CHAPT~RIV

For CheS~PERCONDUCTIVITV. . . -
( m1stry .& Mom
- e.Sc1ence
. Mains)

The resistivity of a typical metalli . .

However, the resistivity does not fali co nd uctor·decreases as the temperature decreases.
The resistivity of a conducto . . . o zero, even as the temperature approaches O K.
they move through the la~:~~~=es ~r?m comsio~s made by ~e·conducting ~electrons
35
lectrons to have collisfo d ?~nties .and lattice defects mcrease the chaµces for
e . . b .b . alns, colhsions of electrons with atoms displaced from their
lattice s11es y. v1 rauon. mouon contribute to the rests . t·1v1ty.
. .
In cert:am
.h matenals,
· the. res·istance 1 alls gradually with . . .
decreasmg temperature, as ··
.expected, · ow.ever, at a cert:am temperature, the crztlca
. . 1temperature Tc, the resistivity drops
suddenly
. to zero.
. Such
, . substances are called superconductors. ii) Below Tc, electrons move
W!thout any hindrance through the material. · -·
The critical temperature is different for differ- ordinary metal
ent materials. For mercury Tc = 4.2 K, for lead
Tc = 7.25 K, for niobium Tc = 9.2 K and for
ceramic semiconduct~r Tl2Ba2Ca2C'ii3 ci 10 , 'Tc =
125 K. .
~uperconductivity has been observed in 27 el- T(K)-
ements and in numerous compounds, but it bas not
Fig. 1
been observed for the best metallic conductors like
. - . .

(superconductivity was first observed by Kammerlingh Onnes in the year 191 l in mer-
cury) He fomkl ~at the electtjcal re~~ance of mercury dro ed _bru12tly to zero-at about
-1.K.._and below that temperature mercury exhi~ited no resistance. This new state with hith-
erto unknown electrical properties was named superconductivi~ In 1913 lead was fo~nd.
to e~ bilsuperconductivity below 7.2 K.1t took 17 more years to find Niobium wh.tch be-
came superconductor below 9.2 K.([ was subsequently discovered that superconductivity
could be destroyed and resistance restored by the aQplication of sufficiently strong magnetic.
fie19 · · ·

Properties of superconductors /
1. Zero resistance
§t~n cooled sufficiently, the electrical res~stance o~ ~ .aterials disappear~ c_o~plete~y
at a certain temperature called the ,,superconductmg transition temperature_or cntical tem-
T--:}
J)erature Below this temperatur~ there is no voltage drop for a current through the
conductor. fn other words, no power 1s generated on passage of a current.

2. Meissener-Oschsenfeld effect --./

a
@_n cooling a superconductor ·below its criti~al temperature in· weak magnetic fi<~ld,
the flux lines are expelled out of the material. This ~eans superconquctor exhibits perfect

231
 232 SUPERCONDUCTIVITY

diamagnetism below Tc. The flux inside a superconductor is zero irres e · oolin i
the presence or absence of~agnetic field. ts phenomenon was discovered by Meiss_en
and Oschsenfeld in 1933 and hence the namg
. @hen a sufficiently strong magnetic field ·
is applied to a superconductor in the diamag-
netic state, the flux is found ' to penetrate at a
certain critical field B = Be destroying super-
,5onductivit~ Such superconductors are called
_!ype I superconductors. However in other ma-
terials, including the recen~ly discovered high
Tc ceramic superconductors, perfect diamag-
-netism is exhibited up to a lower critical field
T:>TC T<Tc
Bc1. Above Bc1 flux penetration takes place,
but the specimen partly remains superconduct- ·Fig. 2
ing. It exists in a mixed state in which super-.
conducting regions are enclosed by normal regions in tlie form of_ ".~rtices upto a field Bc2
ca.J.ied the upper critical field. In this mixed state the material has still zero resistanc~. Above
Bc2 the material returns to the normal ·state. 'Superconductivity of this type is called Type II
super:conductivi_o/ . ..
3. Penetration depth and coherence length · · · ··
Superconductors are characterised by penetration depth ). c·oherence length ~d the e
dim~n~io~ess ·paramete~ l( ~ --X/e. The depth to which magnetic filed can penetrate into
01~~ a· superconductor is called penetration depth. The electron w~ve fuQct:ions. in _supercQ!.1- .
___ductor are coherent over macroscopic distances called cohe~)\ce !ength. In type I super-.
e
conductors < .,x and in type fl;~ ::> .X. ~ pically K > 1/2 for typel' and·K < 1/2 for
type II.
ntical field( Be_) .. . .· . ,
7
. (lhe ~ _nimum applied magnetic field to des~roy :superconductivity and to restore the
no~al resistivity is called the critical .fi~ld BVBc ·dx~hcts·; on.the .temperature: ·1t-can be
shown that · ·
2
Be =·Bqli _: (T/Tc) ];
'
where 130 is the_critical field a_t ab~olute zero.
~ritical curren~ 0· ..
· The minimum current that can be pas~ed through a superconductor without destroying_
its superconductivity is called critical'currenfic . .

