Superconductors at Radio Frequencies – SURF

The aim of this project is to elucidate a number of outstanding fundamental problems in superconductivity involving the application and development of microwave technology based on High Temperature Superconductors (HTSC). These include: the origin of losses and nonlinearities in radiofrequency devices (such as filters, resonators and delay lines); pinpointing the physical location of lossy regions in rf devices; the generation and detection of plasma oscillations and sub THz emission in layered HTSC. Within this scope, we aim to establish new links between researchers of the superconductivity community that, until now, have worked using different approaches. Our strategy is to apply high frequency and broadband experimental techniques common in applied superconductivity to basic science. Conversely, we expect our results to yield new strategies for the improvement of superconducting microwave components. Thus, the three parts of the project will be tackled in the following. A link between losses and nonlinearities in microwave devices and the rf frequency-and power dependence of the dynamics of flux vortices will be studied by fashioning HTSC thin films of known microstructure into devices, operating over specific frequency ranges (or combs of frequencies). These devices will then be used for critical current and surface resistance measurements, with the aim of establishing the behavior of the vortex lattice (in the presence of a given defect structure) over a broad frequency range. In such a manner, we obtain, simultaneously, information on the optimisation of devices, and a data base allowing for the verification of presently accepted paradigms for vortex dynamics. Studies will be performed on HTSC of both the YBCO and BSCCO families of compounds. Second part of the project is the visualisation of the penetration of microwaves into real superconducting structures using the magneto-optical technique for flux imaging. In a first step, direct and differential magneto-optical imaging will be used to detect changes in previously established flux distributions by a microwave field. In this manner, the nature of flux penetration (Critical state , i.e. lossy vs Meissner-like) can be established. In a later stage, we wish to directly image microwave magnetic field distributions in HTSC films. The third part of the project consists of the application of heterodyne detection techniques common in astrophysics to the characterization of layered HTSC (of the BSCCO family of compounds). The sample, placed in a variable-temperature copper block, will be exposed to black-body radiation. The radiation spectrum, minus the part absorded by the sample, will be detected either directly, of by a heterodyne mixing method, using SIS (Nb-AlO3-Nb) junctions. Tunable and Phase-locked Gunn oscillators will be used as a local oscillator. The method is characterized by the physical and thermal separation of the sample block and the detector block, allowing one to apply magnetic fields of different magnitude and direction to the sample, but no to the detector chip. In this way, phenomena such as the Josephson Plasma Resonance, Coherent locking between flux-flow and plasma waves, and THz emission can be studied with high sensitivity and spectral resolution, in an unambigous way.