The transport properties of ''interface-engineered'' edge-type YBa 2 Cu 3 O 7 Josephson junctions are investigated in detail. We have investigated the dependence of the current-voltage characteristics on external magnetic field, temperature, and microwave irradiation and compare them to the resistively shunted junction model. The temperature dependence of the critical current and the normal resistance allows us to draw conclusions to the transport of quasiparticles and Cooper pairs in the investigated ''interface-engineered'' junctions. We have studied the properties of junctions for which La doped YBa 2 Cu 3 O 7 is used for the superconducting electrodes. We will propose a model for the undoped and the La doped case which takes into account a barrier which consists of a series connection of a normal conducting layer and an insulator, containing superconducting microconstrictions.
A waveguide mixer based on a Josephson junction made from a high-temperature superconductor (HTS) has been analysed at frequencies of 90 GHz and 115 GHz. The mixer consisted of a single-grain-boundary bicrystal junction which was integrated with a bow-tie antenna and a microstrip filter structure fabricated on an MgO substrate. Two different designs of mixer block were tested. Using the first design the best noise performance was measured at a local oscillator (LO) frequency of about 90 GHz. At the projected LO frequency of 115 GHz significant deterioration of performance was observed. A computer simulation of the mixer chip and substrate channel structure was performed using the 3D simulation program HFSS. Sufficient RF signal rejection by the filter structure was confirmed at 90 GHz. After a simulation cycle at 115 GHz the substrate channel in the mixer block was modified. Using this second design we measured a double-sideband mixer noise temperature of 1090 K at 10 K operating temperature which is the best performance of an HTS Josephson mixer to date obtained at this frequency and temperature.
We have investigated the dependence of the critical current I C on the value and orientation of an externally applied magnetic field H for interface-engineered YBa 2 Cu 3 O 7Ϫx ramp-type Josephson junctions. The results are compared with measurements of Nb ramp-type junctions with a PdAu interlayer. The I C versus H dependences are similar to Fraunhofer patterns and their modulation period changes several orders of magnitude with the orientation of the magnetic field. For both junction types, the dependence of the modulation period on the orientation of the magnetic field can be well described by the change of the relevant projection of the junction area and the influence of flux-focusing. Therefore the features of the I C (H) curves have to be attributed to the ramp geometry and not to specific properties of the superconducting material.
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