R. G. Humphreys

Nonlinear microwave surface impedance of YBCO films: latest results and present understanding

Abstract While passive high-temperature superconductors (HTS) microwave devices are finding application in wireless communication systems, the nonlinear surface impedance and intermodulation distortion (IMD) limit wider application. Understanding and controlling the nonlinear microwave response of HTS is important for narrow band, sharp cut-off receive filters, which are susceptible to intermodulation, and high-power transmit filters, which suffer from power-dependent absorption. The sources of these two related nonlinear microwave responses are not understood at present. We discuss the latest experimental results directed toward understanding the nonlinearities. Several experiments have ruled out some of the most likely origins. We discuss measurements of the impedance of films on bicrystals showing that low-angle (

Nonlinear dielectric microwave losses in MgO substrates

We have investigated the nonlinear surface impedance and two-tone intermodulation distortion of ten epitaxial YBa2Cu3O7−δ films on MgO substrates, using stripline resonators, at frequencies f=2.3–11.2u2009GHz and temperatures T=1.7u2009K−TC. The power dissipation decreased by up to one order of magnitude as the microwave electric field was increased to 100 V/m for T<20u2009K. The reactance showed only a weak increase. The minimum of the losses correlated with a plateau in the intermodulation signal. The same features were observed for a Nb film on MgO but not for YBa2Cu3O7−δ and Nb on LaAlO3 or sapphire. The anomalous response results from dielectric losses in MgO, most probably due to defect dipole relaxation.

Intermodulation distortion and third-harmonic generation in YBCO films of varying oxygen content

We have measured the nonlinear surface impedance, intermodulation distortion (IMD), and third-harmonic generation (THG) in a series of identically prepared YBaCuO films that have been carefully annealed to produce a controlled oxygen stoichiometry. These are also compared with an unannealed film. The measurements were performed using a stripline-resonator technique as a function of temperature at a fundamental frequency of 2.3 GHz; the IMD tone separation was 10 kHz. We have found that overdoping films with oxygen substantially lowers the IMD relative to optimally and underdoped films. We have also observed differences in the slopes of the THG and IMD, with IMD slope of close to 2:1 while the same film shows THG slope of 3:1. A possible explanation of the differences lies in the different time scales to which IMD and THG are sensitive.

Nonlinear Microwave Response of Epitaxial YBaCuO Films of Varying Oxygen Content on MgO Substrates

We have investigated the nonlinear microwave properties of electron-beam coevaporated YBa2Cu3O7−x films on MgO, using stripline resonators at 2.3 GHz and temperatures 1.7–80 K. The oxygen content of the films ranged from strongly underdoped to overdoped. Above 20 K, the nonlinear response of the resonators was dominated by the superconductor. We could establish clear correlations between the nonlinear surface resistance, two-tone intermodulation (IMD), and oxygen content of the films, which indicate that the superconducting order parameter is important for the nonlinearities. A power-law representation of the nonlinear current-voltage relation would not be appropriate to explain our results phenomenologically. Below 20 K, the dielectric loss tangent of MgO dominated the nonlinear response of the resonators. With increasing power, the dissipation losses decreased markedly, accompanied by enhanced IMD. The surface reactance passed through a shallow minimum at about 5 K, independent of power. We attribute these effects to resonant absorption by impurity states in MgO.

Scanning Hall probe microscopy of flux penetration into a superconducting YBa2Cu3O7−δ thin film strip

A high-resolution scanning Hall probe microscope has been used to measure flux profiles across one “wire” of a long YBa2Cu3O7−δ thin-film meander line as a function of both transport current density and applied magnetic field. Flux bundle penetration due to an applied current or magnetic field is demonstrated to occur at the same regions at the edge of the strip. A correlation between the surface topography of the meander line edges and the regions of penetration has been established. Penetrating flux profiles at low temperatures are in qualitative agreement with theories of dynamical instability of the order parameter.

