector to map the spatial non-uniformity in electromagnetic properties of the sample including loss, dielectric constant, and surface morphology. Macroscopic defects in YBa2Cu3O7-delta (YBCO) films were clearly identified and imaged using this technique with adequate sensitivity and resolution.

Fabrication and characterization of two-pole X-band HgBa2CaCu2O6+δ microstrip filters

HgBa2CaCu2O6+δ (Hg-1212) two-pole X-band microstrip filters of 5% 3dB bandwidth have been fabricated and evaluated. The Hg-1212 filters exhibited a 449±3MHz bandwidth and a center frequency of 10.86±0.02GHz below the superconducting temperature Tc of 121K with low insertion loss of 0.70±0.04dB at 110K. This performance represents the best so far achieved in superconductor filters at above 100K. The better performance of the Hg-1212 filter at higher temperatures, as compared to its YBa2Cu3O7 and Cu counterparts at 77K, was attributed mainly to its higher Tc, which makes Hg-1212 a promising alternative material for passive microwave devices.

Application of near-field scanning microwave microprobe to electrical current density mapping

A near-field scanning microwave microprobe (NSMM) has been integrated with electrical current-voltage (I–V) characterization for mapping electrical current density (J) in conducting films at room temperature (RT). In this technique, the focused microwave emitted at the tip of the NSMM was used to alter the local resistance of the sample via heating which results in a voltage response ΔV that depends linearly on the local J and the applied microwave power. In addition to the ΔV map generated during the NSMM scan over a sample, a microwave response map can be recorded simultaneously using the NSMM, providing complementary information of sample morphology and surface impedance. On silver microbridges with predefined patterns, a 10‐μm spatial resolution has been demonstrated and better resolution may be achieved with further optimization. Hence, this technique provides a simple and nondestructive method for identification of current-limiting defects in conducting films and may be scaled for reel-to-reel quali...

Fabrication of Three-Pole

The increased research interest in the microwave applications of high temperature superconductors (HTS) was brought about by the perceived potential on the marketability of superconducting electronics, especially in the wireless communications industry. Excellent surface morphology and high reproducibility of the HgBa2CaCu2O6+delta (Hg-1212) thin films made from cation exchange process has motivated us to develop microwave bandpass filters, which can be operated at above 77 K and therefore are much more cost effective. An Hg-1212 three-pole hairpin filter of 5% 3-dB bandwidth has been fabricated and characterized. The transmission properties and third-order inter modulation (IM3) measurements on the filter have demonstrated Hg-1212 as a promising alternative material for passive microwave devices at above 77 K operating temperature.

{\rm HgBa}_{2}{\rm CaCu}_{2}{\rm O}_{6+\delta}

Third-order intermodulation is studied in two-pole X-band high-temperature superconducting (HTS) HgBa2CaCu2O6+δ microstrip filters at >77K. The third-order intercept (IP3) of the HgBa2CaCu2O6+δ filters is consistently higher than that of the YBa2Cu3O7 filter of the same geometry in this temperature range. At 110K, the IP3 of 38dBm remains for the Hg-1212 filters, the best so far achieved at T>100K. The dc critical current density Jc and the rf one JIP3 derived from the IP3 have a similar reduced temperature dependence, suggesting that the magnetic vortex depinning in HTS materials dominates the microwave nonlinearity at elevated temperatures.

Hairpin Filter and Characterization of Its Third Order Intermodulation

A dual-channel scanning microprobe (dual probe) has been developed for simultaneous mapping of microwave and optical properties of a sample. It employs a tunable open-ended coaxial resonator with a tapered and metal-coated fiber optic tip, from which microwave and light can be emitted/collected simultaneously. The microwave channel of this probe, operating at 1.5 GHz, can detect changes in sheet resistance (Rx) above 293 mΩ with a spatial resolution ranging from 5 to 10 μm. The optical channel, on the other hand, has a spatial resolution ∼1 μm. The poorer spatial resolution in the microwave channel was attributed to its lower sensitivity. Imaging of dielectric grid and high-Tc superconducting microwave resonators was carried out at ambient temperature.

