Types of Superconductors - Pedagogy Zone

Based on the value of superconducting transition temperature, superconductors are divided into two categories:

(i) Low temperature superconductors
(ii) High temperature superconductors

S.No	Low Temperature superconductor	High Temperature superconductor
1.	Superconductors having low superconducting transition temperature (<20 K).	Superconductors having high superconducting transition (>100 K).
2.	Due to the formation of superconductivity from holes, it is also called as n-type superconductors,	Due to the formation of superconductivity from cooper pairs or electron pairs, it is also called as p-type superconductors,
3.	It can explained only by BSC theory.	It obey RVB theory.
4.	It is not so useful due to maintaining of ultra low temperature.	It is very useful for commercial and other engineering application because of high transition temperature.

Depending on the magnetizing behaviour of superconductors in an external magnetic, superconductors can be classified as type I and type II superconductors.

Based on critical magnetic field, superconductors can be classified as types I and type II superconductors.

(i) Type – I Superconductor (or) Soft Superconductors

In type I Superconductor, the transition from superconducting state to normal state in the presence of magnetic field occurs sharply at the critical value of H_c as shown in fig.

Type I superconductors are perfectly diamagnetic below H_c and completely expel the magnetic field from the interior of the superconducting phase up to the critical field strength, the magnetization of the material groups in penetration to the external field and then abruptly drops to zero at the transition to the normal state as show in fig.

The critical field H_C is relatively low for type I superconductors.

The magnetic field can penetrate only the surface layer and current can flow only in this layer.

Consequently type I superconductor are poor carriers of electrical current.

They would generator magnetic fields of about 100 to 2000 G only. Hence they are not of much use in production of high magnetic field.

Type I superconductors are also called soft superconductors.

Aluminum, lead and Indium.

Characteristics of Type-I superconductors

They are perfectly diamagnetic.
They have only one critical field, at the critical field the magnetization drops to zero.
They produce low magnetic field.

Disadvantages

Type I superconductors cannot carry large current and hence are not of much use in producing high magnetic fields.

(ii) Type -II Superconductor (or) hard Superconductors

Type II superconductivity was discovered by Schubnikov in 1930s and was explained by Abrikosov in 1957.

The material which losses its superconducting property gradually due to the increase in magnetic field are called type II superconductors.

Type II superconductors are characterized by two critical field H_C1 and H_C2.

The transition from superconducting state to normal state occur gradually as the magnetic field is increased from H_c1 to H_c2 as shown in fig.

The magnetization of the material grows in proportion to the external field up to lower critical field H_c1.

The external magnetic flux is expelled from the interior of the material till then. At H_c1 the magnetic field lines begin to penetrate the material.

As the magnetic field increase further, the magnetic flux through the material increases.

At the upper critical field H_c2, the magnetization vanishes completed and the external field has completely penetrated and destroyed the superconductivity.

In the region between H_c1 and H_c2 the material is in a magnetical mixed state but electrically it is a superconductor. H_c2 can be as high as 20 to 50 Wb/m² and the retention of superconductivity in such high magnetic fields make type II materials very useful in applications of creating very high magnetic fields.

Examples: Transition metals and alloys consisting of niobium, silicon and vanadium.

Characteristics of Type-II superconductors

They have two magnetic fields.
The upper critical field is very high.

Difference between type I and II superconductor

S.No	Type I superconductor	Type II superconductor
1.	They are called as soft super conductors	They are called as hard super conductors.
2.	It has only one critical field.	It has two critical fields. i.e., lower critical field (H_c1) and upper critical field (H_C2)
3.	No mixed state exist.	Vortex (or) Mixed state is present.
4.	They strictly follow the Meissner effect.	They do not strictly follow the Meissner effect.
5.	They have limited technical applications due to lower magnetic field strength. (0.1 Tesla)	They have unlimited technical applications due to higher magnetic field strength. (i.e., order of 10 Tesla)
6.	Type I superconductors become normal conductor abruptly at critical magnetic field.	Type II superconductors losses its superconducting property gradually due to increase of magnetic field.
7.	Examples: Lead, Aluminium, Mercury, tin etc.,	Examples: Niobium, Vanadium, Certain copper oxide superconductors etc.,