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 S uper conductivity 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.
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.
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