Physica C 235-240 (1994)3371-3372 PllI$1gA North-Holland High-To Superconducting Thin Film Resonators with Buffer Layers Farhat Abbas 1, L.E.Davis 1, J.C.Gallop2 tDept, of Electrical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, UIC 2National Physical Laboratory, Teddington TWl 1 0LW, UK We show that the addition of buffer layers to HTS planar resonator structures may be used to improve Q-factor as well as solving superconductor/substrate incompatibilities.. 1. INTRODUCTION 2. THEORY In addition to their potential for applications, microwave resonators made from high quality two- sided YBCO thin films on sapphire with CeO buffer layers may provide experimental data to elucidate appropriate theoretical models for high temperature superconductivity. Here we use London's electrodynamic equations to calculate a sinusoidal solution for a planar superconducting transmission line, including the effects of buffer layers. The solution provides expressions for the phase velocity and attenuation coefficient for a range of combinations of materials properties. The Q factor of a resonator constructed from such a transmission line is also calculated and some specific cases are considered which demonstrate how the use of low- loss buffer layers may enhance the Q factr.. Such resonators may find practical application as the stabilising element in low phase noise microwave oscillators. Superconductor Dielectric I (Buffer Layer) I)iclcclrit ~ I),t!lcclri~: _.2 (~tl|)~tr ale ~ Fig.1 Schematic of resonator using HTS thin films and buffer layers. The structure of the resonator, shown in fig.l, consists of a pair of thin buffer layers (dielectric 1) separated by a central substmte (dielectric 2) and a pair of superconducting thin films separated by the buffer layers from the substrate. Thicknesses of the thin films, buffer Myers and central substrate are 1, d~ and d2 respectively. The dielectric regions 3 are considered to be very thick so that the fields in these regions can be assumed to decay exponentially away from the interface. The dispersion relation for the resonator of fig.1 can be written as follows [3]: a2= c°2P'oe~e2 [2~,coth(/)÷2dt.hd2] (2dlz2+d2sl) ;Z Here a is the propagation constant along the z direction (taking em), 0~ is the angular frequency (assuming ei°~) e0 and Ix 0 are the permittivity and permeability of free space respectively, Et~ is the dielectric constant of dielectrics 1 and 2 respectively, and c are the penetration depth and normal state conductivity of the superconductors. The total unloaded Q-factor Qo of the resonator can be writtten in terms of Q-factors arising from losses in conductors, buffer layers, dielectric substrate and radiation: Qo, Qb, Qd & Q~ respectively, as follows: 1 1 1 1 1 ~_ .... _ +.... _.. + ÷ Qo Qc Qb Qd Qr where Qo = o~/2aov s, Qb = co/2o~vg, Qd = ¢o/2aavs, vg---v22/vpand v2 ffi Re(e21Xo) "tt2 where Vp is the phase velocity in the corresponding material (for details see [3]). Q, has been discussed elsewhere [4]. 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)02251-8