oResistivitySuperconductivity 93
UNIT
Superconductivity
6
To Temperature
are type I and tye »
Q. 1. Explain superconductivity. What of the electrons become super conducting as these have infinite colision times.
superconductors ?
much more than th. The electrons undergo no scattering and are responsible for super condutivity.
Ans. Superconductors are the materials which posses special condition The normal resistivity is recovered on heating above T.
infinite conductivity, i.e., perfect electrical conductors under
current can be induced in it . Superconductivity occurs in metallic elements (silver, lead etc.) of the
Ifa superconductor has the form of a ring, a temperatur periodic system and also in alloys and semiconductors. The range of transition
electromagneticinduction, cool the ring in a magnetic field from athe field. The temperature at present extends from 23.,2 Kfor the alloy Nb, Ge to 0.01K for
above the critical temperature T to below T, and then to remove some semiconductors.
Such currents
current continues to persist with undiminished strength for days. The super conductors can be classified into the following two categories -
are called Persistent Currents"
is 1. Type I (or Soft) super conductors.
When strong magnetic field is applied to a super conductor, there normal 2. Type II (or Hard) super conductors.
destruction of super conductivity, i.e., there is restoration of their
conducting state. 1. Type-I Superconductors : The superconductors, in which the
Let us denote the critical value of the magnetic field for the
destruction of magentic field is totally excluded from the interior of
superconductivity by H, This will be functionally related to temperature. below a certain magnetising field, Hand, at H, thesuperconductors
material loses
superconductivity and. the magnetic field penetrates fully are termed'
as type I or soft superconductors.
H,- H,1-} Examples : Tin, Lead, Aluminium,
The magnetization curve for type I Mercury, Zinc,
Magnesium etc.
super-conducting materials is shown
The highest
where H, =Critical field at 0°K. H, istypical for each material.
i.e., T = T when
in Figure.
critical temperature occurs when there is no magnetic field,superconductivity is
The important characteristics of type I superconductorsare :
H =0. The important factor causing the destruction of
the current iteself. Magnetisation
(M)
the magnetic field associated with the current than
increases
The effect is field-controlled. The entropy in all superconductor
to the normal state: The
as their specimens go from the superconductors state
normal state. The difference
super conducting state is more ordered than the
re-arrangement of the system on
in entropy is very small suggesting that the Supercond
-ucting state
Normal
becoming superconducting is relatively small. state
Applied field (H) H
The heat capacity in the superconducting state varies with the temperature
(i) They exhibit complete Meissner
in an exponential manner. effect.
(ü) The critical values of magnetic field H at
The superconductor critical temperature for various isotopes of a super are very low. The maximum known wvhich magnetisation drops
conductor is different. critical field for type I
superconductor is of the order of 0.1 T. The low
Thermal conductivity of an ideal super conductor drops considerably when these materials unsuitable for use in high fieldvalue of H makes
superconductivity sets in. magnets. superconducting
electrica
The superconducting state is marked by a sudden fall of the The magnetisation curve shows that
means that if the magnetic field is transition
at His reversible. This
resistivity of the material to near zero, when it is cooled to a sufficiently 10" reduced below H, the material
temperature. again acquires superconducting property and the field is
(iiü) Below H expelled.
As the temperature is lowered below the critical temperature (T), afractu C the material is
superconductor and above H it becomes a
(92)
SOTIO
94 Amar : B.Sc. (Prog.) II Year Physics (Semester-VII
conductor.
