Epitaxial HTS Research Articles

Novel Design of Band-Pass Waveguide Filter With HTS E-Plane Insert

In this paper, the idea of a band-pass filter (BPF) with HTS E-plane insert in the cross waveguide is implemented in Ka-band. The advantage of this approach is to solve the problem of mounting HTS insert into the body of the metal waveguide in order to ensure good electrical contact. A three-step method for the synthesis and design of such a filter is proposed and implemented. The method includes synthesis of the normalized prototype filter, sequentially iterative synthesis of filter geometry, and the final synthesis with optimization using numerical electrodynamic analysis of the entire geometry. It is shown that for the narrowband BPF, resonance windows in the E-plane insert should be performed with accuracy better than ±0.2 μm. The characteristics of such filters are expected better or not worse than their counterparts on the basis of rectangular waveguides, but at less stringent conditions of fastening HTS insert in the body of the waveguide. For fabricating the BPF, epitaxial HTS YBa2Cu3 O7-δ films sputtered on both sides of MgO single crystal dielectric substrate of 0.5 mm thickness were used. The difference between losses in the produced filter and its simulated sample is explained with insufficient accuracy of making resonance windows in the HTS insert.

On the mechanism of thickness dependence of the critical current density in HTS cuprate epitaxial films

The critical current density, Jc, in epitaxial HTS YBCO films strongly depends on the film thickness, degrading from 7-8 MA/cm2 in very thin films (50-70 nm) to less than 1 MA/cm2 in much thicker ones. Electron backscattering diffraction and high-resolution transmission electron microscopy reveal that the nanostructure of pulsed laser deposited epitaxial HTS YBCO films is evolving during growth due to high dislocation mobility. An analysis of existing pinning models leads to a conclusion that any other pinning centres but dislocations can not provide observed Jc. Extended linear defects provide a maximum elementary pinning force, if vortices are parallel to dislocation lines, while randomly distributed nanoparticles produce much lower pinning. Thus, the thickness dependence of Jc is determined mainly by the evolution of dislocation structure. Embedding of nanoparticles into HTS cuprates films results in Jc enhancement if extra dislocations are being formed during growth, preserving a high density of dislocations. Nanoparticles reduce the dislocation mobility, fix the dislocation cellular nanostructure and prevent its polygonization.

Thickness dependence mechanisms of the critical current density inhigh-Tc cuprate superconductor films

The obtained electron backscattering diffraction data and high-resolution transmissionelectron microscopy have shown that the nanostructure of epitaxial HTS YBCO filmsevolves essentially with the film thickness. This happens due to a high depositiontemperature and a high dislocation mobility that induce polygonization, dislocationrearrangement and a remarkable reduction of the out-of-plane dislocation density.The analysis of experimental thickness dependences of the critical current inthe framework of existing pinning models leads to the conclusion that thec-axis-correlated pinning by out-of-plane edge dislocations plays a dominant role. Thus, theevolution of the dislocation nanostructure is responsible for the critical current densityreduction. Embedding of nanoparticles into HTS films should be useful only if they arecoherently coupled with the matrix and extra dislocations are being formed duringthe film growth. The nanoparticles just preserve a high density of dislocations.Another effect is self-assembling of nanoparticles within dislocation cores forminga ‘bamboo structure’. This phenomenon may result in an enlargement of thedislocation normal core and an essential increase of the elementary pinning force.

Nature of magnetic field and angular dependencies of the critical current density in epitaxial HTS YBa2Cu3O7−δ films

Abstract Out-of-plane edge dislocations (mean 2D density is 10 11 lines/cm 2 ) in low angle tilt domain boundaries of single-crystalline YBCO films are shown to play a crucial role in J c ( H ∥ c , T ) behavior and J c angular dependence. Dislocation cores and their vicinity, where T c is locally suppressed, are shown to provide an extremely strong pinning force resulting in J c (77 K )≈2×10 6 A/cm 6 . Peculiarities of J c ( H ∥ c ) are found in the low-field range. A simple model of pinning by dislocation domain boundaries is developed. The model consistently describes the J c ( H ∥ c , T )/ J c (0, T )-dependencies, accounting for the fraction of vortices pinned by dislocations. Angular dependencies of J c in rotating magnetic field were measured by four-probe transport technique. A satisfactory agreement with theoretically predicted behavior has been found.

