Small volume high-Tc super-conducting YBa2Cu3O7 (YBCO) thin films are used as low power, very wide bandwidth mixers in the frequency range of 75 GHz to 2.5 THz. The YBCO films are patterned into lattice-cooled hot-electron bolometers (HEB) coupled to an integrated Au thin-film antenna and transmission line. Near 77 K, these mixers have responsivity as high as 780 V/W using only 8 nW of local oscillator (LO) power at 585 GHz. The responsivity can be shown to be truly bolometric. Direct heterodyne and homodyne down-conversion mixing using local-oscillator frequencies of 75 GHz and 585 GHz show overall conversion gains of -35 dB, which includes a -18 dB coupling loss, using only approximately 1 (mu) W of LO power. The gain bandwidth shows a simple Lorentzian roll-off with -3 dB point of 5 to 8 GHz. The large gain bandwidth and small power requirements make these high-Tc superconducting mixers an attractive alternative to existing Schottky diode and conventional superconducting receiver technologies.
Continuous single crystal (001)PbTiO3 thin films under a step-flow growth mode were epitaxially grown on the miscut (001)SrTiO3 substrate with miscut angle of 1.7 degrees. A planar magnetron sputtering system was used for the epitaxial growth. The film thickness was ranged from 5 to 250 nm. The surface of the sputtered PbTiO3 thin films comprised periodic striped patterns with step lines and atomically flat terraces. The typical step height and terrace width were 3 nm and 200 nm, respectively. The partial oxygen pressure during sputtering deposition affected the stability of the step-flow growth mode. Lower partial oxygen pressure stabilized the step-flow growth mode resulting in a layer growth. The PT thin films include the stress due to the tetragonal lattice deformation.
We have deliberately controlled the initial stage nucleation, growth mechanism and domain structure of epitaxial SrRuO3 thin films, using substrate etching, miscut and lattice mismatch. The use of buffered HF etched substrates results in more uniform coverage and two-dimensional growth of ultrathin SrRuO3 films, as compared to polished substrates. For thicker films, the use of exact (001) SrTiO3 substrates results in growth by two-dimensional nucleation, leading to two 90 degree domains in the plane. As the miscut angle of vicinal (001) SrTiO3 substrates is increased, step flow growth occurs leading to single domain films. An incoherent three dimensional island growth occurs on (001) LaAlO3 substrates, resulting in multi domain films.
Thin films and heterostructures of a new family of copper- oxide-based isotropic metallic oxides such as La6.4Sr1.6Cu8O20, La5BaCu6O13 and La6BaYCu8O20 have been investigated. These metallic oxides are oxygen deficient pseudo-cubic perovskites that exhibits Pauli paramagnetism, which could be ideal normal metals for SNS junctions in high Tc superconducting devices. We have grown epitaxial thin films of these metallic oxides and SNS superconducting heterostructures in situ by 90 degrees off-axis sputtering. X-ray diffraction and cross-sectional transmission electron microscopy reveal these heterostructures to have high crystalline quality and clean interfaces. This material will facilitate fabrication of ideal SNS Josephson junctions with low boundary resistance due to its excellent chemical compatibility and lattice match with cuprate superconductors and will be useful for determining the source of interface resistance in such heterostructures.
We report recent results on the synthesis and transport properties of a-axis oriented YBa2Cu307/ PrBa2Cu307 (YBCO/ PBCO) superlattices. A-axis oriented YBCO is of great interest in fabricating sandwich tunnel junctions since the coherence length in this direction is much greater than that in the caxis direction. There have been reports of supercurrent in YBCO/PBCO/YBCO junctions across up to 1500Å of PBCO.1,2 We use the YBCO/PBCO multilayer system to investigate the decoupling length of the YBCO across PBCO by changing the modulation wavelength. The resistive transitions of these superlattices in magnetic field show a crossover temperature above which they are insensitive to magnetic fields and below which broadening occurs; this may be related to the disappearance of the coupling between YBCO layers above T*.
Single target in situ sputter deposition using 9O off-axis geometry has made it possible to study various physical
properties of superconducting films of YBaCu3O'x under well controlled conditions. The superconducting DC and high
frequency properties, and the normal state properties can all be optimized under conditions which do not necessarily produce
the highest Tc's. Surface resistances at 4.2 K and 10 GHz are reproducibly less than 20 iQ. Correlations between the
processing parameters, DC properties, microstructures and high frequency properties of these films as they are presently
known are summarized.
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