MICROWAVE SUPERCONDUCTIVITY

Superconductivity is a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect). It was discovered by Heike Kamerlingh Onnes in 1911. Applying the principle of Superconductivity in microwave and millimeter-wave (mm-wave) regions, components with superior performance can be fabricated. Major problem during the earlier days was the that the cryogenic burden has been perceived as too great compared to the performance advantage that could be realized. There were very specialized applications, such as low-noise microwave and mm-wave mixers and detectors, for the highly demanding radio astronomy applications where the performance gained was worth the effort and complexity. With the discovery of high temperature superconductors like copper oxide, rapid progress was made in the field of microwave superconductivity.

This topic describes the properties of superconductivity that can be exploited in microwave and mm-wave technologies to yield components with appreciable performance enhancement over conventional systems. Superconducting signal transmission lines can yield low attenuation, zero-dispersion signal transmission behavior for signals with frequency components less than about one tenth the superconducting energy gap. No other known microwave device technology can provide a similar behavior. Superconductors have also been used to make high speed digital circuits, josephsons junction and RF and microwave filters for mobile phone base stations.

Superconductivity

Superconductivity is a phenomenon occurring in certain materials generally at very low temperatures characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect). It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetism and atomic spectral lines superconductivity is a quantum mechanical phenomenon. The electrical resistivity of a metallic conductor decreases gradually as the temperature is lowered. The temperature at which the transition to superconducting state occurs is known as the critical temperature.

However, in ordinary conductors such as copper and silver, impurities and other defects impose a lower limit. Even near absolute zero a real sample of copper shows a non-zero resistance. The resistance of a superconductor, despite these imperfections, drops abruptly to zero when the material is cooled below its critical temperature. Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors. The common examples are niobium with Tc=9.2K. however superconductivity doesnot occur noble metals like gold ,silver etc and pure samples of ferromagnetic materials.The major properties shown by the super conductors are zero electrical resistance and meissner effect.

Microwave superconductivity

According to BCS theory cooper pairs are formed during superconducting state and it is having energy slightly less than the normal electrons.so there exist a  superconducting energy gap between normal electrons and cooper pairs. The band gap ‘E’ related to transition temperature by relation,

                    E (at t=0K) =3.52*Kb*Tc

Where    Kb – Boltzman’s constant

              Tc – Critical temperature and

              3.52 is a constant for ideal superconductor and may vary from 3.2 to 3.6 for most superconductors.

            

If a microwave or a millimeter wave photon with energy greater than superconducting energy gap incident on a sample and is absorbed by the cooper pair, it will be broken with two normal electron created above the energy gap and zero resistance property is lost by material. This property is shown in fig below. For ideal with a transition temperature of Tc = 1K, the frequency of the mm wave photon with energy equal to superconducting energy gap at T=0K would be about 73GHz. For practical superconductors the photon energy corresponding to energy gap would scale with Tc. For niobium (Tc=9.2K) the most common material in LTS devices and circuits, the frequency of radiation corresponding to energy gap is about 670GHz.

The zero resistance property of the superconductor is true for dc (f=0). For finite frequencies there are finite but usually very small electrical losses. The origin of these losses at non zero frequency is due to the presence of two type of charge carriers in the superconductor. Although cooper pairs move without resistance, the carriers in normal state, those above energy gap behave as electrons in normal conductor. As long as the operating frequency is below energy gap the equivalent circuit for the superconductor is simply the parallel combination of resistor and inductor, where resistor indicate normal electrons and inductor the cooper pairs. These two carriers contribute separately to the screening of fields. The characteristic decay length of fields into a super conductor as determined by cooper pair current is superconducting penetration depth.  The penetration depth get larger with increased temperature but only slightly close to Tc.

As operating temperature closer to Tc the band gap  also reduces. Hence the superconductor will be more sensitive to temperature variations. So inorder to avoid this operating temperature must be less than 2/3 of Tc. Thus for high frequency application of superconducting materials the operating frequency of device should be 10% or less of the frequency corresponding to energy gap of the material and temperature must be less than 2/3Tc. For example in case of niobium (Tc=9.2K) the band gap is about 670 GHz and can be operated at a maximum frequency of 70GHz and temperature below 6K.

Conclusion

Superconductivity is one of the most exotic phenomena observed in nature and it can have an impressive impact on the performance of passive and active devices operating throughout the microwave and mm wave region of the spectrum. In these frequency ranges,

The electrical losses are superconductors are significantly less than the losses for normal conducting metallization in device and component applications.

Active superconducting Josephson device technology is inherently low loss and has demonstrated operation in excess of 700 GHz.

There has been much progress in the recent years to exploit superconductivity in selected microwave and mm-wave system. The HTS filters having low loss, sharp roll off have been designed for wireless communication systems to filter out of band interference and reduce noise. HTS found application in high energy particle accelerators, ADC’s etc. In addition to advances in superconducting technology there have been gains in cryogenic refrigeration community that can provide energy efficient, reliable cryogenic refrigeration systems.