Flux structures in mesoscopic YBa2CU3O7-delta discs

Scanning Hall probe microscopy has been used to study flux Structures and dynamics in 5 mu m x 5 mu m YBCO thin film squares, which are mesoscopic with respect to the magnetic penetration depth, lambda(T), at temperatures close to T-C. A number of unusual vortex phenomena are observed in these microstructures which differ qualitatively from the expected behaviour of more macroscopic pieces of film. In field-cooled (FC) experiments to similar to 65 K a full Meissner state is generated for cooling fields less than similar to 6 Oe, reflecting the relatively small demagnetization factors in our samples. Cooling in higher fields, however, results in only a very weak diamagnetic response at low temperatures whose magnitude is almost independent of the cooling field. In contrast we observe considerable trapped flux upon field-removal whose magnitude grows monotonically with cooling field. Remarkably, all FC flux distributions exhibit almost perfect rotational symmetry, and can be nearly completely cancelled in a reversible fashion by tuning the field applied to the initially FC state. Our field-cooled and zero-field-cooled results have been analysed in terms of a Bean-like critical state model containing constant edge and bulk current densities, and most of the observed phenomena can be explained by considering the relative weight of these two components. Not all flux profiles call be described by our simple model, however, and under certain circumstances symmetry-breaking dipole-like flux structures can form in several adjacent YBCO squares. We speculate that these are related to the unidirectional At-ion milling process which was used to pattern the squares and could have broken the expected four-fold symmetry. We note that our results could have important implications for the miniaturization of thin film HTS devices.

Effect of Ca doping on the nonlinear microwave properties of YBCO thin films

We have investigated the microwave properties of a set of four identical YBa2Cu3O7−δ (YBCO) films. One of the films has a 30% Ca-substituted YBCO overlayer, whereas the three others are used as references to highlight the effect of the Ca substitution. The microwave characterization was carried out using the stripline-resonator technique. The Rs of the sample that has a Ca-rich overlayer increased slightly in the linear regime, whereas no significant effect on the nonlinear components and the intermodulation distortion has been observed. We conclude that Ca doping does not significantly improve the nonlinear microwave properties of YBCO films, but the small changes in the linear surface resistance that we observed are discussed in terms of the physics of Ca doping of YBCO.

Scanning hall probe microscopy of nanostructured superconductors

Abstract High resolution scanning Hall probe microscopy has been used to study flux structures in various nanostructured superconducting samples. Five micron square YBCO disks exhibit rotationally symmetric flux structures at high temperatures and after field-cooling to low temperatures. We find that their magnetic response can be almost perfectly compensated by tuning the applied magnetic field. High temperature measurements of the ‘local magnetisation’ above the disks as a function of applied field display pronounced sawtooth structures with very sharp jumps. In contrast asymmetric flux structures are observed after cooling in H=0 and the application of a field at low temperatures. We speculate that we may possibly be observing the formation of Giant Vortex states at high temperatures and multi-vortex states at low temperatures. A thin Pb film containing a square antidot pinning array has also been studied. Close to T c ‘local magnetisation’ measurements exhibit pronounced peaks at rational fractional filling factors reflecting strongly pinned commensurate flux line structures. These special flux structures have been imaged directly at lower temperatures (5.5K) where we find that a maximum of two flux lines can be pinned at each antidot.

Nonlinear microwave properties of superconducting YBaCuO and dielectric MgO

We have investigated the nonlinear properties of high-temperature superconducting films on MgO substrates for microwave applications in the UHF-range. Surface impedance and two-tone frequency intermodulation are different sources of nonlinear microwave response, which are subtly related. We have found correlations between the microwave response of the superconducting films and their electronic and structural properties, which are essential to optimise the device performance. Furthermore, for the first time, we have observed a nonlinear dielectric loss tangent of MgO at temperatures up to 70 K. The dielectric losses contribute significantly to intermodulation distortion. Our findings are relevant for conventional and super-conducting microwave components involving usual dielectric materials like MgO. The results may also prove helpful for the development of novel RF dielectrics.

Flux penetration and critical current limited by material inhomogeneities in a YBCO superconducting strip

Abstract A state-of-the-art scanning Hall probe microscope has been used to measure flux profiles across one ‘wire’ of a long YBCO thin film meander line as a function of both transport current density and applied magnetic field. Flux bundle penetration due to an applied current or magnetic field is demonstrated to occur at the same disordered regions at the edge of the film. Vortices trapped in the strip during field cooling appear to play no role in the process of first flux penetration.