Czochralski growth of (La,Sr)(Al,Ta)O3 single crystal

Abstract Twin- and crack-free single crystals of (La,Sr)(Al,Ta)O3 with mixed-perovskite structure have been grown using the Czochralski method. These new crystals with a typical size of 55 mm in diameter and 50 mm in length are potential substrate candidates for growing large size and epitaxial HTS and GaN films. Their dielectric constant and dielectric loss at room temperature are 23 and 1×10−4, respectively.

Epitaxial thick film of YBCO by high temperature spray pyrolysis for coated conductors

YBCO thick films have been synthesized using a low cost spray pyrolysis method. We used an all nitrate precursor approach to grow epitaxial HTS oxide film on SrTiO 3 (0 0 1) substrate at a high deposition rate and at high temperature (over 800 °C). In these experiments thick YBCO films (μm) were grown in situ at 850 °C within 4 min; the final sintering process is performed at the same temperature for 2 h under oxygen atmosphere before low temperature oxygen loading. X-ray diffraction results ( θ–2 θ scans and ϕ scans) show that the films have not only a strong c-axis texture (mis-orientation ∼0.4°) but also a sharp in-plane biaxial orientation (mis-orientation is ∼1.4°). The critical temperature ( T c) was 91 K and the critical current density ( J c) higher than 1.4×10 6 A/cm 2 (at 77 K, 0 T), while film thickness is over 1 μm. Spray pyrolysis is a promising technique for coated conductors.

Magnetic field dependence of the critical current density in thin epitaxial HTS YBa 2Cu 3O 7− δ films with dislocation low-angle domain boundaries

Highly biaxially oriented YBa 2Cu 3O 7− δ (YBCO) films exhibit critical current densities, J c, two orders of magnitude higher than any sort of bulk samples. Out-of-plane edge dislocations (mean 2D density is 10 11 lines/cm 2 in the film area) in low-angle tilt domain boundaries are shown to play a crucial role in J c( H∥ c, T)-behavior. Measurements of the magnetic field dependence of J c for the pulse-laser deposited YBCO films are carried out by AC magnetic susceptibility technique. Dislocation cores are shown to provide an extremely strong pinning force leading to J c (77 K)≅2×10 6 A/cm 2. Peculiarities of J c( H∥ c) are found in the low-field range. A simple model of pinning by dislocation domain boundaries is developed. The model consistently describes the J c( H∥ c, T)/ J c(0, T)-dependencies, accounting for the fraction of vortices pinned by dislocations.

Fabrication of HgBa2CaCu2Oy grain boundary junctions using the cation exchange method

Grain boundary Josephson junctions have been fabricated using high-quality epitaxial Hg-1212 HTS films grown on bicrystal SrTiO3 substrates with a 24° misorientation angle. The films are prepared by the cation exchange method, with Tc between 120 K and 124 K. I-V characteristics of the junctions show typical RSJ behaviour. Critical current density obtained from junctions with different widths is uniform over a length scale of a few micrometres and is reproducible. The relatively high IcRn product at 77 K and 100 K and stability against thermal cycles show that the Hg-1212 junctions are promising for operations at temperatures higher than 77 K.

Non-linear microwave surface impedance of epitaxial HTS thin films in low DC magnetic fields

We have carried out non-linear microwave (8 GHz) surface impedance measurements of three YBaCuO thin films in dc magnetic fields $H_{dc}$ (parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero dc field the three films, deposited by the same method, show a spread of low-power residual surface resistance, $R_{res}$ and penetration depth, $\lambda$ (T=15 K) within a factor of 1.9. However, they exhibit dramatically different microwave field, $H_{rf}$ dependences of the surface resistance, $R_s$, but universal $X_s(H_{rf})$ dependence. Application of a dc field was found to affect not only absolute values of $R_s$ and $X_s$, but the functional dependences $R_s(H_{rf})$ and $X_s(H_{rf})$ as well. For some of the samples the dc field was found to decrease $R_s$ below its zero-field low-power value.

