The microwave power and frequency dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by the breakdown in the ω2 law for poor-quality samples at all temperatures. The performance of MgB2 at 10 GHz and 21 K was compared directly with that of high-quality YBCO films. The surface resistance of MgB2 was found to be approximately three times higher at low microwave power and showed an onset of nonlinearity at microwave surface fields ten times lower than the YBCO film. It is clear that MgB2 films are not yet optimized for microwave applications.
The two tone intermodulation arising in MgB2 thin films deposited in-situ by
planar magnetron sputtering on sapphire substrates is studied. Samples are
characterised using an open-ended dielectric puck resonator operating at 8.8
GHz. The experimental results show that the third order products increase with
the two-tone input power with a slope ranging between 1.5 and 2.3. The
behaviour can be understood introducing a mechanism of vortex penetration in
grain boundaries as the most plausible source of non linearities in these
films. This assumption is confirmed by the analysis of the field dependence of
the surface resistance, that show a linear behaviour at all temperatures under
test.Comment: 13 pages, 3 figures; to be published in Appl. Phys. Let
The resonance curve shape of a thin film parallel plate HTS resonator was observed as a function of delay time from 0.3 to 12 s at each frequency step across the resonance at various microwave surface magnetic field levels up to 28 mT. Skewed resonance curves and a pronounced shift of resonance frequency were encountered as the microwave input power was increased. A simple phenomenological model was set up in order to explore whether the skewed shape and frequency shift observed could originate from a global temperature rise. The simulation produces resonance curve shapes that agree well with the experimental observation whether the frequency is swept up or down. However, the model fails to produce a sufficiently large resonance frequency shift unless input powers two orders of magnitude larger than in experiment or unrealistically small values for the thermal conductivity are used. The delay time experiments in combination with the simulation suggests that the films fail due to a complex combination of mechanisms.
In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB 2 . The powder sample has a T c = 39 K and the films have T c = 29 K and 38 K. These samples show different temperature dependences of the field penetration depth. Over a period of six months, the film with T c = 38 K degraded to a T c of 35 K. We compare the results on all samples with data obtained elsewhere and discuss the implications as far as is possible at this stage.
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