What are Super Conductors?

Certain Metals when cooled, their electrical resistance decreases in usual way as in normal conductors, but on reaching a temperature a few degrees above absolute zero they suddenly lose all trace of electrical resistance. Then they are said to have passed into superconducting state.

Kamarlingh Onne was the first to observe this peculiar property in case of Mercury in 1911. In the course of his investigation of electrical resistance of a sample of Mercury dropped from 0.08Ω at about 4K to less than 3x10-6Ω over a temperature interval of 0.01K.

Super Conductors are extra ordinary because they take no energy at all to make current flow in a conductor and no energy is lost to friction to sustain the current either. Its electrical resistance is precisely zero.

Critical or Transition Temperature

One of the important characteristic property of super conductors is that their electrical resistance, for all practical purposes is zero below a well defined temperature T_c, called Critical or Transition Temperature. For instance, semiconducting mixed oxides of Barium, Lead and Bismuth.

Important Properties of Super Conductors

1. The current in super conductors persists for a very long time.

2. The magnetic field does not penetrate into the body of super conductor. This property known as Meissner Effect is fundamental characterization of super conductivity.

3. When the applied magnetic field ‘B’ is greater than Critical value B_c(T), the super conductor becomes a normal conductor.  B_c(T) is zero at T=Tc and has the largest value at T=0.

4. When the current through Super Conductor is increased beyond a Critical value I_c(T), Super Conductor becomes a normal conductor.

5. Specific heat of the Super Conducting materials shows an abrupt change at T=T_c jumping to a larger value for T<T_c.  

Effect of Magnetic Field on Super Conductors

   The super conducting state of metal exists only in a particular range of temperature and field strength. Super conductivity state will disappear if the temperature of specimen is raised above its critical temperature Tc or if a sufficient strong magnetic field is employed.

The critical field for which super conducting property loses is a function of temperature.

Hc =H0[1-(T/Tc)^2]

Where

Hc is Maximum Critical Field Strength at temperature T

H0 is Maximum Critical Field Strength at temperature absolute zero.

Tc is Critical Temperature, the highest temperature for super conductivity

The Meissner Effect
 

    Superconductors which are resistance less materials have an additional property of exclusion of applied magnetic field on it i.e. inside a superconducting material, we always have B=0.





 

    The property of perfect diamagnetism arises in super conductor because when a magnetic field ‘Ba’ is applied surface screening currents circulate so as to produce a flux density ‘Bi’ which every where inside the metal exactly cancels the flux density due to applied field Bi=-Ba.

For a Super Conductor μ_r=0; i.e. B=μ_rB_a=0

This property of exhibiting perfect diamagnetism by super conductor is known as Meissner Effect.

 

PAULI's NEUTRINO HYPOTHESIS

Pauli postulated the existence of new particle, “Neutrino” as early as in 1930. According to Pauli, an additional particle called a neutrino denoted by ‘ν’ is emitted in process of β-decay.

This particle according to Pauli, carries away an amount of energy equal to difference between the observed energy for a β-particle and maximum energy of continuous beta spectrum. The principle of conservation of energy is thus satisfied.

To satisfy Principle of Conservation of Angular Momentum, Neutrino must be assigned following properties

i) It must have zero charge; because in a β-decay process the charge is conserved without neutrino. Also if Neutrino is charged, it would produce ionization which certainly could have been detected. Zero charge in turn implies negligible magnetic moment.

ii) It must have zero or almost zero mass: the mass-energy balance of β-decay processes shows that neutrino rest mass is negligible.

iii) It must have a spin of ½: This will satisfy Law of Conservation of Angular Momentum in β-decay process. Further Neutrino must be a Fermion, so that nuclear statistical requirements are fulfilled.

iv) A neutrino has an antiparticle called anti-neutrino which has zero rest mass, zero charge and spin-1/2.