History of superconductors

1911: K.Onnes finds that at 4.2K the resistance of mercury suddenly drops to zero. He called this effect superconductivity and the temperature at which this occurs, critical temperature Tc.

1933:

Walter Meissner and Robert Ochsenfeld discover that a superconducting material repels a magnetic field (Meissner effect).

Superconductivity:

When a sample of metal is cooled below a critical temperature, its resistivity totally vanishes and allow current to pass through it without any power loss. This phenomenon is called as superconductivity.

The temperature below which superconductivity is attained is known as the critical temperature (TC).

Superconductor:

Super conductors are the material having almost zero resistivity and behave as diamagnetic below the super conducting transition critical temperature.

General properties of superconductors

Virtually zero electrical resistance.

Super conductors are PERFECT conductors.

Perfect diamagnetic property (m=-1).

A superconductor excludes any magnetic field that comes near it.

Critical field depends upon temperature of superconducting material.

Temperature, T

Superconductor (e.g. Pb)

Tc

Normal metal(e.g. Ag)

residual

00

R e s i s t i v i t y

Meissner Effect:

The vanishing of resistivity is not only characteristics of superconductor. A superconductor cannot view simply as a substance that has infinite conductivity below critical temperature. If a superconductor cools below its critical temperature, it expels all the magnetic field lines from the sample by setting up surface current. This phenomenon is called the Meissner Effect.

Fig. 8.44: The Meissner effect. A superconductor cooled below itscritical temperature expels all magnetic field lines from the bulk bysetting up a surface current. A perfect conductor ( =) showsno Meissner effect.

B

I

T > Tc T < Tc

Perfect conductor

Superconductor

B off

T < Tc

B

I

Comparison between a perfect conductor and superconductor:

i) Superconductor:

If we place a superconductor in a magnetic field above TC the magnetic lines will penetrate the sample. However, when the superconductor is cooled below TC, it rejects all the magnetic flux in the sample. The superconductor developed a magnetization (M) by developing surface currents, such that M and applied electric field cancel everywhere in the sample. So, M is opposite direction to the H and equal magnitude. Thus below TC a superconductor is a perfect diamagnetic material (m=-1). (See in Fig 8.44)

If we switch of the field, the field around the superconductor simply disappears.

ii) Perfect Conductor:

If we place a perfect conductor in a magnetic field and cool it below TC, The magnetic field is not rejected.

If we switch off the field on the perfect conductor means decreasing the applied field. This change in the field induces a current in the perfect conductor by virtue of Faradays law of induction. These currents generate a field along the same direction as the applied field. As the current can be sustained (=0) without power loss, it keeps on flowing and maintaining the magnetic field. (See in Fig 8.44)

Critical Magnetic Field:

The superconductivity below the critical temperature has been observed to disappear in the presence of applied magnetic field exceeding a critical value. This critical value is called as critical magnetic field (BC).

Superconductivity can be destroyed by a critical external magnetic field Bc.

This critical field depends on the temperature and is a characteristic of the material.

= ()

As long as the applied magnetic field below BC at the temperature, the material is in superconducting state, but when the field exceeds BC, the material reverts to the normal state.

Types of Superconductor:

There are two types of superconductors, Type I and Type II, according to their behaviour in a magnetic field.

Type I Superconductor:

In type I superconductor, there is one critical magnetic field (BC). As long as the applied magnetic field below BC at the temperature, the material is in superconducting state, but when the field exceeds BC, the material reverts to the normal state.

So below BC it expels all the magnetic field lines from the interior of the sample and behaves as a perfect diamagnetic material.

0 2 4 6 8Temperature (K)

Lead

Tin

0

0.02

0.04

0.06

0.08

Mercury

B c (

T)

Fig. 8.47: The critical field vs temperature in three examples of Type Isuperconductors.

0 2 4 6 8 1 00

0 .1

L e ad

S u p e rco n d u c tin gsta te

N o rm al s ta te

T c

T em p e ra tu re (K )

Fig. 8.46: The critical field vs temperature in Type I superconductors.

Bc ( T e s l a )

Type II Superconductor:

Type II superconductors have two critical fields BC1 and BC2. Below lower critical field (BC1), it behaves as a perfect diamagnetic material and in

superconducting state. When the applied field is between BC1 and BC2, it is in mixed or vortex state. This state is

mixture of normal and superconducting state. Above upper critical field (BC2), It goes to normal state. So, the type II superconductors have much higher critical magnetic fields than Type I, but

for most of that field range they are mixtures of normal and superconducting state.

Tc0

Normal stateBc2

Bc1

Vortex state

Meisner stateC r i t i c a l m a g n e t i c f i e l d

Fig. 8.50: Temperature dependence of Bc1 and Bc2.

Bc1

m = 1

0

o M

Mixed state

Bc2B = o HB = o H

Bc

m = 1

o MType I

0

Fig. 8.48: Characteristics of Type I and Type II superconductors. B= o H is the applied field and M is the overall magnetization ofthe sample. Field inside the sample would be Binside = o H + o M,which is zero only for B < B (I) and B < B (II).