This type of super conductors cxist in two states: superconducting and
specimen, the
eiection
the flux as trapped fluX.
of Superconductivity
flux can be 95
normal. n a series of incomplete leaving acertain fraction of
2. Type II (or hard) Superconductors:The superconductors in
the material loses magnetisation gradually rather than suddenlywhich
experiments on superconducting cylinders, they
termed as type II or hard superconductors. Normal
sphere demonstrated
The magnetisation curve for these superconductors is shown. T>T Superconduct
sohere ing
The important characteristics are: Superconducting
Magnetisation
(M)
State
Type il that, as the
expelled, as temnperature
-Vortex
State
is
lowered to T, the flux is suddenly and
the specimen becomes
expulsion continues superconducting,
for allT<T. They as shown in Fig.completely
the magnetic field in the established this by carefully The flux
Normal state
demonstrated that the neighbourhood of the specimen. Furtur, measuring
Hc He
H from below T. the flux effect is reversible: When the temperature they
and the suddenly penetrates the specimen after it is raised
Applied Magnetic Field (H) substance in the normal
is reaches T
This effect shows that a bulk magnete
i) They do not show complete Meissner effect. super conductor behaves in an
external
(ü) These superconductors have two critical fields : H (the lower magnetic field Has if inside the specimen B = H+4 rM= 0 or 1
criticaB ffeld) and H, (the upper critical field). The specimen is x=
47 that is, a
diamagnetic below H, i.e., the magnetic field as is completely excluded superconductor exhibits perfect diamagnetism. The perfect diamagnetism of
below H,. At HC, the flux begins to penetrate the specimen, and the superconductors is an independent property not related to zero resistivity.
penetration of flux increases until the upper eritical field HC, is reached. Q.3. Discuss very briefly the applications of
At H the magnetisation vanishes and specimens returns to normal Ans. Application of Super Conductivity : The Superconductivity.
various
conducting state. The value of H, isgreater than H The value of super conductivity can be listed as follows of applications
critical field for type II materials be 100 times or more higher than the 1. Construction of superconducting electromagnets which carry
value of H, for type I superconductors Critical fields Hpo upto 30T resistance less currents and produce large magnetic fields.
large
have been observed. 2. Use of superconducting cables to transmit electric
power over long
The mnaterials which display type II behaviour are essentially distances without resistive losses by keeping the cable below the critical
inhomogeneous and include, Nb-Zr, Nb-Ti alloys and Va-Ga and Nb-Sn inter temperature of the material used.
metallic compounds. These are technically more useful than type 3. Superconductors are used for amplifying very small direct currents
superconductors due to tolerating high magnetic fields. and voltages.
This type of superconductorsexist in three states:superconducting, moxed 4. Superconductors are used in switching devices.
superconducting
and normal. The mixed state is resistance-less but unlike the 5. Being diamagnetic in nature, superconductors can be used to shield
state flux from an applied magnetic field penetrates through it. out unwanted magnetic flux. This is particlarly important in shaping
Q. 2. Explain Meissner Effect in case of super conductors. the magnetic lens system of an electron microscope.
Ans. The Meissner Effect : This was proposed and verified by Meissner Q. 4. Discuss very briefly the principle of Magnetic Levitation.
Ans. Principle of Magnetic Levitation : The diamagnetic property of
and Ochsenfeld in 1933. When material is
levitation.
a superconductor is the basis of magnetic
Whenever a long super conductor is cooled in alongitudnal magnetic held the magnetic flux lines will be expelled from the
superconducting,
material or
to below the value of eritical temperature corresponding to thatconductor t
field, then the material. For example, if a small light and
at
lines of magnetic field are pushed out of the body of the super complete onlyif the flux lines cannot penetrate the
superconducting material, the magnet will be
transition. Exclusion of flux from the bulk super conductor is Powerful magnet is kept over a
exist in the
the specimen is pure and strain - free. If impurities and stress
DTO
96 Amar : B.Sc. (Prog.) III Year Physics [Semester-VI
levitated (lifted up) and it willfloat in air. This is known
known as
It can be demonstrated using ahigh T, superconductor magnetic
levitation.,
like Y BBa,u,
taken in bulk and a light weight and powerful magnet Y,(Samarium
compoundItaken
cobalt magnet). The Y-123 compound is cooled by pouring liquid
it. The magnet kept over it just floats in air. In I Japan, nitrogenn
magnets have been used to levitate an experimental train above its
drive it at great speed. superconducting
rails and