Third-order intermodulation in two-pole X-band HgBa2CaCu2O6+δ microstrip filters

An improved near-field scanning microwave microscope (NSMM) combined with electrical transport measurement was applied for characterizing nonuniformity of electrical dissipation in YBa2Cu3O7−δ (YBCO) films. We demonstrate identification of the current-obstructing defects in thin (thickness below 300nm) YBCO films by mapping microwave-induced electrical voltage (ΔV) and reflected microwave power on the sample. In addition, the technique was also found to be suitable for characterizing electrical dissipation in thick YBCO films of a few micrometer thick. In order to improve the spatial resolution to submicron regime, we have employed a hybrid probe tip with a submicron tip diameter and operate the NSMM to its second harmonic frequency to increase hot spot microwave absorption. A much improved spatial resolution in the submicrometer range for the microwave maps was achieved while maintaining the sensitivity in the ΔV measurement.

Development of a dual-channel scanning microwave/optical microprobe

The effect of localized microwave absorption in conducting thin films had been simulated using a heat diffusion model to quantitatively evaluate the electrical voltage maps obtained recently using a near-field scanning microwave microprobe (NSMM) in combination with the I-V characterization. In the experiment, the focused microwave emitted from the NSMM at a sample spot of dimension comparable to the NSMM tip dimension changes the local resistivity of the sample via microwave heating. By measuring the voltage response while scanning the NSMM, nonuniformity in the electrical current distribution in the sample can be resolved. To quantitatively assess the effect of various parameters, a theoretical simulation of the temperature profile has been made and related to the voltage response profile obtained experimentally. The variation of the induced voltage due to microwave absorption with time, microwave input power, and the thickness of the film has also been studied and the simulation compares well with expe...

Improved near-field scanning microwave microscope combined with electrical transport measurement for characterizing nonuniformity of electrical dissipation in YBa2Cu3O7−δ films of variable thickness

Near-field scanning microwave microscopy (NSMM) was combined with electrical transport measurements to detect the dynamic evolution of low-level dissipation and thermal instability in Y Ba2Cu3O7 − δ microbridges in the superconducting state driven by temperature, applied current and self-heating. High sensitivity NSMM enables quantitative detection of dissipation evolution starting from 3 to 4 orders of magnitude lower than the transport critical current criterion. The nonlinear step-like feature in the dynamic development of dissipation suggests a bi-modal evolution pattern of nucleation of isolated hot spots followed by their spreading/coalescence. A linear behavior prevails near the superconducting-to-normal transition.

A comparative study of simulated and experimentally obtained nonuniformity in thermal and electrical properties of conducting films

Near-field scanning microwave microscopy (NSMM) provides a unique nondestructive approach for detection of local dissipation with high sensitivity and high spatial resolution. With recently improved NSMM probes of spatial resolution of up to 400 nm ( ~ 10^-6 wavelength), detection of dissipation was achieved on YBCO microbridges at currents more than three orders of magnitude below the <i>Jc</i>(<i>T</i>). In this work, we report characterization of the dynamic behavior of low-level dissipation at the grain boundary of YBa₂Cu₃O_7-δ microbridges as function of time and applied electrical current. On higher-angle grain boundary, the dissipation develops rapidly with increasing current and shows approximately linear dependence on current. On lower-angle grain boundary, nonlinear features were observed and attributed to bi-modal pattern of dissipation evolution of nucleation of isolated hot spots and their evolution. Comparison with the similar NSMM+<i>IV</i> measurement made on the “bulk” part of the same YBa₂Cu₃O_7-δ microbridges on a reduced temperature scale shows higher dissipation on the grain boundary can be mostly attributed to the lower <i>Tc</i> values on grain boundaries.