Farhat Abbas

Tunable microwave components based on dielectric nonlinearity by using HTS/ferroelectric thin films

This study provides the possibility of developing tunable microwave components based on the dielectric substrate nonlinearity, with the conducting surfaces made of a superconductor. The displacement vector, the dipole moment, polarization, polarizability, susceptibility, and relative permittivity concepts are used for ferroelectrics; and for superconductors, Bose statistics and the Gorter and the Casimir model for a two-fluid model, Londons equations, and the classical skin effect for the normal component of the current are used. A sinusoidal wave solution is found for a planar superconducting transmission line. This solution gives expressions for the phase velocity and attenuation coefficient which are used to characterize the tunability of microwave components. The measured data in the literature have been used to compute the relative phase velocities and phase shift per cm versus temperature and the dc bias electric field E (kV/cm). It is shown that with a ferroelectric film of thickness of 140 nm, with /spl epsiv//sub /spl tau//=2/spl times/10/sup 3/ and tan /spl delta/=10/sup -3/ phase shifts and attenuation of the order of tens of degrees per centimeter and 5.76/spl times/10/sup -3/ dB/cm, respectively, at 10 GHz, can be obtained with tens of millivolts at 4 K.

Field solution for a thin-film superconducting parallel-plate transmission line

Abstract By using Bose statistics and the Gorter and Casimir model for a two-fluid model, Londons equations, and the classical skin effect for the normal component of the current, a sinusoidal wave solution is found for a superconducting transmission line. This solution gives expressions for the phase velocity and attenuation coefficient which are used to compare the behaviour of a wave in LTSs and HTSs and to investigate the value of the Q -factor of a parallel-plate resonator. The computed values of the Q -factor are compared with measured data in the literature.

Dielectric loaded HTS resonators as frequency standards and low phase noise oscillators

High-quality, high-temperature superconducting (HTS) thin-films exhibit very low losses at microwave frequencies and, as a result, allow very high Q resonators to be produced. The size of such resonators may be significantly reduced by loading with low-loss single crystal dielectric. The potential for HTS dielectric loaded resonators as practical frequency standards and reference elements for low-phase-noise oscillators is assessed, with emphasis on operation at 60 K, a temperature readily attainable with compact Stirling cycle coolers.

Propagation coefficient in a superconducting asymmetrical parallel‐plate transmission line with a buffer layer

A sinusoidal wave solution is found for a superconducting asymmetrical parallel‐plate transmission line with a buffer layer, by using Maxwell’s and London’s equations, the two‐fluid model, and the classical skin effect for the normal component of the current. Expressions for phase velocity and attenuation coefficient of the wave are derived. Values for the attenuation coefficient are computed for various combinations of material properties, and losses in the buffer layer are discussed.

The effect of buffer layers on slow wave propagation in superconducting planar transmission lines

Slow wave propagation in superconducting planar transmission lines is dependent on the materials properties and thicknesses of superconductors, dielectric (substrate) and the buffer layers. The wave velocity ratio (with respect to free space) is computed for various combinations of material properties. The dependence of the relative wave velocity on the dielectric constant and thicknesses of the buffer layers is discussed.

Ultra-high-Q resonators for low-noise, microwave signal generation using sapphire buffer layers and superconducting thin films

An analysis has been completed to provide guidelines for producing planar microwave resonators with Q-factors of the order of 106-108 at a temperature of 4 K and a frequency of 1-20 GHz using high-Tc superconducting thin films on sapphire shielded by suitable buffer layers. Buffer layers not only overcome the problems of film-substrate interactions and reduction of microcracks, but can also confine the field into the sapphire which enhances the Qr and Qc of the resonator.

Propagation in a multilayer structure of superconductors and dielectrics

Abstract A numerical solution is obtained for a sinusoidal wave propagating along a multilayered structure of high- T c superconductors and dielectrics. Attenuation coefficients and phase velocities are computed and for certain modes the attenuation coefficients are relatively insensitive to losses in the dielectrics and the outer superconductors.

A distributed ferroelectric superconducting transmission-line phase shifter

A numerical solution is obtained for a sinusoidal wave propagating along a multilayered structure of superconductors, ferroelectric and dielectrics. An example of a distributed coupled strip-transmission-line phase shifter has been investigated, and such phase shifters may find applications in low-loss tunable microwave components for satellite and ground-based communications.

Microwave Q-factor of a dielectric resonator sandwiched between two thin films of superconductor

This analysis provides a comparison between the computed and the measured Q-factors of a microwave resonator consisting of a dielectic resonator sandwiched between two thin films of superconductor using the Bose statistics and the Gorter and Casimir model for a two-fluid model. Such comparisons are helpful not only to design ultrahigh-Q resonators but also in considering the theory of thin films of HTS (high-temperature superconductors).

Radiation Q factor of high-T c superconducting parallel-plate resonators

Abstract A parallel-plate resonator may radiate spherical waves and/or cylindrical waves, or in some intermediate manner. Expressions are derived for the radiation Q-factor of a high-T c superconducting parallel-plate half-wavelength open-ended resonator as a function of frequency. Values of Q r are computed for the cases of spherical and cylindrical radiation. Such estimates are required in the design of ultra-high Q-factor resonators, together with data on other losses.