Erland Wikborg

HTS/ferroelectric devices for microwave applications

High Temperature Superconducting (HTS, e.g. YBCO) microwave devices based on bulk or thin film ferroelectrics (e.g. Strontium Titanate-STO) are studied theoretically and experimentally. YBCO/STO/YBCO parallel-plate resonators and based on bulk single crystal STO are for electrically tunable high power filters, phase shifters and other devices in the frequency band 0.5-2.0 GHz of advanced microwave communication systems. Thin film YBCO/STO Inter-digital Capacitors (IDC), Coplanar Waveguides (CPW) phase shifters/delay lines are also designed and studied experimentally for low microwave power applications. Modelling problems of these devices and microwave losses in ferroelectrics are also discussed.

Tunable microwave devices based on bulk and thin film ferroelectrics

Electrically tunable microwave resonators and filters based on bulk single crystal SrTiO3 and KTaO3 with superconducting plates are presented and discussed. Filters are designed for operation at frequencies 0.2 – 2.0 GHz. Thin film varactors and phase shifters, designed for cryogenic or room temperature operation and based on epitaxial SrTiO3 and BaSrTiO3 are also discussed. The frequency band for the thin film devices is 0.5 - 40 GHz.

Encoders and decimation filters for superconductor oversampling ADCs

Modulators of superconductor ADCs can operate at very high sampling rates, ranging from 20 GHz (todays technologies) to 400 GHz (prospective sub-micron technologies). Such devices are too fast and therefore impractical without other superconductor devices capable of processing the data streams at such rates. We discuss various structures of such devices, in particular demultiplexers and decimation filters. We also analyze possible hybrid devices, in which only the front stages contain superconductor components. Encoders and decimation comb filters for single-comparator and time-interleaved modulators were designed, fabricated (at HYPRES, Inc.) and successfully tested. The most recent designs of the decimation filters have a unique modular structure, which allows an assembly of various comb filters handling clock frequencies of about 20 GHz (for a 1 kA/cm/sup 2/ fabrication technology).

Voltage divider based on submicron slits in a high Tc superconducting film and two bicrystal grain boundaries

Experiments on a model of rapid single flux quantum (RSFQ) flip‐flop cell, based on high‐Tc (HTS) Josephson junctions show that it can operate as a voltage divider at frequency up to 400 GHz. The junctions were formed in YBaCuO film, deposited on novel Y–ZrO2 bicrystals with two asymmetric 32° grain boundaries, about 10 μm apart, and allow a new design of RSFQ logic based on a single HTS layer. Small inductances (≂10 pH) were made as narrow, submicron size slits. The junction widths were between 4 and 10 μm and for ten junctions located close to the tested circuits, the linear critical current densities at T=4.4 K were 10.7 μA/μm±50% for one grain boundary and 8.3 μA/μm±50% for the other one. IcRn was about 1 mV±50%. A current density of half the expected value meant that the test circuit did not act as an ideal flip–flop down to the lowest frequency. As a voltage divider it gave a half value division up to 0.82 mV at T=4.4 K and to 0.4 mV at 30 K.

RSFQ front-end for a software radio receiver

We present results of a preliminary analysis of possible front-ends of a software defined radio base station receiver containing superconductor Rapid Single-Flux-Quantum (RSFQ) components. The main component of such a front-end, an oversampling analog-to-digital converter (ADC) with expected internal clock rate up to 20 GHz has already been developed and fabricated with commercially available 3.5-/spl mu/m, 1-kA/cm/sup 2/ niobium-trilayer technology. Simulations show that, even with such crude feature size, the superconductor ADC components can outperform their semiconductor counterparts.

Tailoring the temperature coefficient of capacitance in ferroelectric varactors

Two BaxSr1-xO3, films with different contents of Ba (Ba0.25Sr0.75O3 and Ba0.75Sr0.5O3), separated by a MgO film, are used to fabricate planar varactors with low temperature coefficient of capacitance (TCC), high tunability and low losses. The TCC of the varactors are small in the temperature interval between the dielectric permittivity peaks of the Ba0.25Sr0.75O3 and Ba0.75Sr0.5O3 films

Lower order modes of YBCO/STO/YBCO circular disk resonators

Lower order modes in a single crystal strontium titanate (STO) circular disk resonator are studied experimentally. Superconducting epitaxial YBCO films form the parallel-plates of the resonator. Due to the extremely high dielectric constant of STO, the electric fields are concentrated between the plates, while there is a substantial magnetic fringing field which affects both the resonant frequencies, Q-factors, and tunability of all modes, especially the TM/sub 110/ and TM/sub 210/.

Novel design of rapid single flux quantum logic based on a single layer of a high‐Tc superconductor

We suggest a new design of rapid single flux quantum (RSFQ) logic circuits which is based on a single superconducting layer and does not require a superconducting ground plane. Small inductances of about 10 pH, obligatory for RSFQ circuits, are formed as narrow slits of widths comparable to the London penetration depth (≂0.15 μm). The design allows us to decrease the geometric size of the RSFQ cell. Test circuits with YBaCuO grain boundary junctions on asymmetric 32° Y– ZrO2 bicrystals were used to measure the slit inductance per unit length and the mutual inductance of neighboring slits. A typical inductance of a 0.4 μm slit was found to be 0.7–0.8 pH/μm.

A new design approach for High-T/sub c/ based RSFQ logic

We suggest a new design of Rapid Single Flux Quantum (RSFQ) logic circuits which is based on a single superconducting layer and does not require a superconducting ground plane. The small inductances of about 10 pH, that are obligatory for the RSFQ applications, are formed as narrow slits with width comparable to the London penetration depth (/spl cong/0.15 /spl mu/m). The design allows us to decrease the geometric size of the RSFQ cell and can be applied to low-T/sub c/ circuits as well. Test circuits have been implemented using YBaCuO grain boundary junctions on assymmetric 32/spl deg/Y-ZrO/sub 2/ bi-crystals to measure the slit inductance per unit length and the mutual inductance of neighboring slits. A typical inductance of a 0.4 pm slit was determined to be 0.7pH//spl mu/m. We present a new design and a computer simulation of a flip-flop circuit based on these inductance measurements. To realize these circuits experimentally one needs at least two grain boundaries, separated by a distance of 10 to l5 /spl mu/m, or to use bi-epitaxial or step-edge junction technology.<<ETX>>

Electric field dependent microwave losses in KTaO3 single crystal with YBa2Cu3O7-x electrodes

We have measured the voltage dependence of the resonant frequency and the microwave losses for parallel-plate resonators based on single crystals KTaO3, with YBa2Cu3O7-x electrodes. The losses increase and the dielectric constant decrease monotonically with applied voltage. Both the dielectric constant and the losses show a hysteretic effect in the voltage dependence. The hysteretic effects may be explained by electric field assisted charge trapping both in the KTaO3, crystal and at the KTaO3/YBa2Cu3O7-x interfaces