Epitaxial HTS thin films grown by PLD with a beam homogenizer

A beam homogenizer has been used in combination with a standard pulsed laser deposition (PLD) system. This combination reproducibly resulted in laser spots of precisely predetermined shapes and energy densities, thus providing control over one of the most difficult parameters in PLD. Thin films of YBa 2Cu 3O 7− δ (YBCO) were grown on LaAlO 3 substrates, and the new possibilities arising from the excellent control over the laser spot parameters were explored. Films with high transition temperatures ( T c≈90 K, T c≈0.5 K) and smooth surfaces were obtained. It has been shown that target wear is greatly reduced, resulting in significantly increased lifetime of the target and possibly reduced particle density on the films.

Nonlinear microwave properties of epitaxial HTS films

The surface resistance Rs of epitaxial HTS films of YBa2Cu3O7−δ on sapphire is studied experimentally at a frequency of 8.95 GHz in the presence of an additional powerful microwave signal of frequency 9.4 GHz ensuring the ac magnetic field amplitude up to 20 Oe on the film. It is found that the surface resistance Rs of the HTS film increases with the amplitude of the additional signal. The value of Rs of the HTS film virtually coincides with the value of the nonlinear surface resistance of the film at the same amplitude of the ac magnetic field. In other words, the surface resistances of the film for a weak signal in the presence of an additional powerful signal and for the powerful signal alone are close. The observed phenomena are explained under the assumption that the HTS film is a Josephson medium containing various types of Josephson junctions. Under the influence of a powerful microwave signal, the properties of this medium can vary due to switching of a part of the junctions, causing a transition of the medium to a new state which is manifested identically both for a weak and for a powerful signal.

Materials basis for a six level epitaxial HTS digital circuit process

We have developed a process for fabrication of HTS single-flux-quantum logic circuits based on edge SNS junctions which requires six epitaxial film layers and six mask levels. The process was successfully applied to fabrication of small-scale circuits (/spl les/10 junctions). This paper examines the materials properties affecting the reproducibility of YBCO-based SNS junctions, the low inductance provided by an integrated YBCO ground plane, and electrical isolation by SrTiO/sub 3/, or Sr/sub 2/AlTaO/sub 6/, ground-plane and junction insulator layers. Some of the critical processing parameters identified by electrical measurements, TEM, SEM, and AFM were control of second-phase precipitates in YBCO, oxygen diffusion, Ar ion-milling parameters, and preparation of surfaces for subsequent high-temperature depositions.

High-temperature superconductor antennas: Utilization of low rf losses and of nonlinear effects

The low radio frequency (r.f.) losses in epitaxial HTS thin films allow the realization of novel antenna structures which have to be excluded in conventional antenna techniques with normal conductors because of the highly reduced radiation efficiency. Thus, the design of miniaturized but nevertheless highly efficient antennas down to a lower limit determined by both the required order of radiation pattern and the frequency bandwidth becomes possible. For a bandwidth of more than about 1%, a considerable margin for a size reduction below the “critical size” is restricted to the case of electrically small antennas and of superdirective antennas with a relatively low order of the radiation pattern, e.g. antennas with a beam of less than 15 dB maximum gain. If the size approaches the lower limit, the antennas show a sharp bandpass frequency response. This is demonstrated by means of experimental results for a novel HTS meander antenna. These bandpass characteristics can be utilized in compact multiport antenna systems in order to decouple subantennas for adjacent frequency bands. Besides the low losses in HTS's, their nonlinear properties can be used in order to realize current-controlled HTS switches for antenna systems.