Spatially-resolved nonlinearity measurements of YBa2Cu3O7−δ bicrystal grain boundaries

Abstract

We have developed a near-field microwave microscope to locally excite a superconducting film and measure second and third harmonic responses at microwave frequencies. We study the local nonlinear response of a YBa2Cu3O 7−δ thin film grown on a bi-crystal SrTiO3 substrate. The location of the bi-crystal grain boundary is clearly identified by the microscope through higher harmonic response, and the spatial resolution is on the order of the magnetic loop probe size, about 500µm. The harmonic power and spatial resolution are successfully modeled with a onedimensional extended Josephson junction simulation. From the model, the 2nd order harmonic response is dominated by Josephson vortex generation and flow. A geometry-free nonlinear scaling current density JNL ≃ 10 4 ∼ 10 5 A/cm 2 is also extracted from the data, indicating that the grain boundary weak link is the dominant nonlinear source in this case.PACS numbers: PACS: 74.25. Fy, 74.25.Nf, 74.62.Dh, 74.72.Bk The nonlinear properties of high-T c superconductors have been of great concern in microwave applications, although the microscopic origins of the nonlinear response still remain uncertain. [7] Many experiments have studied the intermodulation power, [8,9] harmonic generation, [10,11] or the nonlinear surface impedance of superconductors [12,13] as a function of applied microwave power. However, most nonlinear experiments are done with resonant techniques, which by their nature study the averaged nonlinear response from the whole sample rather than locally. Such techniques usually have difficulty in either avoiding edge effects, which give undesired vortex entry, or in identifying the microscopic nonlinear sources. Therefore, a technique capable of locally measuring nonlinear properties of samples is necessary for understanding the physics of different nonlinear mechanisms. In addition, most existing experimental techniques focus on 3rd order nonlinearities, which can be conveniently studied by intermodulation techniques, but rarely address the 2nd order nonlinear response. Here we present a technique to locally characterize 2nd and 3rd order nonlinearities through spatially localized harmonic generation.In prior work, [9] we studied the intermodulation signal from a high-T c superconducting microwave resonator * Electronic address: sycamore@wam.umd.edu † Electronic address: anlage@squid.umd.edu using a scanned electric field pick-up probe. Both the "global" and the "local" intermodulation power measured with the open-end coaxial probe were presented. However, the local measurements were actually a superposition of nonlinear responses that were generated locally but propagated throughout the microstrip and formed a resonant standing-wave pattern. To avoid this loss of spatial information, we have developed a non-resonant near-field microwave microscope, to nondestructively measure the local harmonic generation from un-patterned samples. This technique has the additional advantage of operating equally well through T c and into the normal state of the sample...

Microwave nonlinearities of an isolated longYBa2Cu3O7−δbicrystal grain boundary

“…2. A strong peak in P 3f (T) is observed around T c at all locations on the sample, 26,28,29 and these peaks show similar magnitudes as shown in Fig. 2.…”

“…2 as a solid line. 26,28 A small shift in temperature ~ 0.5K of this peak with location is also observed, which is most likely due to inhomogeneity at the grain boundary. Temperature (K) P 3f at GB; P 3f at GB-fit P 3f at Non-GB; P 3f at Non-GB-fit P f = 8 dBm A significant difference in the third harmonic response with position is observed for T < 0.9T c , where the GB shows significant enhancement (P 2f ~ -70 dBm and P 3f ~ -55 dBm) in both P 2f and P 3f , but non-GB regions show only noise-level signals.…”

“…Temperature (K) P 3f at GB; P 3f at GB-fit P 3f at Non-GB; P 3f at Non-GB-fit P f = 8 dBm A significant difference in the third harmonic response with position is observed for T < 0.9T c , where the GB shows significant enhancement (P 2f ~ -70 dBm and P 3f ~ -55 dBm) in both P 2f and P 3f , but non-GB regions show only noise-level signals. 26 The enhanced P 2f and P 3f at the GB are due to the nonlinearity of the Josephson tunneling effect across the boundary, and our technique is capable of detecting and quantifying this local nonlinear mechanism, as discussed below.…”

“…If there are any local nonlinear mechanisms present in the range of the induced microwave currents, additional microwave currents at multiples of f will be generated. 26,28 We detect the lowest-order harmonic signals at 2f and 3f by measuring the microwave signal coupled back from the sample surface through the probe with the spectrum analyzer. Since the harmonic signals are extremely weak, they are amplified by ~ 60dB before entering the spectrum analyzer.…”

“…26 In this work, we present additional data and much more comprehensive analysis and simulations. The simulation results are examined to understand the vortex dynamics taking place in the middle of an infinite superconducting GB with AC currents passing through.…”