~ top~c 1effect . . . · . ·
n );las been found by early experiments that.__,the critic.al temperature is strongly depen-
2. dent on the average isotopic mass M of the constituents of a superconductor.
Tc ex M -:: I /2
More recent_eiperimen~ have s_uggested the following general form
Tc ex M- 0 ;
sUPERCONDUCTMTY

-
where a is called the isotope effect coefficient. It is defined by

'Iype I and type II superconductors

~ed on critical magnetic fi~ld, superconductors are classified as (i) Type I (soft) and
(ii) r~ II (hard) Superconducto.Ii) B ()J).Q__d_ o--n <!ha u-i ; ric.J •rtrO-{\I

. G':) ~ ~ ..i
(i) 'fype I superconductor ([, \ ~~ fr~
0 superconductor which ~xhibits com- ·
plete Meiss~ner ~ffect is called Type!) That
t .conducting
Super~ : Normal
, state
Bb +'- state ---+ ' /

is, the specimen 1s a normal conductor above

the critical magnetic field Be; the specim~n be-
-;omes a diamagnetic material below the critical
Ba
~.@ese superconductors are also called soft
superconductor?)(ine value of Be is always too Fig. 3
low for these material_V Eg: Al, Zn, G~ .
. The Fig: 3 shows the relation between Ute magnetic fi,eld Bb produced by the induced
superconducting currents and applied external magnetic field Ba, i.e., the magnetisation
curve for Type I semiconductor.

(ii) Type II $.Uperconductor

(jhe Type II superconductor is a super-

conductor in which the magnetic flux starts t
Bb
''
, Mixed : Norma(
Super~ :+'- state -..+:~ state
'

to pt:netrate the se_'ecimen at a field Bc1 conducang: :

+- state ·-+: .... ,:
which is lower tha~he critical field Jf;)__ The ., ..... :
specimen is in a mixed state between . Be1 ,.''
and Be2- ft has superconducting electrical '
p~rties upto Be2- Above Bc.2 the speci-
men is a normal conductor.
Be, Be
Fig. 4 is the· magnetisation curve for
Type II superconductor. · Fig. 4
The Type_II semiconductor i called hard semiconductor which has a large amount of
~ netic hysteresis induced by mechanical treatme~t. Sole1_101 wouna 'With wir~ of a hard
superconductor can produce magnetic field over 10 T. .
. Incomplete Meissener effect occurs. in the region bet~een Bc1 &nd Be2 • This region
is Called, vortex region. It exists _in a mi:ted state in which superconducting regions are
enclosed by normal regions·in the form of vortices upto Be2 - this mixed state the material
has zero resistan~ ove Be2 the spec~en return~ to the no~al ~tat~
(Type II superconductors can carry ~ b SY.Qer current den_s1t1es m high magnetic fields. ·
They are of great commercial importance;.) ' · •
Eg: Zr, Nb, 60% Nb - 40'!° Ti alloy.
 High Tc superconductors (High Temp Ceramic Super
In 1986 Bednorz and Muller discovered superconductivity above 20 Kin La-Ba-Cu-
O system. The superconducting phase was soon identified. as _La2~xBaxCu04. A full
description of the crystal structure is given by Pickett. Studies m the effect o! eleme~tal
substitution as well as in different processing techniques led to a host of cerarmc matenals
of similar structure and properties.
. In e~ly 1987 scientists at the University of Tokyo and at Bell Communi~ation Group
found that substitution of Sr and Ba raised _Tc to about 40 K. These matenals have the
pervoskite AB03 structure. Although the exact mechanism was not known, it was apparent
that the CuO planes forming part of the crystal lattice played the crucial role.
In 1987 itself Paul Chu and other workers observed superconductivity above 70 Kin
YBaCuO system. The result was immediately reproduced and the phase was identified as
YBa2Cu3O7_«5. In May 1987, Michel et al reported. superconductivity above 22 Kin BiSr-
CuO system. In 1988 Maeda et al and Chu et al reported that ·adding Ca to BiSrCuO system
raised Tc above liquid nitrogen temperatures. The phase Bi2Ca2Sr2Cu3O10+x showed a
Tc of 110 K.
Sheng and Hermann_reported superconductivity above liquid nitrogen teJllperatures in
TIBaCuO system in 1988. They later showed :that addition of Ca can raise the teniperature
to 107 K. Parkin processed the phase Tl2Ca2Ba2Cu3010+x and got a Tc of 125 K .
Superconductivity below Tc of 94 K was observed in HgBa2CuO4+d by Pu1itin et
al during April 1993. In May 1983, Schilling et al reported a Tc of 133 Kin multiphase
Hg-Ba-Ca-CuO system and pointed out that the phase HgBa2Ca2Cu3O1+x (Hg 1223) may
be responsible for th~ high Tc, In September 1993, Chu et al reported a- Tc up to 153 K in
Hg 1223 at 150 kbar pressure. A lot of work is being done on the ·m ~ury based materi-
als.
The latest member of the high Tc cerami~_~uprates is ~opper b ased Barium Calcium
Cuprates. A Tc of more ~ail 117 Kin Ag1-:i;CuxBa2C¾-l CunQ2n+3d is -reported by
Ibara et al. Tc of more than 116 K i_s reported in Cu1-xBa2C¾-l O 2n+ 4-d by the same
· group. ~, · ·
. Q rtlu..l '( (),LU)
~ ) \ l)lJ-, VL,(J5• : . - - ~
~ tions .
_ Low temperature liquid helium siipercondm;;tors have been used to fabricate high field
magnets and so~1ie electronic and radio frequency devices. Superconducting magnets have
~n employed m NMR spectrometers and NMR imaging used in medical diagnostics. Su-
~roonductors have ~n used to produce various devices based on superconducting quan-
tum effec~. The~e mclude ~QUIDs and Josephson devices such as square law detector,

T-;; .
parametenc amplifier and nnxer. The SQUID magnetometer .can detect magnett· fi ld
low as 10-21 c e as